abstract stringlengths 3 192k | title stringlengths 4 857 |
|---|---|
context. since the orbital insertion of the rosetta spacecraft, comet 67p/churyumov-gerasimenko (67p) has been mapped by osiris camera and virtis spectro-imager, producing a huge quantity of images and spectra of the comet's nucleus.aims: the aim of this work is to search for the presence of h2o on the nucleus which, in general, appears very dark and rich in dehydrated organic material. after selecting images of the bright spots which could be good candidates to search for h2o ice, taken at high resolution by osiris, we check for spectral cubes of the selected coordinates to identify these spots observed by virtis.methods: the selected osiris images were processed with the osiris standard pipeline and corrected for the illumination conditions for each pixel using the lommel-seeliger disk law. the spots with higher i/f were selected and then analysed spectrophotometrically and compared with the surrounding area. we selected 13 spots as good targets to be analysed by virtis to search for the 2 μm absorption band of water ice in the virtis spectral cubes.results: out of the 13 selected bright spots, eight of them present positive h2o ice detection on the virtis data. a spectral analysis was performed and the approximate temperature of each spot was computed. the h2o ice content was confirmed by modeling the spectra with mixing (areal and intimate) of h2o ice and dark terrain, using hapke's radiative transfer modeling. we also present a detailed analysis of the detected spots. | detection of exposed h2o ice on the nucleus of comet 67p/churyumov-gerasimenko. as observed by rosetta osiris and virtis instruments |
hh 212 is one of the well-studied protostellar systems, showing the first vertically resolved disk with a warm atmosphere around the central protostar. here we report a detection of nine organic molecules (including newly detected ketene, formic acid, deuterated acetonitrile, methyl formate, and ethanol) in the disk atmosphere, confirming that the disk atmosphere is, for hh 212, the chemically rich component, identified before at a lower resolution as a “hot corino.” more importantly, we report the first systematic survey and abundance measurement of organic molecules in the disk atmosphere within ∼40 au of the central protostar. the relative abundances of these molecules are similar to those in the hot corinos around other protostars and in comet lovejoy. these molecules can be either (i) originally formed on icy grains and then desorbed into gas phase or (ii) quickly formed in the gas phase using simpler species ejected from the dust mantles. the abundances and spatial distributions of the molecules provide strong constraints on models of their formation and transport in star formation. these molecules are expected to form even more complex organic molecules needed for life and deeper observations are needed to find them. | first abundance measurement of organic molecules in the atmosphere of hh 212 protostellar disk |
xenon (xe) is an exceptional tracer for investigating the origin and fate of volatile elements on earth. the initial isotopic composition of atmospheric xe remains unknown, as do the mechanisms involved in its depletion and isotopic fractionation compared with other reservoirs in the solar system. here we present high precision analyses of noble gases trapped in fluid inclusions of archean quartz (barberton, south africa) that reveal the isotopic composition of the paleo-atmosphere at ~3.3 ga. the archean atmospheric xe is mass-dependently fractionated by 12.9+/-2.4 ‰ u-1 (+/- 2σ, s.d.) relative to the modern atmosphere. the lower than today 129xe excess requires a degassing rate of radiogenic xe from the mantle higher than at present. the primordial xe component delivered to the earth's atmosphere is distinct from solar or chondritic xe but similar to a theoretical component called u-xe. comets may have brought this component to the earth's atmosphere during the last stages of terrestrial accretion. | the origin and degassing history of the earth's atmosphere revealed by archean xenon |
carbon dioxide (co2) is one of the most abundant species in cometary nuclei, but because of its high volatility, co2 ice is generally only found beneath the surface. we report the infrared spectroscopic identification of a co2 ice-rich surface area located in the anhur region of comet 67p/churyumov-gerasimenko. spectral modeling shows that about 0.1% of the 80- by 60-meter area is co2 ice. this exposed ice was observed a short time after the comet exited local winter; following the increased illumination, the co2 ice completely disappeared over about 3 weeks. we estimate the mass of the sublimated co2 ice and the depth of the eroded surface layer. we interpret the presence of co2 ice as the result of the extreme seasonal changes induced by the rotation and orbit of the comet. | seasonal exposure of carbon dioxide ice on the nucleus of comet 67p/churyumov-gerasimenko |
