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the discovery of 2012 vp113 initiated the debate on the origin of the sedna family of planetesimals in orbit around the sun. sednitos roam the outer regions of the solar system between the egeworth-kuiper belt and the oort cloud, in extraordinary wide (a > 150 au) orbits with a large perihelion distance of q > 30 au compared to the earth's (a ≡ 1 au and eccentricity e ≡ (1 - q/a) ≃ 0.0167 or q ≃ 1 au). this population is composed of a dozen objects, which we consider a family because they have similar perihelion distance and inclination with respect to the ecliptic i = 10°-30°. they also have similar argument of perihelion ω = 340° ± 55°. there is no ready explanation for their origin. here we show that these orbital parameters are typical for a captured population from the planetesimal disc of another star. assuming that the orbital elements of sednitos have not changed since they acquired their orbits, we reconstruct the encounter that led to their capture. we conclude that they might have been captured in a near miss with a 1.8 m⊙ star that impacted the sun at ≃ 340 au at an inclination with respect to the ecliptic of 17°-34° with a relative velocity at infinity of ∼4.3 km s-1. we predict that the sednitos region is populated by 930 planetesimals and the inner oort cloud acquired ∼440 planetesimals through the same encounter. | how sedna and family were captured in a close encounter with a solar sibling |
in the analysis of thermal infrared data of asteroids by means of thermophysical models (tpms) it is a common practice to neglect the uncertainty of the shape model and the rotational state, which are taken as an input for the model. here, we present a novel method of investigating the importance of the shape model and the pole orientation uncertainties in the thermophysical modeling - the varied shape tpm (vs-tpm). our method uses optical photometric data to generate various shape models that map the uncertainty in the shape and the rotational state. the tpm procedure is then run for all these shape models. we apply the implementation of the classical tpm as well as our vs-tpm to the convex shape models of several asteroids together with their thermal infrared data acquired by the nasa's wide-field infrared survey explorer (wise) and compare the results. these show that the uncertainties of the shape model and the pole orientation can be very important (e.g., for the determination of the thermal inertia) and should be considered in the thermophysical analyses. we present thermophysical properties for six asteroids - (624) hektor, (771) libera, (1036) ganymed, (1472) muonio, (1627) ivar, and (2606) odessa. | thermophysical modeling of asteroids from wise thermal infrared data - significance of the shape model and the pole orientation uncertainties |
with the recent discovery of transiting planetary material around wd 1145+017, a critical target has been identified that links the evolution of planetary systems with debris disks and their accretion onto the star. we present a series of observations, five epochs over a year, taken with keck and the vlt, which for the first time show variability of circumstellar absorption in the gas disk surrounding wd 1145+017 on timescales of minutes to months. circumstellar absorption is measured in more than 250 lines of 14 ions among 10 different elements associated with planetary composition, e.g., o, mg, ca, ti, cr, mn, fe, and ni. broad circumstellar gas absorption with a velocity spread of 225 km s-1 is detected, but over the course of a year blueshifted absorption disappears, while redshifted absorption systematically increases. a correlation of equivalent width and oscillator strength indicates that the gas is not highly optically thick (median τ ≈ 2). we discuss simple models of an eccentric disk coupled with magnetospheric accretion to explain the basic observed characteristics of these high-resolution and high signal-to-noise observations. variability is detected on timescales of minutes in the two most recent observations, showing a loss of redshifted absorption for tens of minutes, coincident with major transit events and consistent with gas hidden behind opaque transiting material. this system currently presents a unique opportunity to learn how the gas causing the spectroscopic, circumstellar absorption is associated with the ongoing accretion evidenced by photospheric contamination, as well as the transiting planetary material detected in photometric observations. | spectroscopic evolution of disintegrating planetesimals: minute to month variability in the circumstellar gas associated with wd 1145+017 |
sunskirting asteroid (3200) phaethon has been repeatedly observed in solar terrestrial relations observatory (stereo) heliospheric imager 1 (hi1) imagery to anomalously brighten and produce an antisunward tail for a few days near each perihelion passage, phenomena previously attributed to the ejection of micron-sized dust grains. color imaging by the solar and heliospheric observatory (soho) large angle spectrometric coronagraph (lasco) during the 2022 may apparition indicates that the observed brightening and tail development instead capture the release of sodium atoms, which resonantly fluoresce at the 589.0/589.6 nm d lines. while hi1's design bandpass nominally excludes the d lines, filter degradation has substantially increased its d line sensitivity, as quantified by the brightness of mercury's sodium tail in hi1 imagery. furthermore, the expected fluorescence efficiency and acceleration of sodium atoms under solar radiation readily reproduce both the photometric and morphological behaviors observed by lasco and hi1 during the 2022 apparition and the 17 earlier apparitions since 1997. this finding connects phaethon to the broader population of sunskirting and sungrazing comets observed by soho, which often also exhibit bright sodium emission with minimal visible dust, but distinguishes it from other sunskirting asteroids without detectable sodium production under comparable solar heating. these differences may reflect variations in the degree of sodium depletion of near-surface material and thus the extent and/or timing of any past or present resurfacing activity. | sodium brightening of (3200) phaethon near perihelion |
the first interstellar object, ‘oumuamua, was discovered in the solar system by pan-starrs in 2017, allowing for a calibration of the abundance of interstellar objects of its size and an estimation of the subset of objects trapped by the jupiter-sun system. photographing or visiting these trapped objects would allow us to learn about the conditions in other planetary systems, relieving the need to send interstellar probes. here, we explore the orbital properties of captured interstellar objects in the solar system using dynamical simulations of the jupiter-sun system and initial conditions drawn from the distribution of relative velocities of stars in the solar neighborhood. we compare the resulting distributions of orbital elements to those of the most similar population of known asteroids, namely centaurs, to search for a parameter space in which interstellar objects should dominate and therefore be identifiable solely by their orbits. we find that there should be thousands of ‘oumuamua-size interstellar objects identifiable by centaur-like orbits at high inclinations, assuming a number density of ‘oumuamua-size interstellar objects of ∼1015 pc-3. we note eight known objects that may be of interstellar origin. finally, we estimate that the large synoptic survey telescope will be able to detect several hundreds of these objects. | identifying interstellar objects trapped in the solar system through their orbital parameters |
context. a high fraction of carbon bound in solid carbonaceous material is observed to exist in bodies formed in the cold outskirts of the solar nebula, while bodies in the region of terrestrial planets contain only very small mass fractions of carbon. most of the solid carbon component is lost and converted into co during the spiral-in of matter as the sun accretes matter from the solar nebula.aims: we study the fate of the carbonaceous material that entered the proto-solar disc by comparing the initial carbon abundance in primitive solar system material and the abundance of residual carbon in planetesimals and planets in the asteroid belt and the terrestrial planet region.methods: we constructed a model for the composition of the pristine carbonaceous material from observational data on the composition of the dust component in comets and of interplanetary dust particles and from published data on pyrolysis experiments. this material entered the inner parts of the solar nebula during the course of the build-up of the proto-sun by accreting matter from the proto-stellar disc. based on a one-zone evolution model of the solar nebula, we studied the pyrolysis of the refractory and volatile organic component and the concomitant release of hydrocarbons of high molecular weight under quiescent conditions of disc evolution, while matter migrates into the central parts of the solar nebula. we also studied the decomposition and oxidation of the carbonaceous material during violent flash heating events, which are thought to be responsible for the formation of chondrules. to do this, we calculated pyrolysis and oxidation of the carbonaceous material in temperature spikes that were modeled according to cosmochemical models for the temperature history of chondrules.results: we find that the complex hydrocarbon components of the carbonaceous material are removed from the disc matter in the temperature range between 250 and 400 k, but the amorphous carbon component survives to temperatures of 1200 k. without efficient carbon destruction during flash-heating associated with chondrule formation, the carbon abundance of terrestrial planets, except for mercury, would be of several percent and not as low as it is found in cosmochemical studies. chondrule formation seems to be a crucial process for the carbon-poor composition of the material of terrestrial planets. | spatial distribution of carbon dust in the early solar nebula and the carbon content of planetesimals |
proximity observations by osiris-rex and hayabusa2 provided clues on the shape evolution processes of the target asteroids, (101955) bennu and (162173) ryugu. their oblate shapes with equatorial ridges, or the so-called top shapes, may have evolved due to their rotational conditions at present and in the past. different shape evolution scenarios were previously proposed; bennu's top shape may have been driven by surface processing, while ryugu's may have been developed due to large deformation. these two scenarios seem to be inconsistent. here, we revisit the structural analyses in earlier works and fill a gap to connect these explanations. we also apply a semi-analytical technique for computing the cohesive strength distribution in a uniformly rotating triaxial ellipsoid to characterize the global failure of top-shaped bodies. assuming that the structure is uniform, our semi-analytical approach describes the spatial variations in failed regions at different spin periods; surface regions are the most sensitive at longer spin periods, while interiors fail structurally at shorter spin periods. this finding suggests that the shape evolution of a top shape may vary due to rotation and internal structure, which can explain the different evolution scenarios of bennu's and ryugu's top shapes. we interpret our results as the indications of top shapes' various evolution processes. | spin-driven evolution of asteroids' top-shapes at fast and slow spins seen from (101955) bennu and (162173) ryugu |
the acm multimedia 2023 computational paralinguistics challenge addresses two different problems for the first time in a research competition under well-defined conditions: in the emotion share sub-challenge, a regression on speech has to be made; and in the requests sub-challenges, requests and complaints need to be detected. we describe the sub-challenges, baseline feature extraction, and classifiers based on the usual compare features, the audeep toolkit, and deep feature extraction from pre-trained cnns using the deepspectrum toolkit; in addition, wav2vec2 models are used. | the acm multimedia 2023 computational paralinguistics challenge: emotion share & requests |
solar system small bodies were the first objects to accrete inside the protoplanetary disk, giving insights into its composition and structure. the p-/d-type asteroids are particularly interesting because of the similarity of their spectra, at visible and near infrared wavelengths (vis-nir), with cometary nuclei, suggesting that they are the most primitive types of small bodies. there are various indications that (1) their low albedo in the visible (vis) and mid-infrared (mir) wavelength ranges seems mainly controlled by the presence of opaque minerals (iron sulfides, fesbnd ni alloys etc.) (quirico et al., 2016; rousseau et al., 2018); and (2) their surfaces are made of intimate mixtures of these opaque minerals and other components (silicates, carbonaceous compounds, etc.) in the form of sub-micrometre-sized grains, smaller than the wavelength at which they are observed, so-called "hyperfine" grains. here, we investigate how the vis-nir-mir (0.55-25 μm) spectral and v-band (0.53 μm) polarimetric properties of surfaces made of hyperfine grains are influenced by the relative abundance of such hyperfine materials, having strongly different optical indexes. mixtures of grains of olivine and iron sulfide (or anthracite), as analogues of silicates and opaque minerals present on small bodies, were prepared at different proportions. the measurements reveal that these mixtures of hyperfine grains have spectral and polarimetric vis-nir properties varying in strongly nonlinear ways. when present at even a few percent, opaque components dominate the vis-nir spectral and polarimetric properties, and mask the silicate bands at these wavelengths. the vis-nir spectral slope ranges from red (positive slope), for pure opaque material, to blue (negative slope) as the proportion of silicates increases, which is reminiscent of the range of spectral slopes observed on p/d/x/c- and b-types asteroids. the spectra of the darkest mixtures in the vis-nir exhibit the absorption bands of sisbnd o in olivine around 10 μm in the mir, which is observed in emission for several small bodies. the samples studied here have macro- and micro-porosities lower than 78%, indicating that surfaces more compact than "fairy castle" hyperporous (80-99%) ones can also exhibit a blue spectral slope or a silicate signature at 10 μm. remarkably, some mixtures exhibit altogether a red spectral slope in the vis-nir, a 10-μm feature in the mir, and a v-band polarimetric phase curve similar (but not identical) to p-/d-type asteroids, reinforcing the hypothesis that these bodies are made of powdery mixtures of sub-micrometre-sized grains having contrasted optical indexes. this work shows that both the contrasted optical indexes of the components, and the dispersion or aggregation -depending on their relative proportions- of their hyperfine grains, induce different light scattering regimes in the vis-nir and mir, as observed for primitive small bodies. the optical separation of hyperfine grains seems to be a major parameter controlling the optical properties of these objects. | reflection, emission, and polarization properties of surfaces made of hyperfine grains, and implications for the nature of primitive small bodies |
this paper proposes a novel adaptive guidance system developed using reinforcement meta-learning with a recurrent policy and value function approximator. the use of recurrent network layers allows the deployed policy to adapt in real time to environmental forces acting on the agent. we compare the performance of the dr/dv guidance law, an rl agent with a non-recurrent policy, and an rl agent with a recurrent policy in four challenging environments with unknown but highly variable dynamics. these tasks include a safe mars landing with random engine failure and a landing on an asteroid with unknown environmental dynamics. we also demonstrate the ability of a rl meta-learning optimized policy to implement a guidance law using observations consisting of only doppler radar altimeter readings in a mars landing environment, and lidar altimeter readings in an asteroid landing environment thus integrating guidance and navigation. | adaptive guidance and integrated navigation with reinforcement meta-learning |
space weathering alters the optical properties of exposed surfaces over time, complicating the interpretation of spectroscopic observations of airless bodies like asteroids, mercury, and the moon. solar wind and micrometeoroids are likely the dominant agents of space weathering, but their relative contributions are not yet well understood. based primarily on clementine mosaics, we report a previously unrecognized systematic latitudinal variation in the near-infrared spectral properties of the lunar maria and show that the characteristics of this latitudinal trend match those observed at 'lunar swirls', where magnetic fields alter local solar wind flux without affecting the flux of micrometeoroids. we show that the observed latitudinal color variations are not artifacts of phase angle effects and cannot be accounted for by compositional variation alone. we propose that reduced solar wind flux, which should occur both at swirls and toward higher latitudes, is the common mechanism behind these color variations. this model helps us quantify the distinct effects of solar wind and micrometeoroid weathering and could aid in interpreting the spectra of airless bodies throughout the solar system. | latitudinal variation in spectral properties of the lunar maria and implications for space weathering |
landslides have been observed in different terrestrial environments and also on planets, satellites, and asteroids. long runout landslides are strongly dependent on the initial mass position, material and slope path properties, topographic relief, and presence of volatiles. therefore, landslides represent a means for the description of rock properties and environment of deposition prevailing at the time of occurrence and may assist in understanding the geological and climatological history of the planetary surfaces. concerning mars, previous studies have concentrated on valles marineris, where among the largest and longest landslides have been observed. using different imagery, we present and analyze an original database of 3,118 martian landslides of deposit area greater than 0.1 km2 throughout the planet between 60°n and 60°s, resulting in a data set far richer than previously done. after a distinction is made between different typologies of landslides, their position and the statistical distribution of their geometrical properties are examined. large landslides cluster along the noctis labyrinthus-valles marineris-margaritifer terra system. rock avalanches within craters are widespread, but no significant large landslides have been found at latitudes higher than 40°s and 46°n. the magnitude-frequency distribution follows a power law with scaling exponent ranging between 1.02 and 1.57, for the entire data set and varies according to the geomorphological settings, the landslide typology, and mobility. a volume-area power law relationship (exponent: 1.12-1.24) is proposed, based on the reconstruction of 222 landslide geometries, and compared to those for similar terrestrial landslides (1.39). similarities with respect to terrestrial landslide, distribution with respect to impact craters and impact energy, and cryosphere extent are also discussed. | introducing a new inventory of large martian landslides |
by means of a varied-shape thermophysical model of hanuš et al. (2015) that takes into account asteroid shape and pole uncertainties, we analyze the thermal infrared data acquired by the nasa's wide-field infrared survey explorer of about 300 asteroids with derived convex shape models. we utilize publicly available convex shape models and rotation states as input for the thermophysical modeling. for more than one hundred asteroids, the thermophysical modeling gives us an acceptable fit to the thermal infrared data allowing us to report their thermophysical properties such as size, thermal inertia, surface roughness or visible geometric albedo. this work more than doubles the number of asteroids with determined thermophysical properties, especially the thermal inertia. in the remaining cases, the shape model and pole orientation uncertainties, specific rotation or thermophysical properties, poor thermal infrared data or their coverage prevent the determination of reliable thermophysical properties. finally, we present the main results of the statistical study of derived thermophysical parameters within the whole population of main-belt asteroids and within few asteroid families. our sizes based on tpm are, in average, consistent with the radiometric sizes reported by mainzer et al. (2016). the thermal inertia increases with decreasing size, but a large range of thermal inertia values is observed within the similar size ranges between d ∼ 10-100 km. we derived unexpectedly low thermal inertias ( < 20 j m-2 s- 1 / 2 k-1) for several asteroids with sizes 10 < d < 50 km, indicating a very fine and mature regolith on these small bodies. the thermal inertia values seem to be consistent within several collisional families, however, the statistical sample is in all cases rather small. the fast rotators with rotation period p ≲ 4 h tend to have slightly larger thermal inertia values, so probably do not have a fine regolith on the surface. this could be explained, for example, by the loss of the fine regolith due to the centrifugal force, or by the ineffectiveness of the regolith production(e.g., by the thermal cracking mechanism of delbo' et al. 2014). | thermophysical modeling of main-belt asteroids from wise thermal data |
to characterize the meteoroid environment around mercury and its contribution to the planet’s exosphere, we combined four distinctive sources of meteoroids in the solar system: main-belt asteroids, jupiter-family comets, halley-type comets, and oort cloud comets. all meteoroid populations are described by currently available dynamical models. we used a recent calibration of the meteoroid influx onto earth as a constraint for the combined population model on mercury. we predict vastly different distributions of orbital elements, impact velocities, and directions of arrival for all four meteoroid populations at mercury. we demonstrate that the most likely model of mercury’s meteoroid environment—in the sense of agreement with earth—provides good agreement with previously reported observations of mercury’s exosphere by the messenger spacecraft and is not highly sensitive to variations of uncertain parameters such as the ratio of these populations at earth, the size-frequency distribution, and the collisional lifetime of meteoroids. finally, we provide a fully calibrated model consisting of high-resolution maps of mass influx and surface vaporization rates for different values of mercury’s true anomaly angle. | a comprehensive model of the meteoroid environment around mercury |
this is a pilot paper serving as a launching pad for study of orbital and spin evolution of binary asteroids. the rate of tidal evolution of asteroidal binaries is defined by the dynamical love numbers kl divided by quality factors q. common in the literature is the (oftentimes illegitimate) approximation of the dynamical love numbers with their static counterparts. since the static love numbers are, approximately, proportional to the inverse rigidity, this renders a popular fallacy that the tidal evolution rate is determined by the product of the rigidity by the quality factor: {k}l/q\propto 1/(μ q). in reality, the dynamical love numbers depend on the tidal frequency and all rheological parameters of the tidally perturbed body (not just rigidity). we demonstrate that in asteroidal binaries the rigidity of their components plays virtually no role in tidal friction and tidal lagging, and thereby has almost no influence on the intensity of tidal interactions (tidal torques, tidal dissipation, tidally induced changes of the orbit). a key quantity that overwhelmingly determines the tidal evolution is a product of the effective viscosity η by the tidal frequency χ . the functional form of the torque’s dependence on this product depends on who wins in the competition between viscosity and self-gravitation. hence a quantitative criterion, to distinguish between two regimes. for higher values of η χ , we get {k}l/q\propto 1/(η χ ), {while} for lower values we obtain {k}l/q\propto η χ . our study rests on an assumption that asteroids can be treated as maxwell bodies. applicable to rigid rocks at low frequencies, this approximation is used here also for rubble piles, due to the lack of a better model. in the future, as we learn more about mechanics of granular mixtures in a weak gravity field, we may have to amend the tidal theory with other rheological parameters, ones that do not show up in the description of viscoelastic bodies. this line of study provides a tool to exploring the orbital history of asteroidal pairs, as well as of their final spin states. | tidal evolution of asteroidal binaries. ruled by viscosity. ignorant of rigidity. |
wide-field microscopy of optically thick specimens typically features reduced contrast due to spatial cross-talk, in which the signal at each point in the field of view is the result of a superposition from neighbouring points that are simultaneously illuminated. in 1955, marvin minsky proposed confocal microscopy as a solution to this problem. today, laser scanning confocal fluorescence microscopy is broadly used due to its high depth resolution and sensitivity, but comes at the price of photobleaching, chemical and phototoxicity. here we present artificial confocal microscopy (acm) to achieve confocal-level depth sectioning, sensitivity and chemical specificity non-destructively on unlabelled specimens. we equipped a commercial laser scanning confocal instrument with a quantitative phase imaging module, which provides optical path-length maps of the specimen in the same field of view as the fluorescence channel. using pairs of phase and fluorescence images, we trained a convolution neural network to translate the former into the latter. the training to infer a new tag is very practical as the input and ground truth data are intrinsically registered and the data acquisition is automated. the acm images present much stronger depth sectioning than the input (phase) images, enabling us to recover confocal-like tomographic volumes of microspheres, hippocampal neurons in culture, and three-dimensional liver cancer spheroids. by training on nucleus-specific tags, acm allows for segmenting individual nuclei within dense spheroids for both cell counting and volume measurements. in summary, acm can provide quantitative, dynamic data, non-destructively from thick samples while chemical specificity is recovered computationally. | artificial confocal microscopy for deep label-free imaging |
interplanetary dust particles are important samples of dust-producing objects in the solar system, including many primitive and organic-rich bodies that are not sampled by known meteorites. interplanetary dust particles spiral in towards the sun under the influence of poynting-robertson drag forces and are exposed to solar energetic particles that leave tracks of ionization damage in anhydrous silicate grains. here we determine the exposure ages of track-rich interplanetary dust particles using a new calibration of the solar energetic particle track production rate and show that track-rich interplanetary dust particles have long exposure ages (>1 myr) that preclude an origin from main-belt asteroids and jupiter-family comets. we propose that track-rich interplanetary dust particles represent samples of dust produced by collisions among edgeworth-kuiper belt objects and that appreciable amounts of edgeworth-kuiper belt object dust contribute to the zodiacal cloud. many track-rich interplanetary dust particles also contain abundant secondary minerals that provide direct evidence for past aqueous activity on some edgeworth-kuiper belt objects. | evidence for a significant kuiper belt dust contribution to the zodiacal cloud |
occator crater is perhaps the most distinct surface feature observed by nasa's dawn spacecraft on the cerean surface. contained within the crater are the highest albedo features on the planet, cerealia facula and vinalia faculae, and relatively smooth lobate flow deposits. we present hydrocode simulations of the formation of occator crater, varying the water to rock ratio of our pre-impact cerean surface. we find that at water to rock mass ratios up to 0.3, sufficient volumes of occator's post-impact subsurface would be above the melting point of water to allow for the deposition of faculae-like deposits via impact-heat driven hydrothermal effusion of brines. this reservoir of hydrothermally viable material beneath the crater is composed of a mixture of impactor material and material uplifted from 10‧s of kilometers beneath the pre-impact surface, which could sample a deep subsurface volatile reservoir, if present. using a conductive cooling model, we estimate that the lifetime of hydrothermal activity within such a system, depending on choice of material constants, is between 0.4 and 4 myr. our results suggest that impact heating from the occator forming impact provides a viable mechanism for the creation of the observed faculae, with the proviso that the faculae formed within a relatively short time window after the crater itself formed. | post-impact thermal structure and cooling timescales of occator crater on asteroid 1 ceres |
successful sampling on asteroids is challenging due to their weightless environment and unknown material mechanical properties. this work presents a sweeping and grinding combined hybrid sampler (sghs) to improve the sampling success rate. the sghs has two working modes, i.e., using two brush wheels to collect regolith and rock pieces and employing a drill bit to grind rocky surfaces into particles suitable for sweeping. we investigated the sampler-regolith interaction and sample particles contact model and designed the adaptive sampling control method. numerical simulation and prototypical experiments at different structural parameters and speeds of the brush wheels, mechanical properties of the sample, and asteroid surface gravities, were conducted to validate the proposed methods and estimate the amount and speed of sample collection. the 280 g sampler can acquire 24.12 g regolith simulant in 8 s with the "weight on bit" (wob) of 0.2 n. the drill bit can enhance the sampling efficiency by 151% in rocky sample simulant acquiring. the results also indicate that sample collection on asteroids is more challenging than sampling on earth. moreover, the rotational speed of the sghs should be carefully controlled to avoid sample particles with too high a speed spilling out of the sampling site. the sampler has the advantages of small size, light weight, low wob, and high adaptability and can be used for future asteroid sample return missions. | a sweeping and grinding combined hybrid sampler for asteroid sample return mission |
recent investigations of meteorites thought to originate from the asteroid 4 vesta have suggested an early accretion of water on rocky bodies in the inner solar system from a carbonaceous chondrite-like source. however, these studies have been based on the hydrogen isotope compositions (δd) of late-crystallizing apatite grains in eucrites that likely do not record the primary magmatic composition. we have determined the δd and h2o concentrations in some of the earliest-formed silicates (clinopyroxenes) from several eucrites with the goal of constraining the hydrogen isotope composition of their source reservoir on their parent body. the h2o concentrations in clinopyroxenes from eucrites juvinas, stannern and tirhert range from 5 to 18 μg/g, with a weighted average δd of -263 ± 70‰. their apatites and whitlockites exhibit a higher weighted average δd of -165 ± 73‰, possibly as a result of h2 degassing during or after phosphate crystallization. thermal metamorphism of these eucrites has most probably resulted in the loss of h, and an increase in their original δd values. while the weighted average δd value for the eucrite clinopyroxenes reported here is inferred to reflect an upper limit for the isotopic composition of the silicate mantle reservoir on their parent asteroid 4 vesta, the average δd value of stannern clinopyroxenes is considered to be closest to the initial δd of the source mantle (i.e., -373 ± 127‰), which is lighter than that of earth's depleted upper mantle and most carbonaceous chondrites. we suggest that at least some of the water in 4 vesta (and possibly other rocky bodies in the inner solar system) was derived from a relatively deuterium-poor reservoir in the protosolar nebula, which was incorporated into planetesimals formed early in solar system history. | a deuterium-poor water reservoir in the asteroid 4 vesta and the inner solar system |
benthic foraminifera are the most common meiofaunal unicellular deep-sea biota, forming skeletons used as proxies for past climate change. we aim to increase understanding of past non-analog oceans and ecosystems by evaluating deep-sea benthic foraminiferal responses to global environmental changes over latest cretaceous through oligocene times (67-23 million years ago). earth suffered an asteroid impact at the end of the cretaceous (~instantaneous; 66 ma), episodes of rapid global warming during the paleocene-eocene thermal maximum (petm; ~56 ma) and other hyperthermals (millennial timescales), followed by gradual, but punctuated cooling (timescales of hundred thousands of years) from a world without polar ice sheets to a world with a large antarctic ice sheet. here we present the first compilation of quantitative data on deep-sea foraminifera at sites in all the world's oceans, aiming to build a first unique, uniform database that allows comparison of deep-sea faunal turnover across the uppermost cretaceous through paleogene. we document variability in space and time of benthic foraminiferal diversity: lack of extinction at the asteroid impact even though other marine and terrestrial groups suffered mass extinction; major extinction at the petm followed by recovery and diversification; and gradual but fundamental turnover during gradual cooling and increase in polar ice volume (possibly linked to changes in the oceanic carbon cycle). high latitude cooling from ~45 ma on, i.e., after the end of the early eocene climate optimum (53.2-49.2 ma), may have made the middle eocene a critical period of several millions of years of faunal turnover and establishment of latitudinal diversity gradients. this compilation thus illuminates the penetration of global change at very different rates into the largest and one of the most stable habitats on earth, the deep sea with its highly diverse biota. | turnover and stability in the deep sea: benthic foraminifera as tracers of paleogene global change |
the hadean history of earth is shrouded in mystery and it is considered that the planet was born dry with no water or atmosphere. the earth-moon system had many features in common during the birth stage. solidification of the dry magma ocean at 4.53 ga generated primordial continents with komatiite. we speculate that the upper crust was composed of fractionated gabbros and the middle felsic crust by anorthosite at ca. 21 km depth boundary, underlain by meta-anorthosite (grossular + kyanite + quartz) down to 50-60 km in depth. the thickness of the mafic kreep basalt in the lower crust, separating it from the underlying upper mantle is not well-constrained and might have been up to ca. 100-200 km depending on the degree of fractionation and gravitational stability versus surrounding mantle density. the primordial continents must have been composed of the final residue of dry magma ocean and enriched in several critical elements including ca, mg, fe, mn, p, k, and cl which were exposed on the surface of the dry earth. around 190 million years after the solidification of the magma ocean, "abel bombardment" delivered volatiles including h2o, co2, n2 as well as silicate components through the addition of icy asteroids. this event continued for 200 myr with subordinate bombardments until 3.9 ga, preparing the earth for the prebiotic chemical evolution and as the cradle of first life. due to vigorous convection arising from high mantle potential temperatures, the primordial continents disintegrated and were dragged down to the deep mantle, marking the onset of hadean plate tectonics. | hadean earth and primordial continents: the cradle of prebiotic life |
near-earth asteroid (101955) bennu is an active asteroid experiencing mass loss in the form of ejection events emitting up to hundreds of millimeter- to centimeter-scale particles. the close proximity of the origins, spectral interpretations, resource identification, and security-regolith explorer spacecraft enabled monitoring of particles for a 10-month period encompassing bennu's perihelion and aphelion. we found 18 multiparticle ejection events, with masses ranging from near zero to hundreds of grams (or thousands with uncertainties) and translational kinetic energies ranging from near zero to tens of millijoules (or hundreds with uncertainties). we estimate that bennu ejects ~104 g per orbit. the largest event took place on 6 january 2019 and consisted of ~200 particles. the observed mass and translational kinetic energy of the event were between 459 and 528 g and 62 and 77 mj, respectively. hundreds of particles not associated with the multiparticle ejections were also observed. photometry of the best-observed particles, measured at phase angles between ~70° and 120°, was used to derive a linear phase coefficient of 0.013 ± 0.005 magnitudes per degree of phase angle. ground-based data back to 1999 show no evidence of past activity for bennu; however, the currently observed activity is orders of magnitude lower than observed at other active asteroids and too low be observed remotely. there appears to be a gentle decrease in activity with distance from the sun, suggestive of ejection processes such as meteoroid impacts and thermal fracturing, although observational bias may be a factor. | photometry of particles ejected from active asteroid (101955) bennu |
context. gaia is europe's space astrometry mission, aiming to make a three-dimensional map of 1000 million stars in our milky way to unravel its kinematical, dynamical, and chemical structure and evolution.aims: we present a study of gaia's detection capability of objects, in particular non-saturated stars, double stars, unresolved external galaxies, and asteroids. gaia's on-board detection software autonomously discriminates stars from spurious objects like cosmic rays and solar protons. for this, parametrised criteria of the shape of the point spread function are used, which need to be calibrated and tuned. this study aims to provide an optimum set of parameters for these filters.methods: we developed a validated emulation of the on-board detection software, which has 20 free, so-called rejection parameters which govern the boundaries between stars on the one hand and sharp (high-frequency) or extended (low-frequency) events on the other hand. we evaluate the detection and rejection performance of the algorithm using catalogues of simulated single stars, resolved and unresolved double stars, cosmic rays, solar protons, unresolved external galaxies, and asteroids.results: we optimised the rejection parameters, improving - with respect to the functional baseline - the detection performance of single stars and of unresolved and resolved double stars, while, at the same time, improving the rejection performance of cosmic rays and of solar protons. the optimised rejection parameters also remove the artefact of the functional-baseline parameters that the reduction of the detection probability of stars as a function of magnitude already sets in before the nominal faint-end threshold at g = 20 mag. we find, as a result of the rectangular pixel size, that the minimum separation to resolve a close, equal-brightness double star is 0.23 arcsec in the along-scan and 0.70 arcsec in the across-scan direction, independent of the brightness of the primary. to resolve double stars with δg> 0 mag, larger separations are required. we find that, whereas the optimised rejection parameters have no significant impact on the detectability of pure de vaucouleurs profiles, they do significantly improve the detection of pure exponential-disk profiles, and hence also the detection of unresolved external galaxies with intermediate profiles. we also find that the optimised rejection parameters provide detection gains for asteroids fainter than 20 mag and for fast-moving near-earth objects fainter than 18 mag, although this gain comes at the expense of a modest detection-probability loss for bright, fast-moving near-earth objects. the major side effect of the optimised parameters is that spurious ghosts in the wings of bright stars essentially pass unfiltered. | detecting stars, galaxies, and asteroids with gaia |