the data from all rosetta plasma consortium instruments and from the rosina cops instrument are used to study the interaction of the solar wind with the outgassing cometary nucleus of 67p/churyumov-gerasimenko. during 6 and 7 june 2015, the interaction was first dominated by an increase in the solar wind dynamic pressure, caused by a higher solar wind ion density. this pressure compressed the draped magnetic field around the comet, and the increase in solar wind electrons enhanced the ionization of the outflow gas through collisional ionization. the new ions are picked up by the solar wind magnetic field, and create a ring/ring-beam distribution, which, in a high-β plasma, is unstable for mirror mode wave generation. two different kinds of mirror modes are observed: one of small size generated by locally ionized water and one of large size generated by ionization and pick-up farther away from the comet. | mass-loading, pile-up, and mirror-mode waves at comet 67p/churyumov-gerasimenko |
europa likely contains an iron-rich metal core. for it to have formed, temperatures within europa reached ≳1250 k. going up to that temperature, accreted chondritic minerals — for example, carbonates and phyllosilicates — would partially devolatilize. here, we compute the amounts and compositions of exsolved volatiles. we find that volatiles released from the interior would have carried solutes, redox-sensitive species, and could have generated a carbonic ocean in excess of europa's present-day hydrosphere, and potentially an early co2 atmosphere. no late delivery of cometary water was necessary. contrasting with prior work, co2 could be the most abundant solute in the ocean, followed by ca2+, so42-, and hco3-. however, gypsum precipitation going from the seafloor to the ice shell decreases the dissolved s/cl ratio, such that cl>s at the shallowest depths, consistent with recently inferred endogenous chlorides at europa's surface. gypsum would form a 3-10 km thick sedimentary layer at the seafloor. | a metamorphic origin for europa's ocean |
we present a co(2-1) and 1240 μm continuum survey of 23 debris disks with spectral types b9-g1, observed at an angular resolution of 0.″5-1″ with the atacama large millimeter/submillimeter array (alma). the sample was selected for large infrared excess and age ∼10 myr, to characterize the prevalence of molecular gas emission in young debris disks. we identify three co-rich debris disks, plus two additional tentative (3σ) co detections. twenty disks were detected in the continuum at the >3σ level. for the 12 disks in the sample that are spatially resolved by our observations, we perform an independent analysis of the interferometric continuum visibilities to constrain the basic dust disk geometry, as well as a simultaneous analysis of the visibilities and broadband spectral energy distribution to constrain the characteristic grain size and disk mass. the gas-rich debris disks exhibit preferentially larger outer radii in their dust disks, and a higher prevalence of characteristic grain sizes smaller than the blowout size. the gas-rich disks do not exhibit preferentially larger dust masses, contrary to expectations for a scenario in which a higher cometary destruction rate would be expected to result in a larger mass of both co and dust. the three debris disks in our sample with strong co detections are all around a stars: the conditions in disks around intermediate-mass stars appear to be the most conducive to the survival or formation of co. | debris disks in the scorpius-centaurus ob association resolved by alma |
aims: we investigate the nucleus photometric properties of the comet 67p/churyumov-gerasimenko as observed by the visible and infrared thermal imaging spectrometer (virtis) onboard the rosetta spacecraft. both full-disk and disk-resolved images of the comet have been analyzed, deriving light and phase curves as well as a photometric reduction of the radiance factor (i/f) to single scattering albedo (ssa) in the 0.4-3.5 μm range.methods: hyperspectral cubes from virtis were calibrated and corrected for instrumental artifacts. we computed integrated fluxes from full-disk acquisitions to derive nucleus light curves and phase curves at low phase angles (1.2°<α < 14.9°). disk-resolved observations in the phase angle range 27.2°<α< 111.5° were reduced to ssa by means of a simplified hapke model, deriving average spectrophotometric