roughly 1000 white dwarfs are known to be polluted with planetary material, and the progenitors of this material are typically assumed to be asteroids. the dynamical architectures which perturb asteroids into white dwarfs are still unknown, but may be crucially dependent on moons liberated from parent planets during post-main-sequence gravitational scattering. here, we trace the fate of these exomoons, and show that they more easily achieve deep radial incursions towards the white dwarf than do scattered planets. consequently, moons are likely to play a significant role in white dwarf pollution, and in some cases may be the progenitors of the pollution itself. | the fate of exomoons in white dwarf planetary systems |
the main motivation of this research is the analytical exploration of the dynamics of asteroid rotation when it moves in elliptic orbit through space. according to the results of efroimsky, frouard (2016), various perturbations (collisions, close encounters, yorp effect) destabilize the rotation of a small body (asteroid), deviating it from the initial-current spin state. this yields evolution of the spin towards rotation about maximal-inertia axis due to the process of nutation relaxation or to the proper spin state corresponding to minimal energy with a fixed angular momentum. we consider in our research the aforementioned spin state of asteroid but additionally under non-vanishing influence of the effects of non-gravitational nature (yorp effect), which is destabilizing the asteroid rotation during its motion far from giant planets. meanwhile, new solutions for asteroid rotation dynamics in case of negligible (time-dependent) applied torques have been obtained in our development. new method for solving euler's equations for rigid body rotation is suggested; an elegant example for evolution of spin towards the rotation about maximal-inertia axis is calculated. | the dynamics of asteroid rotation, governed by yorp effect: the kinematic ansatz |
dynamicalmass estimates for the main asteroid belt and the trans-neptunian kuiper belt have been found from their gravitational influence on the motion of planets. discrete rotating models consisting ofmovingmaterial points have been used tomodel the total attraction fromsmall or as yet undetected bodies of the belts. the masses of the model belts have been included in the set of parameters being refined and determined and have been obtained by processing more than 800 thousand modern positional observations of planets and spacecraft. we have processed the observations and determined the parameters based on the new epm2017 version of the iaa ras planetary ephemerides. the large observed radial extent of the belts (more than 1.2 au for the main belt and more than 8 au for the kuiper belt) and the concentration of bodies in the kuiper belt at a distance of about 44 au found from observations have been taken into account in the discrete models. we have also used individual mass estimates for large bodies of the belts as well as for objects that spacecraft have approached and for bodies with satellites. our mass estimate for the main asteroid belt is (4.008 ± 0.029) × 10-4/ m ⊕ (3σ). the bulk of the kuiper belt objects are in the ring zone from 39.4 to 47.8 au. the estimate of its total mass together with the mass of the 31 largest trans-neptunian kuiper belt objects is (1.97 ± 0.30) × 10-2 m ⊕ (3σ), which exceeds the mass of the main asteroid belt almost by a factor of 50. the mass of the 31 largest trans-neptunian objects (tnos) is only about 40% of the total one. | masses of the main asteroid belt and the kuiper belt from the motions of planets and spacecraft |
the burrows-wheeler transform (bwt) is an invertible text transformation that permutes symbols of a text according to the lexicographical order of its suffixes. bwt is the main component of popular lossless compression programs (such as bzip2) as well as recent powerful compressed indexes (such as $r$-index [gagie et al., j. acm, 2020]), central in modern bioinformatics. the compression ratio of bwt is quantified by the number $r$ of equal-letter runs. despite the practical significance of bwt, no non-trivial bound on the value of $r$ is known. this is in contrast to nearly all other known compression methods, whose sizes have been shown to be either always within a ${\rm polylog}\,n$ factor (where $n$ is the length of text) from $z$, the size of lempel-ziv (lz77) parsing of the text, or significantly larger in the worst case (by a $n^{\varepsilon}$ factor for $\varepsilon > 0$). in this paper, we show that $r = \mathcal{o}(z \log^2n)$ holds for every text. this result has numerous implications for text indexing and data compression; for example: (1) it proves that many results related to bwt automatically apply to methods based on lz77, e.g., it is possible to obtain functionality of the suffix tree in $\mathcal{o}(z\,{\rm polylog}\,n)$ space; (2) it shows that many text processing tasks can be solved in the optimal time assuming the text is compressible using lz77 by a sufficiently large ${\rm polylog}\,n$ factor; (3) it implies the first non-trivial relation between the number of runs in the bwt of the text and its reverse. in addition, we provide an $\mathcal{o}(z\,{\rm polylog}\,n)$-time algorithm converting the lz77 parsing into the run-length compressed bwt. to achieve this, we develop a number of new data structures and techniques of independent interest. | resolution of the burrows-wheeler transform conjecture |
we present experimental phase function and degree of linear polarization curves for seven samples of cometary dust analogues namely: ground pieces of allende, dag521, fro95002, and fro99040 meteorites, mg-rich olivine and pyroxene, and a sample of organic tholins. the experimental curves have been obtained at the iaa cosmic dust laboratory at a wavelength of 520 nm covering a phase angle range from 3° to 175°. we also provide values of the backscattering enhancement for our cometary analogue samples. the final goal of this work is to compare our experimental curves with observational data of comets and asteroids to better constrain the nature of cometary and asteroidal dust grains. all measured phase functions present the typical behaviour for μm-sized cosmic dust grains. direct comparison with data provided by the osiris/rosetta camera for comet 67p/churyumov-gerasimenko reveals significant differences and supports the idea of a coma dominated by big chunks, larger than one micrometer. the polarization curves are qualitatively similar to ground-based observations of comets and asteroids. the position of the inversion polarization angle seems to be dependent on the composition of the grains. we find opposite dependence of the maximum of the polarization curve for grains sizes in the rayleigh-resonance and geometric optics domains, respectively. | experimental phase function and degree of linear polarization of cometary dust analogues |
in this work we combine several constraints provided by the crater records on arrokoth and the worlds of the pluto system to compute the size-frequency distribution (sfd) of the crater production function for craters with diameter d ≲ 10 km. for this purpose, we use a kuiper belt objects (kbo) population model calibrated on telescopic surveys, that describes also the evolution of the kbo population during the early solar system. we further calibrate this model using the crater record on pluto, charon and nix. using this model, we compute the impact probability on arrokoth, integrated over the age of the solar system. this probability is then used together with other observational constraints to determine the slope of the crater-production function on arrokoth. these constraints are: (i) the spatial density of sub-km craters, (ii) the absence of craters with 1 < d < 7 km; (iii) the existence of a single crater with d > 7 km. in addition, we use our kuiper belt model also to compare the impact rates and velocities of kbos on arrokoth with those on charon, integrated over the crater retention ages of their respective surfaces. this allows us to establish a relationship between the spatial density of sub-km craters on arrokoth and of d ~ 20 km craters on charon. together, all these considerations suggest the crater production function on these worlds has a cumulative power law slope of -1.5 < q < - 1.2. converted into a projectile sfd slope, we find -1.2 < qkbo < - 1.0. these values are close to the cumulative slope of main belt asteroids in the 0.2-2 km range, a population in collisional equilibrium (bottke et al., 2020). for kbos, however, this slope appears to extend from ~2 km down to objects a few tens of meters in diameter, as inferred from sub-km craters on arrokoth. from the measurement of the dust density in the kuiper belt made by the new horizons mission, we predict that the sfd of the kbos becomes steep again below ~10-30 m. all these considerations strongly indicate that the size distribution of the kbo population is in collisional equilibrium. | a re-assessment of the kuiper belt size distribution for sub-kilometer objects, revealing collisional equilibrium at small sizes |
protoplanetary disks are dust-rich structures around young stars. the crystalline and amorphous materials contained within these disks are variably thermally processed and accreted to make bodies of a wide range of sizes and compositions, depending on the heliocentric distance of formation. the chondritic meteorites are fragments of relatively small and undifferentiated bodies, and the minerals that they contain carry chemical signatures providing information about the early environment available for planetesimal formation. a current hot topic of debate is the delivery of volatiles to terrestrial planets, understanding that they were built from planetesimals formed under far more reducing conditions than the primordial carbonaceous chondritic bodies. in this review, we describe significant evidence for the accretion of ices and hydrated minerals in the outer protoplanetary disk. in that distant region highly porous and fragile carbon and water-rich transitional asteroids formed, being the parent bodies of the carbonaceous chondrites (ccs). ccs are undifferentiated meteorites that never melted but experienced other physical processes including thermal and aqueous alteration. recent evidence indicates that few of them have escaped significant alteration, retaining unique features that can be interpreted as evidence of wet accretion. some examples of carbonaceous chondrite parent body aqueous alteration will be presented. finally, atomistic interpretations of the first steps leading to water-mediated alteration during the accretion of ccs are provided and discussed. from these new insights into the water retained in ccs we can decipher the pathways of delivery of volatiles to the terrestrial planets. | accretion of water in carbonaceous chondrites: current evidence and implications for the delivery of water to early earth |
the ci and ci-like chondrites provide a record of aqueous alteration in the early solar system. however, the ci-like chondrites differ in having also experienced a late stage period of thermal metamorphism. in order to constrain the nature and extent of the aqueous and thermal alteration, we have investigated the bulk mineralogy and abundance of h2o in the ci and ci-like chondrites using thermogravimetric analysis and infrared spectroscopy. | characterising the ci and ci-like carbonaceous chondrites using thermogravimetric analysis and infrared spectroscopy |
as the largest magmatic iron meteorite group, the iiiab group is often used to investigate the process of core crystallization in asteroid-sized bodies. however, previous iiiab crystallization models have not succeeded in both explaining the scatter among iiiab irons around the main crystallization trends and using elemental partitioning behavior consistent with experimental determinations. this study outlines a revised approach for modeling the crystallization of irons that uses experimentally determined partition coefficients and can reproduce the iiiab trends and their associated scatter for 12 siderophile elements simultaneously. a key advancement of this revised trapped melt model is the inclusion of an effect on the resulting solid metal composition due to the formation of troilite. the revised trapped melt model supports the previous conclusion that trapped melt played an important role in the genesis of iiiab irons and matches the trace element fractionation trends observed in the cape york suite as due to different amounts of trapped melt. applying the revised trapped melt model to 16 elements as well as s and fe, the bulk composition of the iiiab core is found to have a composition consistent with that expected from a chondritic precursor for refractory siderophile elements but with evidence for depletions of more volatile elements. the bulk s composition of the iiiab core is estimated as 9 ± 1 wt%, implying that a substantial amount of s-rich material from the iiiab core is underrepresented in our meteorite collections. future applications of the revised trapped melt model to other magmatic iron meteorite groups can enable comparisons between the core compositions and crystallization processes across the early solar system. | a revised trapped melt model for iron meteorites applied to the iiiab group |
the binary asteroid 65803 didymos-dimorphos is the target of the first asteroid deflection test (nasa's double asteroid redirection test, dart) and the first binary asteroid system that will be characterized by a rendezvous mission (esa's hera). the cohesive strength of the fast-spin-primary didymos is a key factor that could affect the impact outcome and stability of this system. to support the preparation and data interpretation of these missions and gain a better understanding of the formation and evolution of this system, we investigate the structural stability and cohesive strength of didymos based on current observational information. we use the didymos radar shape model to construct rubble-pile models consisting of ~40,000 to ~100,000 particles with different arrangements and size distributions. to investigate the effect of cohesion on the structural stability and dynamical behaviors of didymos, we explicitly simulate the yorp spin-up process of these rubble-pile models from a slow spin state to didymos' current spin state using a high-efficiency soft-sphere-discrete-element-model code, pkdgrav. we test the creep stability of didymos' rubble-pile representation with different values of cohesion and derive the critical amount of cohesion to maintain stability. the results show that didymos should at least have a minimum bulk cohesion on the order of 10 pa to maintain its structural stability if the interparticle tensile strength is uniformly distributed. since the surface particles are less bonded by cohesive contacts than the interior particles, the internal macroscopic cohesion is about three times the surface macroscopic cohesion. we find that the bulk density and particle arrangement and size distribution of didymos have significant influences on its critical cohesion and failure behaviors, indicating different binary formation pathways. with the critical cohesion, didymos is at the edge of maintaining a stable shape, and a rapid small decrease in its spin period would excite its rubble-pile structure and lead to reshaping or mass shedding. whether the dart impact could partially or globally destabilize this system requires further investigation of the full two-body gravitational dynamics and the ejecta evolution. with the expected measurements returned by dart's onboard cubesat liciacube in 2022 and hera in 2027, the correlations between didymos' physical properties and failure behaviors found in this study may be possible to constrain the mechanical properties and evolutionary history of this binary system. | creep stability of the dart/hera mission target 65803 didymos: ii. the role of cohesion |
the origin of the diverse organic compounds present in carbonaceous chondrites (cc) remains uncertain. we aim at investigating the role that hydrothermal alteration may have had on the molecular evolution of organic matter (om). in particular, within cc matrices, om is intimately embedded within phyllosilicates down to the nanometer scale, which raises the question of the influence of phyllosilicates on om transformation during hydrothermal alteration on parent bodies. we conducted hydrothermal experiments at 150 °c and alkaline ph, using a well-known molecule present in processed interstellar ice analogues, the hexamethylenetetramine (hmt), in the presence of al- and fe-rich smectites. experimental products were characterized by gas chromatography mass spectrometry, infrared spectroscopy, x-ray diffraction and synchrotron-based x-ray absorption near edge structure spectroscopy. within 31 days, the hmt + smectites + h2o system leads to (1) the formation of a diverse suite of soluble organic compounds, yet less abundant and less complex than in the absence of smectite, (2) carbon-rich smectite residues (3.8 wt.% and 2.6 wt.% of carbon for the al- and fe-rich smectite residues, respectively). in addition, the abundance and molecular composition of the final organic compounds depend on the nature of the phyllosilicate (al vs. fe-smectite). various and complex interaction mechanisms could occur between om and smectite. physisorption, chemisorption and intercalation processes have likely entrapped a significant portion of the organic compounds, thereby altering their chemical evolution. the present work demonstrates that the presence and the nature of phyllosilicates influences the reaction pathways of organic compounds during hydrothermal alteration and that the presence of organic compounds may impact the mineral assemblage. this could have had significant importance for the co-evolution of om and mineral phases in primitive bodies during hydrothermal alteration. | influence of phyllosilicates on the hydrothermal alteration of organic matter in asteroids: experimental perspectives |
the collapse of dust particle clouds directly to kilometer-sized planetesimals is a promising way to explain the formation of planetesimals, asteroids, and comets. in the past, this collapse has been studied in stratified shearing box simulations with super-solar dust-to-gas ratio ɛ, allowing for streaming instability (si) and gravitational collapse. this paper studies the non-stratified si under dust-to-gas ratios from ɛ =0.1 up to ɛ =1000 without self-gravity. the study covers domain sizes of l=0.1 {{h}}, 0.01 {{h}}, and 0.001 {{h}} in terms of the gas-disk scale height {{h}} using the pencilcode. they are performed in radial-azimuthal (2d) and radial-vertical (2.5d) extents. the used particles of {st}=0.01 and 0.1 mark the upper end of the expected dust growth. si activity is found up to very high dust-to-gas ratios, providing fluctuations in the local dust-to-gas ratios and turbulent particle diffusion δ. we find an si-like instability that operates in r-φ, even when vertical modes are suppressed. this new azimuthal streaming instability (asi) shows similar properties and appearance as the si. both, si and asi show diffusivity at ɛ =100 only to be two orders of magnitude lower than at ɛ =1, suggesting a δ ∼ {ɛ }-1. relation that is shallow around ɛ ≈ 1. the (a)si ability to concentrate particles is found to be uncorrelated with its strength in particle turbulence. finally, we performed a resolution study to test our findings of the asi. this paper stresses the importance of properly resolving the (a)si at high dust-to-gas ratios and planetesimal collapse simulations, leading otherwise to potentially incomplete results. | azimuthal and vertical streaming instability at high dust-to-gas ratios and on the scales of planetesimal formation |
spacecraft observations of atmosphereless solar system bodies, combined with thermophysical modeling, provide important information about the thermal inertia and degree of surface roughness of these bodies. the thermophysical models rely on various methods of generating topography, the most common being the concave spherical segment. we here compare the properties of thermal emission for a number of different topographies - concave spherical segments, random gaussians, fractals and parallel sinusoidal trenches - for various illumination and viewing geometries, degrees of surface roughness and wavelengths. we find that the thermal emission is strongly dependent on roughness type, even when the degrees of roughness are identical, for certain illumination and viewing geometries. the systematic usage of any single topography model may therefore bias determinations of thermal inertia and level of roughness. we outline strategies that may be employed during spacecraft observations to disentangle thermal inertia, level of roughness and type of topography. we also compare the numerically complex and time consuming full-scale thermophysical models with a simplified statistical approach, which is fairly easy to implement and quick to run. we conclude that the simplified statistical approach is similar to thermophysical models for cases tested here, which enables the user to analyze huge amounts of spectral data at a low numerical cost. | interpretation of thermal emission. i. the effect of roughness for spatially resolved atmosphereless bodies |
magnesium-rich silicate chondrules and calcium-, aluminum-rich refractory inclusions (cais) are fundamental components of primitive chondritic meteorites. it has been suggested that concentration of these early-formed particles by nebular sorting processes may lead to accretion of planetesimals, the planetary bodies that represent the building blocks of the terrestrial planets. in this case, the size distributions of the particles may constrain the accretion process. here we present new particle size distribution data for northwest africa 5717, a primitive ordinary chondrite (ungrouped 3.05) and the well-known carbonaceous chondrite allende (cv3). instead of the relatively narrow size distributions obtained in previous studies (ebel et al., 2016; friedrich et al., 2015; paque and cuzzi, 1997, and references therein), we observed broad size distributions for all particle types in both meteorites. detailed microscopic image analysis of allende shows differences in the size distributions of chondrule subtypes, but collectively these subpopulations comprise a composite "chondrule" size distribution that is similar to the broad size distribution found for cais. also, we find accretionary 'dust' rims on only a subset (∼15-20%) of the chondrules contained in allende, which indicates that subpopulations of chondrules experienced distinct histories prior to planetary accretion. for the rimmed subset, we find positive correlation between rim thickness and chondrule size. the remarkable similarity between the size distributions of various subgroups of particles, both with and without fine grained rims, implies a common size sorting process. chondrite classification schemes, astrophysical disk models that predict a narrow chondrule size population and/or a common localized formation event, and conventional particle analysis methods must all be critically reevaluated. we support the idea that distinct "lithologies" in nwa 5717 are nebular aggregates of chondrules. if ≥cm-sized aggregates of chondrules can form it will have implications for planet formation and suggests the sticking stage is where the preferential size physics is operating. | particle size distributions in chondritic meteorites: evidence for pre-planetesimal histories |
the ubvri broad band photometric system is widely used in ccd astronomy. there are a lot of sets of standard stars for this photometric system, the landolt's and stetson's catalogues being the most precise and reliable. another photometric system, recently considerably spread in ccd observations is ugriz, which originates from the sloan digital sky survey (sdss) and has now many variations based on its 5 broad-band filters. one of the photometric systems based on it is the panoramic survey telescope and rapid response system (pan-starrs). in this paper we compare the bvri magnitudes in the stetson catalogue of standard stars with the magnitudes of the corresponding stars in the pan-starrs1 (ps1) grizyw catalogue. transformations between these two systems are presented and discussed. an algorithm for data reduction and calibration is developed and its functionality is demonstrated in the magnitude determination of an asteroid. | transformation of pan-starrs1 gri to stetson bvri magnitudes. photometry of small bodies observations. |
hayabusa2 is a sample return mission of jaxa launched on 3 december 2014. hayabusa2 is the successor of hayabusa, which returned samples from the asteroid itokawa to the earth. although the design of hayabusa2 follows that of hayabusa, the former is equipped with some new components. the small carry-on impactor (sci) is one of those components. the sci is a compact kinetic impactor designed to remove the asteroid surface regolith locally and create an artificial crater. one of the most important scientific objectives of hayabusa2 is to investigate the chemical and physical properties of the internal materials and structures of the target body, asteroid ryugu. hayabusa2 will attempt to observe the resultant crater with some scientific instruments and to get samples from around the crater. high kinetic energy is required to create a meaningful crater, however, the impact system design needs to fit within strict constraints. complicated functions, such as a guidance and control system, are not permitted. a special type of shaped charge is used for the acceleration of the impactor of the sci in order to make system simpler. using this explosion technique makes it possible to accelerate the impactor very quickly and to hit the asteroid without a guidance system. however, the impact operation will be complicated because the explosive is very powerful and it scatters high-speed debris at the detonation. this paper describes an overview of the sci system, the results of the development testing and an outline of the impact experiment of the hayabusa2 mission. | the small carry-on impactor (sci) and the hayabusa2 impact experiment |
prior to the arrival of the dawn spacecraft at ceres, the dwarf planet was anticipated to be ice-rich. searches for morphological features related to ice have been ongoing during dawn's mission at ceres. here we report the identification of pitted terrains associated with fresh cerean impact craters. the cerean pitted terrains exhibit strong morphological similarities to pitted materials previously identified on mars (where ice is implicated in pit development) and vesta (where the presence of ice is debated). we employ numerical models to investigate the formation of pitted materials on ceres and discuss the relative importance of water ice and other volatiles in pit development there. we conclude that water ice likely plays an important role in pit development on ceres. similar pitted terrains may be common in the asteroid belt and may be of interest to future missions motivated by both astrobiology and in situ resource utilization. | pitted terrains on (1) ceres and implications for shallow subsurface volatile distribution |
near-earth objects (neos) that orbit the sun on or within earth's orbit are tricky to detect for earth-based observers due to their proximity to the sun in the sky. these small bodies hold clues to the dynamical history of the inner solar system as well as the physical evolution of planetesimals in extreme environments. populations in this region include the atira and vatira asteroids, as well as venus and earth co-orbital asteroids. here we present a twilight search for these small bodies, conducted using the 1.2 m oschin schmidt and the zwicky transient facility (ztf) camera at palomar observatory. the ztf twilight survey operates at solar elongations down to 35° with a limiting magnitude of r = 19.5. during a total of 40 evening sessions and 62 morning sessions conducted between 2018 november 15 and 2019 june 23, we detected six atiras, including two new discoveries, 2019 aq3 and 2019 lf6, but no vatiras or earth/venus co-orbital asteroids. neo population models show that these new discoveries are likely only the tip of the iceberg, with the bulk of the population yet to be found. the population models also suggest that we have only detected 5%-7% of the h < 20 atira population over the seven month survey. co-orbital asteroids are smaller in diameter and require deeper surveys. a systematic and efficient survey of the near-sun region will require deeper searches and/or facilities that can operate at small solar elongations. | a twilight search for atiras, vatiras, and co-orbital asteroids: preliminary results |
planets and minor bodies such as asteroids, kuiper-belt objects, and comets are integral components of a planetary system. interactions among them leave clues about the formation process of a planetary system. the signature of such interactions is most prominent through observations of its debris disk at millimeter wavelengths where emission is dominated by the population of large grains that stay close to their parent bodies. here we present alma 1.3 mm observations of hd 95086, a young early-type star that hosts a directly imaged giant planet b and a massive debris disk with both asteroid- and kuiper-belt analogs. the location of the kuiper-belt analog is resolved for the first time. the system can be depicted as a broad (δr/r ∼ 0.84), inclined (30° ± 3°) ring with millimeter emission peaked at 200 ± 6 au from the star. the 1.3 mm disk emission is consistent with a broad disk with sharp boundaries from 106 ± 6 to 320 ± 20 au with a surface density distribution described by a power law with an index of -0.5 ± 0.2. our deep alma map also reveals a bright source located near the edge of the ring, whose brightness at 1.3 mm and potential spectral energy distribution are consistent with it being a luminous star-forming galaxy at high redshift. we set constraints on the orbital properties of planet b assuming coplanarity with the observed disk. | alma 1.3 mm map of the hd 95086 system |
understanding the fate of planetary systems through white dwarfs which accrete debris crucially relies on tracing the orbital and physical properties of exo-asteroids during the giant branch phase of stellar evolution. giant branch luminosities exceed the sun's by over three orders of magnitude, leading to significantly enhanced yarkovsky and yorp effects on minor planets. here, we place bounds on yarkovsky-induced differential migration between asteroids and planets during giant branch mass-loss by modelling one exo-neptune with inner and outer exo-kuiper belts. in our bounding models, the asteroids move too quickly past the planet to be diverted from their eventual fate, which can range from: (i) populating the outer regions of systems out to 104-105 au, (ii) being engulfed within the host star, or (iii) experiencing yarkovsky-induced orbital inclination flipping without any yarkovsky-induced semimajor axis drift. in these violent limiting cases, temporary resonant trapping of asteroids with radii of under about 10 km by the planet is insignificant, and capture within the planet's hill sphere requires fine-tuned dissipation. the wide variety of outcomes presented here demonstrates the need to employ sophisticated structure and radiative exo-asteroid models in future studies. determining where metal-polluting asteroids reside around a white dwarf depends on understanding extreme yarkovsky physics. | speeding past planets? asteroids radiatively propelled by giant branch yarkovsky effects |
dawn's framing camera observed boulders on the surface of vesta when the spacecraftwas in its lowest orbit (low altitude mapping orbit, lamo). we identified, measured, and mapped boulders in lamo images, which have a scale of 20 m per pixel. we estimate that our sample is virtually complete down to a boulder size of 4 pixels (80 m). the largest boulder is a 400 m sized block on the marcia crater floor. relatively few boulders reside in a large area of relatively low albedo, surmised to be the carbon rich ejecta of the veneneia basin, either because boulders form less easily here or live shorter. by comparing the density of boulders around craters with a known age, we find that the maximum boulder lifetime is about 300 ma. the boulder size frequency distribution (sfd) is generally assumed to follow a power law. we fit power laws to the vesta sfd by means of the maximum likelihood method, but they do not fit well. our analysis of power law exponents for boulders on other small solar system bodies suggests that the derived exponent is primarily a function of boulder size range. the weibull distribution mimics this behavior and fits the vesta boulder sfd well. the weibull distribution is often encountered in rock grinding experiments and may result from the fractal nature of cracks propagating in the rock interior. we propose that, in general, the sfd of particles (including boulders) on the surface of small bodies follows a weibull distribution rather than a power law. | the boulder population of asteroid 4 vesta: size frequency distribution and survival time |
context. the centaur (10199) chariklo has the first ring system discovered around a small object. it was first observed using stellar occultation in 2013. stellar occultations allow sizes and shapes to be determined with kilometre accuracy, and provide the characteristics of the occulting object and its vicinity.aims: using stellar occultations observed between 2017 and 2020, our aim is to constrain the physical parameters of chariklo and its rings. we also determine the structure of the rings, and obtain precise astrometrical positions of chariklo.methods: we predicted and organised several observational campaigns of stellar occultations by chariklo. occultation light curves were measured from the datasets, from which ingress and egress times, and the ring widths and opacity values were obtained. these measurements, combined with results from previous works, allow us to obtain significant constraints on chariklo's shape and ring structure.results: we characterise chariklo's ring system (c1r and c2r), and obtain radii and pole orientations that are consistent with, but more accurate than, results from previous occultations. we confirm the detection of w-shaped structures within c1r and an evident variation in radial width. the observed width ranges between 4.8 and 9.1 km with a mean value of 6.5 km. one dual observation (visible and red) does not reveal any differences in the c1r opacity profiles, indicating a ring particle size larger than a few microns. the c1r ring eccentricity is found to be smaller than 0.022 (3σ), and its width variations may indicate an eccentricity higher than ~0.005. we fit a tri-axial shape to chariklo's detections over 11 occultations, and determine that chariklo is consistent with an ellipsoid with semi-axes of 143.8−1.5+1.4, 135.2−2.8+1.4, and 99.1−2.7+5.4 km. ultimately, we provided seven astrometric positions at a milliarcsecond accuracy level, based on gaia edr3, and use it to improve chariklo's ephemeris. tables c.1, c.2 and lightcurves are only available at the cds via anonymous ftp to cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/j/a+a/652/a141. | refined physical parameters for chariklo's body and rings from stellar occultations observed between 2013 and 2020 |
hayabusa2 is the japanese asteroid return mission and targeted the carbonaceous asteroid ryugu, conducted by the japan aerospace exploration agency (jaxa). the goal of this mission was to conduct proximity operations including remote sensing observations, material sampling, and a small carry-on impact experiment, as well as sample analyses. as of september 2020, the spacecraft is on the way back to earth with samples from ryugu with no critical issues after the successful departure in november 2019. here, we propose an extended mission in which the spacecraft will rendezvous with a small asteroid with ~30 m - ~40 m in diameter that is rotating at a spin period of ~10 min after an additional ~10-year cruise phase. we introduce that two scenarios are suitable for the extended mission. in the first scenario, the spacecraft will perform swing-by maneuvers at venus once and earth twice to arrive at asteroid 2001 av43. in the second scenario, it will perform swing-by maneuvers at earth twice to reach asteroid 1998 ky26. in both scenarios, the mission will continue until the early 2030s. jaxa recently released the decision that the spacecraft will rendezvous with 1998 ky26. this paper focuses on our scientific assessments of the two scenarios but leaves the decision process to go to 1998 ky26 for future reports. rendezvous operations will be planned to detail the physical properties and surrounding environments of the target, one of the smallest elements of small planetary bodies. by achieving the planned operations, the mission will provide critical hints on the violent histories of collisions and accumulations of small bodies in the solar system. furthermore, the established scientific knowledge and techniques will advance key technologies for planetary defense. | hayabusa2 extended mission: new voyage to rendezvous with a small asteroid rotating with a short period |
context. the nasa mission osiris-rex (origins, spectral interpretation, resource identification, and security-regolith explorer) has been observing near-earth asteroid (101955) bennu in close proximity since december 2018. in october 2020, the spacecraft collected a sample of surface material from bennu to return to earth.aims: in this work, we investigate spectral phase reddening - that is, the variation of spectral slope with phase angle - on bennu using spectra acquired by the osiris-rex visible and infrared spectrometer (ovirs) covering a phase angle range of 8-130°. we investigate this process at the global scale and for some localized regions of interest (rois), including boulders, craters, and the designated sample collection sites of the osiris-rex mission.methods: spectra were wavelength- and flux-calibrated, then corrected for the out-of-band contribution and thermal emission, resampled, and finally converted into radiance factor per standard ovirs processing. spectral slopes were computed in multiple wavelength ranges from spectra normalized at 0.55 μm.results: bennu has a globally negative spectra slope, which is typical of b-type asteroids. the spectral slope gently increases in a linear way up to a phase angle of 90°, where it approaches zero. the spectral phase reddening is monotonic and wavelength-dependent with highest values in the visible range. its coefficient is 0.00044 μm-1 deg-1 in the 0.55-2.5 μm range. for observations of bennu acquired at high phase angle (130°), phase reddening increases exponentially, and the spectral slope becomes positive. similar behavior was reported in the literature for the carbonaceous chondrite mukundpura in spectra acquired at extreme geometries. some rois, including the sample collection site, nightingale, have a steeper phase reddening coefficient than the global average, potentially indicating a surface covered by fine material with high micro-roughness.conclusions: the gentle spectral phase reddening effect on bennu is similar to that observed in ground-based measurements of other b-type asteroids, but much lower than that observed for other low-albedo bodies such as ceres or comet 67p/churyumov-gerasimenko. monotonic reddening may be associated with the presence of fine particles at micron scales and/or of particles with fractal structure that introduce micro- and sub-micro roughness across the surface of bennu. | phase reddening on asteroid bennu from visible and near-infrared spectroscopy |