properties of the surface and producing ssa maps at different wavelengths. spectral phase reddening in the visible (vis) and infrared (ir) ranges was measured. finally, full-disk and disk-resolved data were used together to derive a phase curve of the nucleus in the 1.2°<α < 111.5° range.results: we measure an asymmetric double-peaked light curve that is due to the elongated shape of the nucleus. the average ssa albedo shows a reddish spectrum with a strong absorption feature centered at 3.2 μm, while the surface exhibits a backscattering behavior. the derived geometric albedo is ageo = 0.062 ± 0.002 at 0.55 μm, indicating a very dark surface. phase reddening is significant both in the vis and ir ranges, and we report spectral slopes of 0.20/kå and 0.033/kå, respectively, after applying photometric reduction. ssa maps indicate that hapi and imothep regions are the brightest in the vis, with the former showing a bluer spectrum with respect to the rest of the surface. the phase curve of the nucleus shows a strong opposition effect, with β = 0.077 ± 0.002 for α < 15°. | photometric properties of comet 67p/churyumov-gerasimenko from virtis-m onboard rosetta |
bathed in intense ionizing radiation, close-in gaseous planets undergo hydrodynamic atmospheric escape, which ejects the upper extent of their atmospheres into the interplanetary medium. ultraviolet detections of escaping gas around transiting planets corroborate such a framework. exposed to the stellar environment, the outflow is shaped by its interaction with the stellar wind and by the planet’s orbit. we model these effects using athena to perform 3d radiative-hydrodynamic simulations of tidally locked hydrogen atmospheres receiving large amounts of ionizing extreme-ultraviolet flux in various stellar environments for the low-magnetic-field case. through a step-by-step exploration of orbital and stellar wind effects on the planetary outflow, we find three structurally distinct stellar wind regimes: weak, intermediate, and strong. we perform synthetic lyα observations and find unique observational signatures for each regime. a weak stellar wind—which cannot confine the planetary outflow, leading to a torus of material around the star—has a pretransit, redshifted dayside arm and a slightly redward-skewed spectrum during transit. the intermediate regime truncates the dayside outflow at large distances from the planet and causes periodic disruptions of the outflow, producing observational signatures that mimic a double transit. the first of these dips is blueshifted and precedes the optical transit. finally, strong stellar winds completely confine the outflow into a cometary tail and accelerate the outflow outward, producing large blueshifted signals posttransit. across all three regimes, large signals occur far outside of transit, offering motivation to continue ultraviolet observations outside of direct transit. | morphology of hydrodynamic winds: a study of planetary winds in stellar environments |
we report the bulk c abundances, and c and o isotopic compositions of carbonates in 64 cm chondrites, 14 cr chondrites, 2 ci chondrites, lew 85332 (c2), kaba (cv3), and semarkona (ll3.0). for the unheated cms, the total ranges of carbonate isotopic compositions are δ13c ≈ 25-75‰ and δ18o ≈ 15-35‰, and bulk carbonate c contents range from 0.03 to 0.60 wt%. there is no simple correlation between carbonate abundance and isotopic composition, or between either of these parameters and the extent of alteration. unless accretion was very heterogeneous, the uncorrelated variations in extent of alteration and carbonate abundance suggests that there was a period of open system behavior in the cm parent body, probably prior to or at the start of aqueous alteration. most of the ranges in cm carbonate isotopic compositions can be explained by their formation at different temperatures (0-130 °c) from a single fluid in which the carbonate o isotopes were controlled by equilibrium with water (δ18o ≈ 5‰) and the c isotopes were controlled by equilibrium with co and/or ch4 (δ13c ≈ -33‰ or -20‰ for co- or ch4-dominated systems, respectively). however, carbonate formation would have to have been inefficient, otherwise carbonate compositions would have resembled those of the starting fluid. a quite similar fluid composition (δ18o ≈ -5.5‰, and δ13c ≈ -31‰ or -17‰ for co- or ch4-dominated systems, respectively) can explain the carbonate compositions of the cis, although the formation temperatures would have been lower (~10-40 °c) and the relative abundances of calcite and dolomite may play a more important role in determining bulk carbonate compositions than in the cms. the cr carbonates exhibit a similar range