one of the key goals of the rosetta mission was to understand how, where and when comets formed in our solar system. there are two major hypotheses for the origin of comets, both pre-rosetta: (1) hierarchical accretion of dust and ice grains in the solar nebula and (2) the growth of pebbles, which are then brought together by streaming instabilities in the solar nebula to form larger bodies. rosetta provided a wealth of new information on comet nuclei and confirmed many past ideas on comets, e.g., high volatile content, lack of aqueous alteration of grains, and the low bulk density of the nucleus. rosetta also provided new data on the nature of cometary activity, the active geology on the nucleus surface and the interior structure and bulk density of the nucleus. supporters of the above-mentioned origin hypotheses each find confirmation of their ideas in the rosetta results. but the question of which hypothesis is preferred, or if there are other, better hypotheses that could be invoked, could not be answered. theoretical studies suggest that comet nuclei were collisionally processed in the primordial disk though it is not clear that the nuclei we see today display the effects of that process. both theoretical and observational studies suggest that the major end-states for cometary nuclei are dynamical ejection, random disruption and disintegration, and/or evolution of nuclei to inactive, asteroidal-appearing objects. rosetta has provided us with many new insights that will help to guide future cometary missions, observations, experiments and theoretical investigations that will lead to answers to the fundamental questions with regard to cometary origin. | origin and evolution of cometary nuclei |
dujmović, joret, micek, morin, ueckerdt and wood [j. acm 2020] proved that for every graph $g$ with euler genus $g$ there is a graph $h$ with treewidth at most 4 and a path $p$ such that $g\subseteq h \boxtimes p \boxtimes k_{\max\{2g,3\}}$. we improve this result by replacing "4" by "3" and with $h$ planar. we in fact prove a more general result in terms of so-called framed graphs. this implies that every $(g,d)$-map graph is contained in $ h \boxtimes p\boxtimes k_\ell$, for some planar graph $h$ with treewidth $3$, where $\ell=\max\{2g\lfloor \frac{d}{2} \rfloor,d+3\lfloor\frac{d}{2}\rfloor-3\}$. it also implies that every $(g,1)$-planar graph (that is, graphs that can be drawn in a surface of euler genus $g$ with at most one crossing per edge) is contained in $h\boxtimes p\boxtimes k_{\max\{4g,7\}}$, for some planar graph $h$ with treewidth $3$. | improved product structure for graphs on surfaces |
the field of asteroid thermophysical modeling has experienced an extraordinary growth in the last 10 years, as new thermal-infrared data became available for hundreds of thousands of asteroids. the infrared emission of asteroids depends on the body's size, shape, albedo, thermal inertia, roughness, and rotational properties. these parameters can therefore be derived by thermophysical modeling of infrared data. thermophysical modeling led to asteroid size estimates that were confirmed at the few-percent level by later spacecraft visits. we discuss how instrumentation advances now allow mid-infrared interferometric observations as well as high-accuracy spectrophotometry, posing their own set of thermal-modeling challenges. we present major breakthroughs achieved in studies of the thermal inertia, a sensitive indicator for the nature of asteroids soils, allowing us to, e.g., determine the grain size of asteroidal regoliths. thermal inertia also governs nongravitational effects on asteroid orbits, requiring thermophysical modeling for precise asteroid dynamical studies. the radiative heating of asteroids, meteoroids, and comets from the sun also governs the thermal stress in surface material; only recently has it been recognized as a significant weathering process. asteroid space missions with thermal-infrared instruments are currently undergoing study at all major space agencies. this will require a high level of sophistication of thermophysical models in order to analyze high-quality spacecraft data. | asteroid thermophysical modeling |
this paper proposes a new global and local weighted signed pressure force (spf) based active contour model (acm) to segment various types of images. first, by introducing the normalized global minimum absolute differences as the coefficients of global inner and outer region fitting centers, a new global weighted spf (gwspf) is defined, which makes the best of the difference information of inner and outer regions and improves segmentation performance. second, by introducing the normalized local minimum absolute differences as the coefficients of local inner and outer region fitting centers similarly, a new local weighted spf (lwspf) is defined and added to the above global weighted spf. third, the global and local within-class variances of the image are used to weight the gwspf and the lwspf, which can automatically adjust the effect degrees of the gwspf and the lwspf. experiments on many kinds of real-world images have validated that the proposed model is superior to popular acms in segmentation accuracy, in addition, it is robust to the initial curve. | active contours driven by global and local weighted signed pressure force for image segmentation |
the recent detection of the gravitational-wave source gw150914 by the ligo collaboration motivates a speculative source for the origin of ultrahigh-energy cosmic rays as a possible byproduct of the immense energies achieved in black hole (bh) mergers, provided that the bhs have spin, as seems inevitable, and there are relic magnetic fields and disk debris remaining from the formation of the bhs or from their accretion history. we argue that given the modest efficiency \lt 0.01 required per event per unit of gravitational-wave energy release, merging bhs potentially provide an environment for accelerating cosmic rays to ultrahigh energies. the presence of tidally disrupted planetary or asteroidal debris could lead to associated fast radio bursts. | ultrahigh-energy cosmic rays and black hole mergers |
the near-earth asteroid (3200) phaethon is the parent body of the geminid meteor stream. phaethon is also an active asteroid with a very blue spectrum. we conducted polarimetric observations of this asteroid over a wide range of solar phase angles α during its close approach to the earth in autumn 2016. our observation revealed that phaethon exhibits extremely large linear polarization: p = 50.0 ± 1.1% at α = 106.5°, and its maximum is even larger. the strong polarization implies that phaethon's geometric albedo is lower than the current estimate obtained through radiometric observation. this possibility stems from the potential uncertainty in phaethon's absolute magnitude. an alternative possibility is that relatively large grains ( 300 μm in diameter, presumably due to extensive heating near its perihelion) dominate this asteroid's surface. in addition, the asteroid's surface porosity, if it is substantially large, can also be an effective cause of this polarization. | extremely strong polarization of an active asteroid (3200) phaethon |
the 90.5-km occator crater, with its peculiar and unique bright spots, is one of the most prominent and renowned feature on ceres. occator attracted broad public attention in scientific media as it is proposed to exhibit signs of post-impact cryovolcanic activity. in order to understand the time sequence of deposition, several attempts were made during dawn's primary mission by different research groups to date geomorphologic key units using superposed crater densities. resulting absolute model formation ages for occator's ejecta and its interior lobate deposits range from 200 ma to 78 ma and about 100 ma to 6.9 ma, but were based on different cratering chronology models, measurements on image data at varying resolution, and different statistical and methodological approaches. here we present the results of a comprehensive approach of determining absolute model formation ages for occator. this is achieved by using the best resolved framing camera image data, by careful treatment of secondary crater admixture and the natural variability of crater detection and sizes by different crater analysts, and by applying appropriate, objective criteria for count area selection. in this context, we evaluate previously published model ages and explain why our results are likely to yield more consistent and robust information about the formation age of occator. we also show that, in contrast to previous publications, csfds measured on occator's less competent ejecta blanket, and its more competent cryovolcanic- or impact melt related interior lobate deposits (ilds), are so similar to each other, that the slight differences might rather be explained by different scaling parameters in different target materials or that the ilds have formed almost contemporaneously or only shortly after the occator forming impact. | the various ages of occator crater, ceres: results of a comprehensive synthesis approach |
early solar system planetesimal thermal models predict the heating of the chondritic protolith and the preservation of a chondritic crust on differentiated parent bodies. petrological and geochemical analyses of chondrites have suggested that secondary alteration phases formed at low temperatures (<300 °c) by fluid-rock interaction where reduced and oxidized vigarano type carbonaceous (cv) chondrites witness different physicochemical conditions. from a thermodynamical survey of ca-fe-rich secondary phases in cv3 chondrites including silica activity (asio2), here we show that the classical distinction between reduced and oxidized chondrites is no longer valid and that their ca-fe-rich secondary phases formed in similar reduced conditions near the iron-magnetite redox buffer at low asio2 (log(asio2) <-1) and moderate temperature (210-610 °c). the various lithologies in cv3 chondrites are inferred to be fragments of an asteroid percolated heterogeneously via porous flow of hydrothermal fluid. putative `onion shell' structures are not anymore a requirement for the cv parent body crust. | reduced and unstratified crust in cv chondrite parent body |
the hayabusa2 asteroid explorer mission focuses principally on the touchdown and sampling on near-earth asteroid 162173 ryugu. hayabusa2 successfully landed on its surface and ejected a projectile for sample collection on february 22, 2019. hayabusa2 later landed near a crater formed by an impactor and executed the sampling sequence again on july 11, 2019. for a successful mission, a thorough understanding and evaluation of spacecraft dynamics during touchdown were crucial. the most challenging aspect of this study was the modeling of such spacecraft phenomena as the dynamics of landing on a surface with unknown properties. in particular, a monte carlo analysis was used to determine the parameters of the operational design for the final descent and touchdown sequence. this paper discusses the dynamical modeling of the simulation during the touchdown of hayabusa2. | modeling and analysis of hayabusa2 touchdown |
there is abundant petrologic evidence for the oxidation of fe during the aqueous alteration of chondrites, and water must have been the oxidant for this process. the h2 lost from the chondrite parent bodies as a result of fe oxidation would have been isotopically very light, enriching any residual water in d. the extents of the d enrichments will have depended on the fractions of water consumed and the temperatures during fe oxidation. here we have estimated the likely ranges of water consumed by fe oxidation in the ci, cm, cr and ll parent bodies, as well as the likely range of changes in water h isotopic compositions this would have produced. we first used fe xanes to determine the fe valences of bulk meteorite powders in orgueil (ci1), a number of cms and crs that experienced varying degrees of alteration, and semarkona (ll3.00). the total ranges of bulk fe valences we obtained were: orgueil 2.77, cms 2.40-2.63, crs 1.46-2.54, and semarkona 2.10. combining previous estimates of the present water/oh contents of our samples with the present bulk fe valences and an estimated range of initial bulk fe valences, we estimate the likely ranges of fractional water losses to have been: orgueil 15-26%, semarkona 73-83%, cms 23-48%, and crs 39-62%. the associated maximum and minimum changes in the h isotopic compositions of the remaining water were estimated assuming the equilibrium h2-h2o isotopic fractionation factor, rayleigh fractionation of the h2, and oxidation temperatures of 0-200 °c. using previous estimates of the water h isotopic compositions in the chondrites, the ranges of estimated δd values for the initial chondritic waters are: orgueil -672‰ to -422‰, cms -676‰ to -493‰, crs -527‰ to -56‰, and semarkona -527‰ to 154‰. the ci, cm, cr and ordinary chondrites all accreted water with similar h isotopic compositions that were distinct from the compositions of comets or saturn's moon enceladus. thus, the carbonaceous chondrites are unlikely to have come from comets or from bodies that were scattered into the asteroid belt from comet forming regions by orbital migration of the giant planets. if the carbonaceous chondrites did form in the outer solar system, as some models predict, it was probably not beyond 7 au. however, based on water isotopic compositions at present it is equally plausible that the carbonaceous chondrites formed in the inner solar system. | the bulk valence state of fe and the origin of water in chondrites |
recent theoretical work in celestial mechanics has revealed that an asteroid may orbit stably in the same region as a planet, despite revolving around the sun in the sense opposite to that of the planet itself. asteroid 2015 bz509 was discovered in 2015, but with too much uncertainty in its measured orbit to establish whether it was such a retrograde co-orbital body. here we report observations and analysis that demonstrates that asteroid 2015 bz509 is indeed a retrograde co-orbital asteroid of the planet jupiter. we find that 2015 bz509 has long-term stability, having been in its current, resonant state for around a million years. this is long enough to preclude precise calculation of the time or mechanism of its injection to its present state, but it may be a halley-family comet that entered the resonance through an interaction with saturn. retrograde co-orbital asteroids of jupiter and other planets may be more common than previously expected. | a retrograde co-orbital asteroid of jupiter |
in the preliminary trajectory design of the multi-target rendezvous problem, a model that can quickly estimate the cost of the orbital transfer is essential. the estimation of the transfer time using solar sails between two arbitrary orbits is difficult and usually requires to solve an optimal control problem. inspired by the successful applications of the deep neural networks in nonlinear regression, this work explores the possibility and effectiveness of mapping the transfer time for solar sails from the orbital characteristics using the deep neural networks. furthermore, the monte carlo tree search method is investigated and used to search the optimal sequence considering a multi-asteroid exploration problem. the obtained sequences from preliminary design will be solved and verified by sequentially solving the optimal control problem. two examples of different application backgrounds validate the effectiveness of the proposed approach. | solar-sail trajectory design for multiple near-earth asteroid exploration based on deep neural networks |
many isolated, old white dwarfs (wds) show surprising evidence of metals in their photospheres. given that the timescale for gravitational sedimentation is astronomically short, this is taken as evidence for ongoing accretion, likely of tidally disrupted planetesimals. the rate of such accretion, {\dot{m}}acc}, is important to constrain, and most modeling of this process relies on assuming an equilibrium between diffusive sedimentation and metal accretion supplied to the wd’s surface convective envelope. building on the earlier work of deal and collaborators, we show that high {\dot{m}}acc} models with only diffusive sedimentation are unstable to thermohaline mixing and that models that account for the enhanced mixing from the active thermohaline instability require larger accretion rates, sometimes reaching {\dot{m}}acc}≈ {10}13 {{g}} {{{s}}}-1 to explain observed calcium abundances. we present results from a grid of mesa models that include both diffusion and thermohaline mixing. these results demonstrate that both mechanisms are essential for understanding metal pollution across the range of polluted wds with hydrogen atmospheres. another consequence of active thermohaline mixing is that the observed metal abundance ratios are identical to accreted material. | increases to inferred rates of planetesimal accretion due to thermohaline mixing in metal-accreting white dwarfs |
rozitis et al. recently reported that near-earth asteroid (29075) 1950 da, whose bulk density ranges from 1.0 g cm-3 to 2.4 g cm-3, is a rubble pile and requires a cohesive strength of at least 44-76 pa to keep from failing due to its fast spin period. since their technique for giving failure conditions required the averaged stress over the whole volume, it discarded information about the asteroid's failure mode and internal stress condition. this paper develops a finite element model and revisits the stress and failure analysis of 1950 da. for the modeling, we do not consider material hardening and softening. under the assumption of an associated flow rule and uniform material distribution, we identify the deformation process of 1950 da when its constant cohesion reaches the lowest value that keeps its current shape. the results show that to avoid structural failure the internal core requires a cohesive strength of at least 75-85 pa. it suggests that for the failure mode of this body, the internal core first fails structurally, followed by the surface region. this implies that if cohesion is constant over the whole volume, the equatorial ridge of 1950 da results from a material flow going outward along the equatorial plane in the internal core, but not from a landslide as has been hypothesized. this has additional implications for the likely density of the interior of the body. | stress and failure analysis of rapidly rotating asteroid (29075) 1950 da |
we introduce a constrained nonlinear least-squares algorithm to be used in estimating the parameters in the h, g1, g2 phase function. as the algorithm works directly in the magnitude space, it will surpass the possible bias problem that may be present in the existing h ,g1 ,g2 fit procedure when applied to low-accuracy observations with large magnitude variations. with constraints on the photometric phase-curve shape parameters g1 and g2, it guarantees a physically reasonable phase-curve estimate. with a new data set of 93 asteroids, we re-assess the two-parameter version of the h ,g1 ,g2 function. finally, we introduce a one-parameter version of the phase function that can give a suggestion of the asteroids taxonomic group based only on its phase curve. a statistical model selection procedure is presented that can automatically select between the different versions of the photometric phase functions. an online tool that implements these algorithms is introduced. | h, g1, g2 photometric phase function extended to low-accuracy data |