of o isotopes, but an almost bimodal distribution of c isotopes between more (δ13c ≈ 65-80‰) and less altered samples (δ13c ≈ 30-40‰). this bimodality can still be explained by precipitation from fluids with the same isotopic composition (δ18o ≈ -9.25‰, and δ13c ≈ -21‰ or -8‰ for co- or ch4-dominated systems, respectively) if the less altered crs had higher mole fractions of co2 in their fluids. semarkona and kaba carbonates have some of the lightest c isotopic compositions of the meteorites studied here, probably because they formed at higher temperatures and/or from more co2-rich fluids. the fluids responsible for the alteration of chondrites and from which the carbonates formed were almost certainly accreted as ices. by analogy with cometary ices, co2 and/or co would have dominated the trapped volatile species in the ices. the chondrites studied are too oxidized for co-dominated fluids to have formed in their parent bodies. if ch4 was the dominant c species in the fluids during carbonate formation, it would have to have been generated in the parent bodies from co and/or co2 when oxidation of metal by water created high partial pressures of h2. the fact that the chondrite carbonate c/h2o mole ratios are of the order predicted for co/co2-h2o ices that experienced temperatures of >50-100 k suggests that the chondrites formed at radial distances of <4-15 au. | carbonate abundances and isotopic compositions in chondrites |
head-on collision among dust acoustic (da) multi-solitons in a dusty plasma with ions featuring non-maxwellian hybrid distribution under the effect of the polarization force is investigated. the presence of the non-maxwellian ions leads to eloquent modifications in the polarization force. specifically, an increase in the superthermality index of ions (via κi) and nonthermal parameter (via α) diminishes the polarization parameter. by employing the extended poincaré-lighthill-kuo method, two sided kdv equations are derived. the hirota direct method is used to obtain multi-soliton solutions for each kdv equation, and all of them move along the same direction where the fastest moving soliton eventually overtakes the others. the expressions for collisional phase shifts after head-on collision of two, four, and six-(da) solitons are derived under the influence of polarization force. it is found that the effect of polarization force and the presence of non-maxwellian ions have an emphatic influence on the phase shifts after the head-on collision of da rarefactive multi-solitons. in a small amplitude limit, the impact of polarization force on time evolution of multi-solitons is also illustrated. it is intensified that the present theoretical pronouncements actually effectuate in laboratory experiments and in space/astrophysical environments, in particular in saturn's magnetosphere and comet tails. | effect of polarization force on head-on collision between multi-solitons in dusty plasma |
low-mass protostars are the extrasolar analogues of the natal solar system. sophisticated physicochemical models are used to simulate the formation of two protoplanetary discs from the initial prestellar phase, one dominated by viscous spreading and the other by pure infall. the results show that the volatile prestellar fingerprint is modified by the chemistry en route into the disc. this holds relatively independent of initial abundances and chemical parameters: physical conditions are more important. the amount of co2 increases via the grain-surface reaction of oh with co, which is enhanced by photodissociation of h2o ice. complex organic molecules are produced during transport through the envelope at the expense of ch3oh ice. their abundances can be comparable to that of methanol ice (few per cent of water ice) at large disc radii (r > 30 au). current class ii disc models may be underestimating the complex organic content. planet population synthesis models may underestimate the amount of co2 and overestimate ch3oh ices in planetesimals by disregarding chemical processing between the cloud and disc phases. the overall c/o and c/n ratios differ between the gas and solid phases. the two ice ratios show little variation beyond the inner 10 au and both are nearly solar in the case of pure infall, but both are subsolar when viscous spreading dominates. chemistry in the protostellar envelope en route to the protoplanetary disc sets the initial volatile and prebiotically significant content of icy planetesimals and cometary bodies. comets are thus potentially reflecting the provenances of the midplane ices in the solar nebula. | cometary ices in forming protoplanetary disc midplanes |