in this work we have estimated 10 collisional ages of 9 families for which for different reasons our previous attempts failed. in general, these are difficult cases that required dedicated effort, such as a new family classifications for asteroids in mean motion resonances, in particular the 1/1 and 2/1 with jupiter, as well as a revision of the classification inside the 3/2 resonance. of the families locked in mean motion resonances, by employing a numerical calibration to estimate the yarkovsky effect in proper eccentricity, we succeeded in determining ages of the families of (1911) schubart and of the "super-hilda" family, assuming this is actually a severely eroded original family of (153) hilda. in the trojan region we found families with almost no yarkovsky evolution, for which we could compute only physically implausible ages. hence, we interpreted their modest dispersions of proper elements as implying that the trojan asteroid families are fossil families, frozen at their proper elements determined by the original ejection velocity field. we have found a new family, among the griquas locked in the 2/1 resonance with jupiter, the family of (11097) 1994 ud1. we have estimated the ages of 6 families affected by secular resonances: families of (5) astraea, (25) phocaea, (283) emma, (363) padua, (686) gersuind, and (945) barcelona. by using in all these cases a numerical calibration method, we have shown that the secular resonances do not affect significantly the secular change of proper a. we have confirmed the existence of the family resulting from cratering on (5) astraea by computing a new set of resonant proper elements adapted to the resonance g +g5 - 2g6 : this new family has a much larger membership and has a shape compatible with simple collisional models. for the family of (145) adeona we could estimate the age only after removal of a number of assumed interlopers. with the present paper we have concluded the series dedicated to the determination of asteroid ages with a uniform method. overall we computed 53 ages for a total of 49 families. for the future work there remain families too small at present to provide reliable estimates, as well as some complex families (221, 135, 298) which may have more ages than we could currently estimate. future improvement of some already determined family ages is also possible by increasing family membership, revising the calibrations, and using more reliable physical data. | on the ages of resonant, eroded and fossil asteroid families |
context. the high-angular-resolution capability of the new-generation ground-based adaptive-optics camera sphere at eso vlt allows us to assess, for the very first time, the cratering record of medium-sized (d 100-200 km) asteroids from the ground, opening the prospect of a new era of investigation of the asteroid belt's collisional history.aims: we investigate here the collisional history of asteroid (6) hebe and challenge the idea that hebe may be the parent body of ordinary h chondrites, the most common type of meteorites found on earth ( 34% of the falls).methods: we observed hebe with sphere as part of the science verification of the instrument. combined with earlier adaptive-optics images and optical light curves, we model the spin and three-dimensional (3d) shape of hebe and check the consistency of the derived model against available stellar occultations and thermal measurements.results: our 3d shape model fits the images with sub-pixel residuals and the light curves to 0.02 mag. the rotation period (7.274 47 h), spin (ecj2000 λ, β of 343°, +47°), and volume-equivalent diameter (193 ± 6 km) are consistent with previous determinations and thermophysical modeling. hebe's inferred density is 3.48 ± 0.64 g cm-3, in agreement with an intact interior based on its h-chondrite composition. using the 3d shape model to derive the volume of the largest depression (likely impact crater), it appears that the latter is significantly smaller than the total volume of close-by s-type h-chondrite-like asteroid families.conclusions: our results imply that (6) hebe is not the most likely source of h chondrites. over the coming years, our team will collect similar high-precision shape measurements with vlt/sphere for 40 asteroids covering the main compositional classes, thus providing an unprecedented dataset to investigate the origin and collisional evolution of the asteroid belt. based on observations made with eso telescopes at the la silla paranal observatory under programme id 60.a-9379 and 086.c-0785. | 3d shape of asteroid (6) hebe from vlt/sphere imaging: implications for the origin of ordinary h chondrites |
determining the size and orbital distribution of the population of near-earth asteroids (neas) is the focus of intense research, with the most recent models converging to a population of approximately 1000 neas larger than 1 km and up to approximately 109 neas with absolute magnitude h < 30. we present an analysis of the combined observations of nine of the leading asteroid surveys over the past two decades, and show that for an absolute magnitude h < 17.75, which is often taken as proxy for an average diameter larger than 1 km, the population of neas is 920 ± 10, lower than other recent estimates. the population of small neas is estimated at (4 ± 1) × 108 for h < 30, and the number of decameter neas is lower than other recent estimates. this population tracks accurately the orbital distribution of recently discovered large neas, and produces an estimated earth impact rate for small neas in good agreement with the bolide data. | the near-earth asteroid population from two decades of observations |
context. in the course of a close approach to planets or stars, the morphological and dynamical properties of rubble-pile small bodies can be dramatically modified, and some may catastrophically break up, as in the case of comet shoemaker-levy 9. this phenomenon is of particular interest for the understanding of the evolution and population of small bodies, and for making predictions regarding the outcomes of future encounters. previous numerical explorations have typically used methods that do not adequately represent the nature of rubble piles. the encounter outcomes and influence factors are still poorly constrained.aims: based on recent advances in modeling rubble-pile physics, we aim to provide a better understanding of the tidal encounter processes of rubble piles through soft-sphere discrete element modeling (ssdem) and to establish a database of encounter outcomes and their dependencies on encounter conditions and rubble-pile properties.methods: we performed thousands of numerical simulations using the ssdem implemented in the n-body code pkdgrav to study the dynamical evolution of rubble piles during close encounters with the earth. the effects of encounter conditions, material strength, arrangement, and resolution of constituent particles are explored.results: three typical tidal encounter outcomes are classified, namely: deformation, mass shedding, and disruption, ranging from mild modifications to severe damages of the progenitor. the outcome is highly dependent on the encounter conditions and on the structure and strength of the involved rubble pile. the encounter speed and distance required for causing disruption events are much smaller than those predicted by previous studies, indicating a smaller creation rate of tidally disrupted small body populations. extremely elongated fragments with axis ratios ~1:6 can be formed by moderate tidal encounters. our analyses of the spin-shape evolution of the largest remnants reveal reshaping mechanisms of rubble piles in response to tidal forces, which is consistent with stable rubble-pile configurations derived by continuum theory. a case study for shoemaker-levy 9 suggests a low bulk density (0.2-0.3 g cc-1) for its progenitor. movies are available at https://www.aanda.org | tidal distortion and disruption of rubble-pile bodies revisited. soft-sphere discrete element analyses |
the article is a review of the state of research on physical processes occurring near the surface of airless bodies, in particular, the moon, asteroids, comets, which lead to their erosion, that is, to the modification of superficial and sometimes even deeper layers. external influences on the bodies are considered-micrometeor streams, solar wind and geomagnetic tail plasma flows, solar electromagnetic radiation, and cosmic rays, which are the main causes of erosion. the main features of airless bodies belonging to different classes are given. the main physical mechanisms that can lead to the release and removal of dust particles from the surface of regolith are analyzed, in particular, the energy aspects of high-speed micrometeor impacts, electrostatic processes of particle release from the surface, and thermodynamic processes occurring in the near-surface layers of cometary nuclei. the conditions for the removal of dust particles from the surface of a rotating body and the conditions under which the body can collapse are considered. depending on the characteristics of these bodies, the processes of erosion, as well as the causes of its occurrence, can vary significantly. the main unresolved problems associated with the processes of erosion of bodies, which require further research, are considered. | physical processes leading to surface erosion and dust particles dynamics of airless bodies |
like most primitive carbonaceous chondrites, the cm chondrites experienced varying degrees of asteroidal aqueous alteration, which may have overprinted pre-accretionary processing. several aqueous alteration scales for cm chondrites (and other carbonaceous chondrites) have been proposed based on alteration-dependent changes in various petrological and geochemical characteristics. given the possibility that the intensity of aqueous alteration could be recorded in the primordial noble gas compositions, we test potential correlations between petrologic, geochemical and noble gas characteristics in a detailed study on 39 cm chondrites, including some of the most pristine cm chondrites identified to date, and 4 cm-related carbonaceous chondrites. we mainly compare our noble gas data with the alteration schemes proposed by alexander et al. (2013) and howard et al. (2015). in addition to the noble gas analyses, we determined the phyllosilicate fractions of 17 of the cm chondrites using x-ray diffraction (xrd) to complement missing data points in the howard alteration scheme. the influence of post-hydration thermal modification on noble gases in cm chondrites is investigated by comparison of heated and unheated samples. cosmic-ray exposure (cre) ages are determined for all samples in this study as well as for 26 more samples based on cm chondrite literature noble gas data. the noble gas inventory in cm chondrites represents a mixture of cosmogenic, radiogenic, and abundant primordially trapped noble gases. additionally, about 50 % of our cm bulk samples contain detectable solar wind (sw), which implies that many but not all cm chondrites are regolith breccias or carry sw from a pre-accretion irradiation phase. aqueous alteration affects primordial noble gas abundances and elemental and isotopic compositions in cm chondrites. in particular, the process causes loss of an ar-rich component, different in elemental and isotopic composition to known noble gas components. this component is lost during the early stages of aqueous alteration until complete degassing of its carrier material (possibly upon at least partial destruction) below petrologic type of ~1.5 on the howard et al. (2015) scale. likely, small amounts of q gases were additionally released by aqueous alteration. strong thermal modification at >750 °c results in a significant additional loss of noble gases, whereas peak temperatures <500 °c likely have minor effects on the noble gas inventories of cm chondrites. some of the described trends of noble gas contents and elemental and isotopic ratios in this study are observable across multiple carbonaceous chondrite groups, in particular also the cr chondrites. hence, these carbonaceous chondrites may have started with similar initial noble gas inventories due to accretion of material from a common reservoir. the cre ages of most of our cm samples fall within the typical range of <10 myr previously observed for cm chondrites. a few cm chondrites, however, show longer cre ages, with the longest cre age of ~20 myr determined for the sw-rich cm allan hills (alh) 85013. the degree of aqueous and thermal alteration is variable among cm chondrites with similar cre ages. | noble gases in cm carbonaceous chondrites: effect of parent body aqueous and thermal alteration and cosmic ray exposure ages |
satellite collisions or fragmentations generate a huge number of space debris; over time, the fragments might get dispersed, making it difficult to associate them to the configuration at break-up. in this work, we present a procedure to back-trace the debris, reconnecting them to their original configuration. to this end, we compute the proper elements, namely dynamical quantities which stay nearly constant over time. while the osculating elements might spread and lose connection with the values at break-up, the proper elements, which have been already successfully used to identify asteroid families, retain the dynamical features of the original configuration. we show the efficacy of the procedure, based on a hierarchical implementation of perturbation theory, by analyzing the following four different case studies associated to satellites that underwent a catastrophic event: ariane 44lp, atlas v centaur, cz-3, titan iiic transtage. the link between (initial and final) osculating and proper elements is evaluated through tools of statistical data analysis. the results show that proper elements allow one to reconnect the fragments to their parent body. | reconnecting groups of space debris to their parent body through proper elements |
unlike the spherical gravitational field of planets and other large solar system bodies, the gravitational field of asteroids is irregular and weak. it is challenging for a planetary rover to obtain sufficient traction forces in this environment. however, this gravitational environment is suitable for legged robots with jumping ability, but it also imposes higher demands on control methods. therefore, this study aimed to address the problem of jump control method for asteroid-exploration quadruped robots. as the robot jumps off the surface of an asteroid, it would fly for a certain amount of time because of the low gravitational acceleration. the prolonged flight phase underscores the significance of the robot's take off and attitude control. a model-free stable jumping control method was devised in this study. this method can satisfy the control requirements for takeoff, attitude adjustment, and soft landing by using end-to-end multi-agent reinforcement learning (marl). marl is more advantageous than single-agent reinforcement learning in dealing with composite motion control problems under similar observation conditions. a simulated training environment was established, incorporating models of the gravitational field, task partitioning for jumping, and design of reward functions, including jump trajectory planning. the efficacy of the proposed jump control method for a quadruped robot was successfully demonstrated in the gravitational environment of an irregular rod-shaped asteroid, 216 kleopatra. | reinforcement learning-based stable jump control method for asteroid-exploration quadruped robots |
we present an eccentric precessing gas disk model designed to study the variable circumstellar absorption features detected for wd 1145+017, a metal polluted white dwarf with an actively disintegrating asteroid around it. this model, inspired by one recently proposed by cauley et al., calculates explicitly the gas opacity for any predetermined physical conditions in the disk, predicting the strength and shape of all absorption features, from the uv to the optical, at any given phase of the precession cycle. the successes and failures of this simple model provide valuable insight on the physical characteristics of the gas surrounding the star, notably its composition, temperature, and density. this eccentric disk model also highlights the need for supplementary components, most likely circular rings, in order to explain the presence of zero velocity absorption as well as highly ionized si iv lines. we find that a precession period of 4.6 ± 0.3 yr can successfully reproduce the shape of the velocity profile observed at most epochs from 2015 april to 2018 january, although minor discrepancies at certain times indicate that the assumed geometric configuration may not be optimal yet. finally, we show that our model can quantitatively explain the change in morphology of the circumstellar features during transiting events. | modeling of the variable circumstellar absorption features of wd 1145+017 |
the defense against near-earth asteroids (neas) using kinetic impact is faced with various challenges, including limited maneuverability of the impactor, inaccurate dynamic models, poor observability of relative navigation, and control execution errors. to address these challenges, an integrated robust navigation and guidance method for the kinetic impact of neas based on deep reinforcement learning (drl) is proposed in this paper, which can directly map angle measurements by a monocular camera to guidance maneuvers. firstly, a discrete partial observable markov decision process (pomdp) is modeled for the integrated navigation and guidance problem of nea interception. to address the situation where impactors are usually only equipped with aiming cameras and only the line of sight (los) angle is measured, past and current los measurements are concatenated into a one-dimensional state observation vector to directly introduce the memory of historical state information. a shaping reward function design based on potential energy has also been proposed to solve the problem of sparse reward as the main goal. subsequently, proximal policy optimization (ppo) is used to solve the established pomdp model, obtaining an integrated navigation and guidance policy that directly maps from the original output of the navigation sensor to the guidance command. the potentially hazard asteroid (pha) bennu is used as the target and a typical kinetic impact defense scenario is designed to simulate and verify the model and method proposed in this paper, considering the influence of multiple factors. numerical simulation results show that the proposed method achieves an interception accuracy of 203.58 m (mean value). the proposed method abandons the traditional separate design of navigation and guidance algorithms, and its robustness has been tested and verified in a wide range of uncertain environments. | integrated robust navigation and guidance for the kinetic impact of near-earth asteroids based on deep reinforcement learning |
defect interactions can be deduced by a variational method.topological defects play a key role in two-dimensional active nematics, and a transient role in two-dimensional active polar fluids. using a variational method, we study both the transient and long-time behavior of defects in two-dimensional active polar fluids in the limit of strong order and overdamped, compressible flow, and compare the defect dynamics with the corresponding active nematics model studied recently. one result is non-central interactions between defect pairs for active polar fluids, and by extending our analysis to allow orientation dynamics of defects, we find that the orientation of +1 defects, unlike that of ±1/2 defects in active nematics, is not locked to defect positions and relaxes to asters. moreover, using a scaling argument, we explain the transient feature of active polar defects and show that in the steady state, active polar fluids are either devoid of defects or consist of a single aster. we argue that for contractile (extensile) active nematic systems, +1 vortices (asters) should emerge as bound states of a pair of +1/2 defects, which has been recently observed. moreover, unlike the polar case, we show that for active nematics, a linear chain of equally spaced bound states of pairs of +1/2 defects can screen the activity term. a common feature in both models is the appearance of +1 defects (elementary in polar and composite in nematic) in the steady state. | defect dynamics in active polar fluids vs. active nematics |
preliminary analyses of asteroid ryugu samples show kinship to aqueously altered ci (ivuna-type) chondrites, suggesting similar origins. we report identification of c-rich, particularly primitive clasts in ryugu samples that contain preserved presolar silicate grains and exceptional abundances of presolar sic and isotopically anomalous organic matter. the high presolar silicate abundance (104 ppm) indicates that the clast escaped extensive alteration. the 5 to 10 times higher abundances of presolar sic (~235 ppm), n-rich organic matter, organics with n isotopic anomalies (1.2%), and organics with c isotopic anomalies (0.2%) in the primitive clasts compared to bulk ryugu suggest that the clasts formed in a unique part of the protoplanetary disk enriched in presolar materials. these clasts likely represent previously unsampled outer solar system material that accreted onto ryugu after aqueous alteration ceased, consistent with ryugu's rubble pile origin. exogenic primitive clasts in ryugu samples have remarkable abundances of presolar grains and isotopically anomalous organics. | abundant presolar grains and primordial organics preserved in carbon-rich exogenous clasts in asteroid ryugu |
diogenites are thought to have originated from asteroid 4 vesta asteroid, and it has been previously shown that they mostly consist of orthopyroxene, chromite, and olivine minerals. in this study, the aioun el atrouss (aat‑1) diogenite was analyzed by vibrational spectroscopic (ft‑ir and micro‑raman), edxrf, and xrd techniques. aat‑001 is mainly composed of pyroxene and with lesser olivine. xrd investigation confirmed magnetite, kamacite, ilmenite, and albite. a troilite‑orthopyroxene intergrowth was revealed by xrf. the olivine composition was estimated to be approximately fo66 by raman olivine characteristic doublets. pyroxenes in aat‑01 were found to be en80fs20. finally, approximate peak shock pressure of more than 86 gpa for aat‑1 was suggested based on previous hypervelocity impact experiments on olivine. | vibrational spectroscopic, crystallographic, and microscopic investigation on the aioun el atrouss diogenite |
hera mission is the european space agency's contribution to the international collaboration with nasa for the planetary defence, i.e. asteroid impact deflection assessment aiming to deflect the trajectory of its target binary asteroid system (65803) didymos. the early characterization phase and the detailed characterization phase of hera mission are two phases of the proximity operations with the objective to physically and dynamically characterize didymos. during these phases, an image processing algorithm is required to estimate the position of the centroid of the primary to enable line of sight navigation. however, the performance of standard image processing algorithms is affected by the disturbances of the image, such as poor illumination conditions, the presence of external bodies and the irregular shape of the target. this research addresses this challenge by developing a robust convolutional neural networks-based image processing algorithm to estimate the position of the centroids of didymos and its moon dimorphos, the pseudorange from the primary and the sun phase angle. the training, validation and testing datasets are generated with the software planet and asteroid natural scene generation utility using the early characterization phase and the detailed characterization phase trajectories as case scenario. the position in the image of the centroids of didymos and dimorphos is estimated using their respective position vectors. to estimate the pseudorange, the developed algorithm regresses a set of keypoints on the visible border of didymos and evaluates its apparent radius. for the sun phase angle, the pixel position of the subsolar point of the primary is leveraged. the high-resolution network is the convolutional neural network architecture applied to detect keypoints with superior spatial precision. even with the considered disturbances, the analysis shows that the proposed algorithm is able to provide an accurate estimation of the mentioned outputs for all the early characterization phase trajectory and for 77.33% of the detailed characterization phase trajectory, improving the robustness and autonomy of the mission navigation. | cnn-based image processing algorithm for autonomous optical navigation of hera mission to the binary asteroid didymos |
the origin of the martian moons, phobos and deimos, remains elusive. while the morphology and their cratered surfaces suggest an asteroidal origin1-3, capture has been questioned because of potential dynamical difficulties in achieving the current near-circular, near-equatorial orbits4,5. to circumvent this, in situ formation models have been proposed as alternatives6-9. yet, explaining the present location of the moons on opposite sides of the synchronous radius, their small sizes and apparent compositional differences with mars2 has proved challenging. here, we combine geophysical and tidal-evolution modelling of a mars-satellite system to propose that phobos and deimos originated from disintegration of a common progenitor that was possibly formed in situ. we show that tidal dissipation within a mars-satellite system, enhanced by the physical libration of the satellite, circularizes the post-disrupted eccentric orbits in <2.7 gyr and makes phobos descend to its present orbit from its point of origin close to or above the synchronous orbit. our estimate for phobos's maximal tidal lifetime is considerably less than the age of mars, indicating that it is unlikely to have originated alongside mars. deimos initially moved inwards, but never transcended the co-rotation radius because of insufficient eccentricity and therefore insufficient tidal dissipation. whereas deimos is very slowly receding from mars, phobos will continue to spiral towards and either impact with mars or become tidally disrupted on reaching the roche limit in ≲39 myr. | dynamical evidence for phobos and deimos as remnants of a disrupted common progenitor |
the stereophotoclinometry (spc) software suite has been used to generate global digital terrain models (dtms) of many asteroids and moons, and was the primary tool used by the origins, spectral interpretation, resource identification, and security-regolith explorer (osiris-rex) mission to model the shape of asteroid bennu. we describe the dedicated preflight testing of spc for the osiris-rex mission using a synthetic "truth" asteroid model. spc has metrics that determine the internal consistency of a dtm, but it was not known how these metrics are related to the absolute accuracy of a dtm, which was important for the operational needs of the mission. the absolute accuracy of an spc-generated dtm cannot be determined without knowing the truth topography. consequently, we developed a realistic, but synthetic, computer-generated representation of asteroid bennu, photographed this synthetic truth model in an imaging campaign similar to that planned for the osiris-rex mission, and then generated a global spc dtm from these images. we compared the spc dtm, which was represented by a radius every 70 cm across the asteroid surface, to the synthetic truth model to assess the absolute accuracy. we found that the internal consistency can be used to determine the 3d root-mean-square accuracy of the model to within a factor of two of the absolute accuracy. | quality assessment of stereophotoclinometry as a shape modeling method using a synthetic asteroid |
the miniaturised asteroid remote geophysical observer (m-argo) is planned to be the first standalone deep-space cubesat mission to rendezvous with and characterise a near-earth asteroid. to this aim, it is essential to assess the attainable set of target asteroids. this work presents the initial results of the mission analysis and design of m-argo. in particular, the original procedure developed to extract the reachable near-earth asteroids and the subsequent down-selection process are shown. hundreds of both time- and fuel-optimal low-thrust trajectory optimisation problems have been solved with an indirect approach, targeting asteroids pre-filtered from the minor planet center database. the method implements a realistic thruster model, featuring variable input power, thrust, and specific impulse, together with an accurate switching detection technique and analytic derivatives. the analysis shows that approximately 150 minor bodies are found potentially reachable by m-argo when departing from the sun-earth lagrange point l2 within a 3-year transfer duration. a manual inspection of the transfer features led to a subset of 41 targets seeming more promising according to mission technological requirements and constraints. initial results indicate mission feasibility for m-argo, which has the potential to enable a completely new class of low-cost deep-space exploration missions. | envelop of reachable asteroids by m-argo cubesat |
line-of-sight (los) navigation is an optical navigation technique that exploits the direction to visible celestial bodies, obtained from an onboard imaging system, to estimate the position and velocity of a spacecraft. the directions are fed to an estimation filter, where they are matched with the actual position of the observed bodies, retrieved from onboard stored ephemerides. as los navigation represents a really promising option for the next-generation deep-space spacecraft, the objective of this work is to provide new insights into the performance. first, the information matrix is analyzed to show the influence of the geometry between the spacecraft and the observed planet(s). then, a monte carlo approach is used to investigate the influence of measurement error (ranging from 0.1 to 100 arcsec), and tracking frequency (ranging from four observations per day to one observation every two days). the effect on navigation performance is quantified by two indicators. the first is the 3d position and velocity root-mean-square-errors, computed once the estimation is considered to be steady-state. the second is the convergence time, which quantifies the required time for the estimation to reach the steady-state behaviour. the simulation is based on a set of four planets, which do not follow the common heliocentric dynamics but rotate around the sun with the same (distance-independent) angular velocity of the spacecraft. this approach allows the separation of scenario-dependent behaviours from navigation intrinsic properties, as the same relative geometry between observer and observed objects is maintained during the whole simulation. the results provide a useful guide for the next-generation autonomous navigation system, for both the definition of hardware requirements and the design of an appropriate navigation strategy. considerations are then applied to near-earth asteroid fly-by mission scenarios for the definition of the navigation strategy and hardware requirements. it is shown the importance of relative angles between the spacecraft and the planets. in the single-planet observation scenario, when the angle between the position vectors of the spacecraft and planet approaches a null value, the estimation error decreases. in the double-planets observation scenario, when the separation angle between the two los directions gets close to 90°, the estimation error decreases. the main influence on the performance is driven by the measurement error, which with current technologies is shown to be able to provide a position error in the order of a few hundred kilometers, while with a lower measurement error (0.1 arcsec) it would be possible to have a position error below 100 km. finally, it is demonstrated that tracking frequency plays a secondary role in the performance, and only influences tangibly the convergence time. | on line-of-sight navigation for deep-space applications: a performance analysis |
a diffractive sail is a solar sail whose exposed surface is covered by an advanced diffractive metamaterial film with engineered optical properties. this study examines the optimal performance of a diffractive solar sail with a sun-facing attitude in a typical orbit-to-orbit heliocentric transfer. a sun-facing attitude, which can be passively maintained through the suitable design of the sail shape, is obtained when the sail nominal plane is perpendicular to the sun-spacecraft line. unlike an ideal reflective sail, a sun-facing diffractive sail generates a large transverse thrust component that can be effectively exploited to change the orbital angular momentum. using a recent thrust model, this study determines the optimal control law of a sun-facing ideal diffractive sail and simulates the minimum transfer times for a set of interplanetary mission scenarios. it also quantifies the performance difference between sun-facing diffractive sail and reflective sail. a case study presents the results of a potential mission to the asteroid 16 psyche. | optimal interplanetary trajectories for sun-facing ideal diffractive sails |
policy gradient (pg) is widely used in reinforcement learning due to its scalability and good performance. in recent years, several variance-reduced pg methods have been proposed with a theoretical guarantee of converging to an approximate first-order stationary point (fosp) with the sample complexity of $o(\epsilon^{-3})$. however, fosps could be bad local optima or saddle points. moreover, these algorithms often use importance sampling (is) weights which could impair the statistical effectiveness of variance reduction. in this paper, we propose a variance-reduced second-order method that uses second-order information in the form of hessian vector products (hvp) and converges to an approximate second-order stationary point (sosp) with sample complexity of $\tilde{o}(\epsilon^{-3})$. this rate improves the best-known sample complexity for achieving approximate sosps by a factor of $o(\epsilon^{-0.5})$. moreover, the proposed variance reduction technique bypasses is weights by using hvp terms. our experimental results show that the proposed algorithm outperforms the state of the art and is more robust to changes in random seeds. | efficiently escaping saddle points for non-convex policy optimization |
162173 ryugu, the target of hayabusa2, has a round shape with an equatorial ridge, which is known as a spinning top shape. a strong centrifugal force is a likely contributor to ryugu’s top-shaped features. observations by the optical navigation camera on board hayabusa2 show a unique longitudinal variation in geomorphology; the western side of this asteroid, later called the western bulge, has a smooth surface and a sharp equatorial ridge, compared to the other side. here, we propose a structural deformation process that generated the western bulge. applying the mission-derived shape model, we employ a finite element model technique to analyze the locations that experience structural failure within the present shape. assuming that materials are uniformly distributed, our model shows the longitudinal variation in structurally failed regions when the spin period is shorter than ∼3.75 hr. ryugu is structurally intact in the subsurface region of the western bulge while other regions are sensitive to structural failure. we infer that this variation is indicative of the deformation process that occurred in the past, and the western bulge is more relaxed structurally than the other region. our analysis also shows that this deformation process might occur at a spin period between ∼3.5 and ∼3.0 hr, providing the cohesive strength ranging between ∼4 and ∼10 pa. | the western bulge of 162173 ryugu formed as a result of a rotationally driven deformation process |
we analyze a set of 760 475 observations of 333 026 unique main-belt objects obtained by the pan-starrs1 (ps1) survey telescope between 2012 may 20 and 2013 november 9, a period during which ps1 discovered two main-belt comets, p/2012 t1 (panstarrs) and p/2013 r3 (catalina-panstarrs). ps1 comet detection procedures currently consist of the comparison of the point spread functions (psfs) of moving objects to those of reference stars, and the flagging of objects that show anomalously large radial psf widths for human evaluation and possible observational follow-up. based on the number of missed discovery opportunities among comets discovered by other observers, we estimate an upper limit comet discovery efficiency rate of ∼70% for ps1. additional analyses that could improve comet discovery yields in future surveys include linear psf analysis, modeling of trailed stellar psfs for comparison to trailed moving object psfs, searches for azimuthally localized activity, comparison of point-source-optimized photometry to extended-source-optimized photometry, searches for photometric excesses in objects with known absolute magnitudes, and crowd-sourcing. analysis of the discovery statistics of the ps1 survey indicates an expected fraction of 59 mbcs per 106 outer main-belt asteroids (corresponding to a total expected population of ∼140 mbcs among the outer main-belt asteroid population with absolute magnitudes of 12 <hv < 19.5), and a 95% confidence upper limit of 96 mbcs per 106 outer main-belt asteroids (corresponding to a total of ∼230 mbcs), assuming a detection efficiency of 50%. we note however that significantly more sensitive future surveys (particularly those utilizing larger aperture telescopes) could detect many more mbcs than estimated here. examination of the orbital element distribution of all known mbcs reveals an excess of high eccentricities (0.1 < e < 0.3) relative to the background asteroid population. theoretical calculations show that, given these eccentricities, the sublimation rate for a typical mbc is orders of magnitude larger at perihelion than at aphelion, providing a plausible physical explanation for the observed behavior of mbcs peaking in observed activity strength near perihelion. these results indicate that the overall rate of mantle growth should be slow, consistent with observational evidence that mbc activity can be sustained over multiple orbit passages. | the main-belt comets: the pan-starrs1 perspective |
the internal structure and strength of small asteroids and large meteoroids is poorly known. observation of bright fireballs in the earth's atmosphere can explore meteoroid structure by studying meteoroid fragmentation during the flight. earlier evaluations showed that the meteoroid's strength is significantly lower than that of the recovered meteorites. we present a detailed study of atmospheric fragmentation of seven meteorite falls, all ordinary chondrites, and 14 other fireballs, where meteorite fall was predicted but the meteorites, probably also ordinary chondrites, were not recovered. all observations were made by the autonomous observatories of the european fireball network and include detailed radiometric light curves. a model, called the semiempirical fragmentation model, was developed to fit the light curves and decelerations. videos showing individual fragments were available in some cases. the results demonstrated that meteoroids do not fragment randomly but in two distinct phases. the first phase typically corresponds to low strengths of 0.04-0.12 mpa. in two-thirds of cases, the first phase was catastrophic or nearly catastrophic with at least 40% of mass lost. the second phase corresponds to 0.9-5 mpa for confirmed meteorite falls and somewhat lower strengths, from about 0.5 mpa, for smaller meteoroids. all of these strengths are lower than the tensile strengths of ordinary chondritic meteorites cited in the literature, 20-40 mpa. we interpret the second phase as being due to cracks in meteoroids and the first phase as a separation of weakly cemented fragments, which reaccumulated at the surfaces of asteroids after asteroid collisions. | two strengths of ordinary chondritic meteoroids as derived from their atmospheric fragmentation modeling |
jbilet winselwan is one of the largest cm carbonaceous chondrites available for study. its light, major, and trace elemental compositions are within the range of other cm chondrites. chondrules are surrounded by dusty rims and set within a matrix of phyllosilicates, oxides, and sulfides. calcium- and aluminum-rich inclusions (cais) are present at ≤1 vol% and at least one contains melilite. jbilet winselwan is a breccia containing diverse lithologies that experienced varying degrees of aqueous alteration. in most lithologies, the chondrules and cais are partially altered and the metal abundance is low (<1 vol%), consistent with petrologic subtypes 2.7-2.4 on the rubin et al. (<link href="#maps13224-bib-0086"/>) scale. however, chondrules and cais in some lithologies are completely altered suggesting more extensive hydration to petrologic subtypes ≤2.3. following hydration, some lithologies suffered thermal metamorphism at 400-500 °c. bulk x-ray diffraction shows that jbilet winselwan consists of a highly disordered and/or very fine-grained phase (73 vol%), which we infer was originally phyllosilicates prior to dehydration during a thermal metamorphic event(s). some aliquots of jbilet winselwan also show significant depletions in volatile elements such as he and cd. the heating was probably short-lived and caused by impacts. jbilet winselwan samples a mixture of hydrated and dehydrated materials from a primitive water-rich asteroid. it may therefore be a good analog for the types of materials that will be encountered by the hayabusa-2 and osiris-rex asteroid sample-return missions. | the alteration history of the jbilet winselwan cm carbonaceous chondrite: an analog for c-type asteroid sample return |
we simulate the formation and evolution of oort clouds around the 200 nearest stars (within ∼16 pc according to the gaia dr2) database. this study is performed by numerically integrating the planets and minor bodies in orbit around the parent star and in the galactic potential. the calculations start 1 gyr ago and continue for 100 myr into the future. in this time frame, we simulate how asteroids (and planets) are ejected from the vicinity of the stars and settle in an oort cloud and how they escape the local stellar gravity to form tidal streams. a fraction of 0.0098 to 0.026 of the asteroids remain bound to their parent star. the orbits of these asteroids isotropize and circularize because of the influence of the galactic tidal field and eventually form an oort cloud between ∼104 and ∼2 × 105 au. we estimate that ≲6% of the nearby stars may have a planet in their oort cloud. the majority of asteroids (and some of the planets) become unbound from their parent star to become free floating in the galactic potential. these interstellar asteroids remain in a similar orbit around the galactic center to their host star, forming dense streams of rogue interstellar asteroids and planets. the solar system occasionally passes through such tidal streams, potentially giving rise to occasional close encounters with objects in this stream. two recently discovered sources, 1i/(2017 q3) 'oumuamua and 2i/(2019 q4) borisov, may be such objects. although the direction from which an individual object originated cannot easily be traced back to the original host, multiple such objects coming from the same source might help to identify their origin. currently, the solar system is in the bow or wake of the tidal stream of approximately ten of the nearby stars, which might contribute considerably to the interaction rate. overall, we estimate that the local density of such leftovers from the planet-formation process contributes to a local density of 1.2 × 1014 per pc−3, or ≳0.1 of the interstellar visitors originate from the obliterated debris disks of such nearby stars. movie is available at https://www.aanda.org the source code, input files, simulation data, and data processing scripts for this manuscript are available at figshare at https://doi.org/10.6084/m9.figshare.12834803.v1 | oort cloud ecology. i. extra-solar oort clouds and the origin of asteroidal interlopers |
psr j2129-0429 is a “redback” eclipsing millisecond pulsar binary with an unusually long 15.2 hr orbit. it was discovered by the green bank telescope in a targeted search of unidentified fermi gamma-ray sources. the pulsar companion is optically bright (mean mr = 16.6 mag), allowing us to construct the longest baseline photometric data set available for such a system. we present 10 years of archival and new photometry of the companion from the lincoln near-earth asteroid research survey, the catalina real-time transient survey, the palomar transient factory, the palomar 60 inch, and the las cumbres observatory global telescope. radial velocity spectroscopy using the double-beam spectrograph on the palomar 200 inch indicates that the pulsar is massive: 1.74 ± 0.18 {m}⊙ . the g-type pulsar companion has mass 0.44 ± 0.04 {m}⊙ , one of the heaviest known redback companions. it is currently 95 ± 1% roche-lobe filling and only mildly irradiated by the pulsar. we identify a clear 13.1 mmag yr-1 secular decline in the mean magnitude of the companion as well as smaller-scale variations in the optical light curve shape. this behavior may indicate that the companion is cooling. binary evolution calculations indicate that psr j2129-0429 has an orbital period almost exactly at the bifurcation period between systems that converge into tighter orbits as black widows and redbacks and those that diverge into wider pulsar-white dwarf binaries. its eventual fate may depend on whether it undergoes future episodes of mass transfer and increased irradiation. | properties and evolution of the redback millisecond pulsar binary psr j2129-0429 |
atmospheric heavy elements have been observed in more than a quarter of white dwarfs (wds) at different cooling ages, indicating ongoing accretion of asteroidal material, whilst only a few per cent of the wds possess a dust disc, and all these wds are accreting metals. here, assuming that a rubble-pile asteroid is scattered inside a wd's roche lobe by a planet, we study its tidal disruption and the long-term evolution of the resulting fragments. we find that after a few pericentric passages, the asteroid is shredded into its constituent particles, forming a flat, thin ring. on a time-scale of myr, tens of per cent of the particles are scattered on to the wd, and are therefore directly accreted without first passing through a circularized close-in disc. fragment mutual collisions are most effective for coplanar fragments, and are thus only important in 103-104 yr before the orbital coplanarity is broken by the planet. we show that for a rubble pile asteroid with a size frequency distribution of the component particles following that of the near earth objects, it has to be roughly at least 10 km in radius such that enough fragments are generated and $\ge 10{{\ \rm per\ cent}}$ of its mass is lost to mutual collisions. at relative velocities of tens of km s-1, such collisions grind down the tidal fragments into smaller and smaller dust grains. the wd radiation forces may shrink those grains' orbits, forming a dust disc. tidal disruption of a monolithic asteroid creates large km-size fragments, and only parent bodies ≥100 km are able to generate enough fragments for mutual collisions to be significant. hence, those large asteroids experience a disc phase before being accreted. | accretion of tidally disrupted asteroids on to white dwarfs: direct accretion versus disc processing |
polluted white dwarfs serve as astrophysical mass spectrometers - their photospheric abundances are used to infer the composition of planetary objects that accrete onto them. we show that due to asymmetries in the accretion process, the composition of the material falling onto a star may vary with time during the accretion of a single planetary body. consequently, the instantaneous photospheric abundances of white dwarfs do not necessarily reflect the bulk composition of their pollutants, especially when their diffusion time-scales are short. in particular, we predict that when an asteroid with an iron core tidally disrupts around a white dwarf, a larger share of its mantle is ejected, and that the core/mantle fraction of the accreting material varies with time during the event. crucially, this implies that the core fraction of differentiated pollutants cannot be determined for white dwarfs with short diffusion time-scales, which sample only brief episodes of longer accretion processes. the observed population of polluted white dwarfs backs up the proposed theory. more white dwarfs have accreted material with high fe/ca than low fe/ca relative to stellar abundance ratios, indicating the ejection of mantle material. additionally, we find tentative evidence that the accretion rate of iron decreases more rapidly than that of magnesium or calcium, hinting at variability of the accreted composition. further corroboration of the proposed theory will come from the upcoming analysis of large samples of young white dwarfs. | asynchronous accretion can mimic diverse white dwarf pollutants i: core and mantle fragments |
exoplanets orbiting in the habitable zone around m dwarf stars have been prime targets in the search for life due to the long lifetimes of the host star, the prominence of such stars in the galaxy, and the apparent excess of terrestrial planets found around m dwarfs. however, the heightened stellar activity of m dwarfs and the often tidally locked planets in these systems have raised questions about the habitability of these planets. in this letter we examine another significant challenge that may exist: these systems seem to lack the architecture necessary to deliver asteroids to the habitable terrestrial planets, and asteroid impacts may play a crucial role in the origin of life. the most widely accepted mechanism for producing a stable asteroid belt and the late-stage delivery of asteroids after gas disk dissipation requires a giant planet exterior to the snow-line radius. we show that none of the observed systems with planets in the habitable zone of their star also contain a giant planet and therefore are unlikely to have stable asteroid belts. we consider the locations of observed giant planets relative to the snow-line radius as a function of stellar mass and find that there is a population of giant planets outside of the snow-line radius around m dwarfs. therefore, asteroid belt formation around m dwarfs is generally possible. however, we find that multiplanetary system architectures around m dwarfs can be quite different from those around more massive stars. | life on exoplanets in the habitable zone of m dwarfs? |
the double asteroid redirection test (dart) is a nasa mission intended to crash a projectile on dimorphos, the secondary component of the binary (65803) didymos system, to study its orbit deflection. as a consequence of the impact, a dust cloud will be be ejected from the body, potentially forming a transient coma- or comet-like tail on the hours or days following the impact, which might be observed using ground-based instrumentation. based on the mass and speed of the impactor, and using known scaling laws, the total mass ejected can be roughly estimated. then, with the aim to provide approximate expected brightness levels of the coma and tail extent and morphology, we have propagated the orbits of the particles ejected by integrating their equation of motion, and have used a monte carlo approach to study the evolution of the coma and tail brightness. for typical power-law particle size distribution of index -3.5, with radii rrmin = 1 μm and rmax = 1 cm, and ejection speeds near 10 times the escape velocity of dimorphos, we predict an increase of brightness of ~3 magnitudes right after the impact, and a decay to pre-impact levels some 10 d after. that would be the case if the prevailing ejection mechanism comes from the impact-induced seismic wave. however, if most of the ejecta is released at speeds of the order of ≳100 m s-1, the observability of the event would reduce to a very short time span, of the order of 1 d or shorter. | ground-based observability of dimorphos dart impact ejecta: photometric predictions |
we have monitored the didymos-dimorphos binary asteroid in spectropolarimetric mode in the optical range before and after the dart impact. the ultimate goal was to obtain constraints on the characteristics of the ejected dust for modeling purposes. before impact, didymos exhibited a linear polarization rapidly increasing with phase angle, reaching a level of ~5% in the blue and ~4.5% in the red. the shape of the polarization spectrum was anticorrelated with that of its reflectance spectrum, which appeared typical of an s-class asteroid. after impact, the level of polarization dropped by about 1 percentage point (pp) in the blue band and about 0.5 pp in the red band, then continued to linearly increase with phase angle, with a slope similar to that measured prior to impact. the polarization spectra, once normalized by their values at an arbitrary wavelength, show very little or no change over the course of all observations before and after impact. the lack of any remarkable change in the shape of the polarization spectrum after impact suggests that the way in which polarization varies with wavelength depends on the composition of the scattering material, rather than on its structure, be this a surface or a debris cloud. | optical spectropolarimetry of binary asteroid didymos-dimorphos before and after the dart impact |
craters formed by the impact of agglomerated materials are commonly observed in nature, such as asteroids colliding with planets and moons. in this paper, we investigate how the projectile spin and cohesion lead to different crater shapes. for that, we carried out discrete element method computations of spinning granular projectiles impacting onto cohesionless grains for different bonding stresses, initial spins, and initial heights. we found that, as the bonding stresses decrease and the initial spin increases, the projectile's grains spread farther from the collision point, and in consequence, the crater shape becomes flatter, with peaks around the rim and in the center of the crater. our results shed light on the dispersion of the projectile's material and the different shapes of craters found on earth and other planetary environments. | impact craters formed by spinning granular projectiles |
studies of material returned from cb asteroid ryugu have revealed considerable mineralogical and chemical heterogeneity, stemming primarily from brecciation and aqueous alteration. isotopic anomalies could have also been affected by delivery of exogenous clasts and aqueous mobilization of soluble elements. here, we show that isotopic anomalies for mildly soluble cr are highly variable in ryugu and ci chondrites, whereas those of ti are relatively uniform. this variation in cr isotope ratios is most likely due to physicochemical fractionation between 54 cr-rich presolar nanoparticles and cr-bearing secondary minerals at the millimeter-scale in the bulk samples, likely due to extensive aqueous alteration in their parent bodies that occurred 5.2 − 1.4 + 1.8 ma after solar system birth. in contrast, ti isotopes were marginally affected by this process. our results show that isotopic heterogeneities in asteroids are not all nebular or accretionary in nature but can also reflect element redistribution by water. nucleosynthetic cr-ti isotopic variations in ryugu are not all of nebular origin, but reflect elemental redistribution by water. | water circulation in ryugu asteroid affected the distribution of nucleosynthetic isotope anomalies in returned sample |
macroscopic dark matter is almost unconstrained over a wide "asteroidlike" mass range, where it could scatter on baryonic matter with geometric cross section. we show that when such an object travels through a star, it produces shock waves that reach the stellar surface, leading to a distinctive transient optical, uv, and x-ray emission. this signature can be searched for on a variety of stellar types and locations. in a dense globular cluster, such events occur far more often than flare backgrounds, and an existing uv telescope could probe orders of magnitude in dark matter mass in one week of dedicated observation. | stellar shocks from dark matter asteroid impacts |
dark compact objects ("clumps") transiting the solar system exert accelerations on the test masses (tm) in a gravitational-wave (gw) detector. we reexamine the detectability of these clump transits in a variety of current and future gw detectors, operating over a broad range of frequencies. tm accelerations induced by clump transits through the inner solar system have frequency content around f ∼μ hz . some of us [fedderke et al., phys. rev. d 105, 103018 (2022), 10.1103/physrevd.105.103018] recently proposed a gw detection concept with μ hz sensitivity, based on asteroid-to-asteroid ranging. from the detailed sensitivity projection for this concept, we find both analytically and in simulation that purely gravitational clump-matter interactions would yield one detectable transit every ∼20 yrs , if clumps with mass mcl∼1014 kg saturate the dark-matter (dm) density. other (proposed) gw detectors using local tms and operating in higher frequency bands are sensitive to smaller clump masses and have smaller rates of discoverable signals. we also consider the case of clumps endowed with an additional attractive long-range clump-matter fifth force significantly stronger than gravity (but evading known fifth-force constraints). for the μ hz detector concept, we use simulations to show that, for example, a clump-matter fifth-force ∼103 times stronger than gravity with a range of ∼au would boost the rate of detectable transits to a few per year for clumps in the mass range 1011 kg ≲mcl≲1014 kg , even if they are a ∼1 % subcomponent of the dm. the ability of μ hz gw detectors to probe asteroid-mass-scale dark objects that may otherwise be undetectable bolsters the science case for their development. | searching for dark clumps with gravitational-wave detectors |
we examine the history of the loss and replenishment of the martian atmosphere using elemental and isotopic compositions of nitrogen and noble gases. the evolution of the atmosphere is calculated by taking into consideration various processes: impact erosion and replenishment by asteroids and comets, atmospheric escape induced by solar radiation and wind, volcanic degassing, and gas deposition by interplanetary dust particles. our model reproduces the elemental and isotopic compositions of n and noble gases (except for xe) in the martian atmosphere, as inferred from exploration missions and analyses of martian meteorites. other processes such as ionization-induced fractionation, which are not included in our model, are likely to make a large contribution in producing the current xe isotope composition. since intense impacts during the heavy bombardment period greatly affect the atmospheric mass, the atmospheric pressure evolves stochastically. whereas a dense atmosphere preserves primitive isotopic compositions, a thin atmosphere on early mars is severely influenced by stochastic impact events and following escape-induced fractionation. the onset of fractionation following the decrease in atmospheric pressure is explained by shorter timescales of isotopic fractionation under a lower atmospheric pressure. the comparison of our numerical results with the less fractionated n (15n/14n) and ar (38ar/36ar) isotope compositions of the ancient atmosphere recorded in the martian meteorite allan hills 84001 provides a lower limit of the atmospheric pressure in 4 ga to preserve the primitive isotopic compositions. we conclude that the atmospheric pressure was higher than approximately 0.5 bar at 4 ga. | a lower limit of atmospheric pressure on early mars inferred from nitrogen and argon isotopic compositions |
we present a series of papers dedicated to modelling the accretion and differentiation of rocky planets that form by pebble accretion within the lifetime of the protoplanetary disc. in this first paper, we focus on how the accreted ice determines the distribution of iron between the mantle (oxidized feo and feo1.5) and the core (metallic fe and fes). we find that an initial primitive composition of ice-rich material leads, upon heating by the decay of 26al, to extensive water flow and the formation of clay minerals inside planetesimals. metallic iron dissolves in liquid water and precipitates as oxidized magnetite fe3o4. further heating by 26al destabilizes the clay at a temperature of around 900 k. the released supercritical water ejects the entire water content from the planetesimal. upon reaching the silicate melting temperature of 1700 k, planetesimals further differentiate into a core (made mainly of iron sulfide fes) and a mantle with a high fraction of oxidized iron. we propose that the asteroid vesta's significant feo fraction in the mantle is a testimony of its original ice content. we consider vesta to be a surviving member of the population of protoplanets from which mars, earth, and venus grew by pebble accretion. we show that the increase in the core mass fraction and decrease in feo contents with increasing planetary mass (in the sequence vesta - mars - earth) is naturally explained by the growth of terrestrial planets outside of the water ice line through accretion of pebbles containing iron that was dominantly in metallic form with an intrinsically low oxidation degree. | anatomy of rocky planets formed by rapid pebble accretion. i. how icy pebbles determine the core fraction and feo contents |
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