the activity of most comets near the sun is dominated by the sublimation of frozen water, the most abundant ice in comets. some comets, however, are active well beyond the water-ice sublimation limit of ∼3 au. three bodies dominate the observational record and modeling efforts for distantly active comets: the long-period comet c/1995 o1 (hale-bopp), and the short-period comets (with centaur orbits) 29p/schwassmann-wachmann 1 and 2060 chiron. we summarize what is known about these three objects with an emphasis on their gaseous comae. we calculate their cn/co and co2/co production rate ratios from the literature and discuss implications, such as hcn and co2 outgassing are not significant contributors to their comae. using our own data we derive co production rates, q(co), for all three objects to examine whether there is a correlation between gas production and different orbital histories and/or size. the co measurements of hale-bopp (4-11 au) and 29p are consistent with a nominal production rate of q(co) = 3.5 × 1029 r-2 superimposed with sporadic outbursts. the similarity of hale-bopp co production rates for pre- and post-perihelion suggests that thermal inertia was not very important and therefore most of the activity is at or near the surface of the comet. we further examine the applicability of existing models in explaining the systematic behavior of our small sample. we find that orbital history does not appear to play a significant role in explaining 29p’s co production rates. 29p outproduces hale-bopp at the same heliocentric distance, even though it has been subjected to much more solar heating. previous modeling work on such objects predicts that 29p should have been devolatilized over a fresher comet like hale-bopp. this may point to 29p having a different orbital history than current models predict, with its current orbit acquired more recently. on the other hand, chiron’s co measurements are consistent with it being significantly depleted over its original state, perhaps due to increased radiogenic heating made possible by its much larger size or its higher processing due to orbital history. observed spectral line profiles for several volatiles are consistent with the development and sublimation of icy grains in the coma at about 5-6 au for 29p and hale-bopp, and this is probably a common feature in distantly active comets, and an important source of volatiles for all comets within 5 au. in contrast, the narrow co line profiles indicate a nuclear, and not extended, origin for co beyond ∼4 au. | co in distantly active comets |
ammonium hydrosulphide has long since been postulated to exist at least in certain layers of the giant planets. its radiation products may be the reason for the red colour seen on jupiter. several ammonium salts, the products of nh3, and an acid have previously been detected at comet 67p/churyumov-gerasimenko. the acid h2s is the fifth most abundant molecule in the coma of 67p followed by nh3. in order to look for the salt nh4+sh-, we analysed in situ measurements from the rosetta/rosina double focusing mass spectrometer during the rosetta mission. nh3 and h2s appear to be independent of each other when sublimating directly from the nucleus. however, we observe a strong correlation between the two species during dust impacts, clearly pointing to the salt. we find that nh4+sh- is by far the most abundant salt, more abundant in the dust impacts than even water. we also find all previously detected ammonium salts and for the first time ammonium fluoride. the amount of ammonia and acids balance each other, confirming that ammonia is mostly in the form of salt embedded into dust grains. allotropes s2 and s3 are strongly enhanced in the impacts, while h2s2 and its fragment hs2 are not detected, which is most probably the result of radiolysis of nh4+sh-. this makes a prestellar origin of the salt likely. our findings may explain the apparent depletion of nitrogen in comets and maybe help to solve the riddle of the missing sulphur in star-forming regions. | abundant ammonium hydrosulphide embedded in cometary dust grains |
co and co2 are the two dominant carbon-bearing molecules in comae and have major roles in driving activity. their relative abundances also provide strong observational constraints to models of solar system formation and evolution but have never been studied together in a large sample of comets. we carefully compiled and analyzed published measurements of simultaneous co and co2 production rates for 25 comets. approximately half of the comae have substantially more co2 than co, about one-third are co-dominated, and about 1/10 produce a comparable amount of both. there may be a heliocentric dependence to this ratio with co dominating comae beyond 3.5 au. eight out of nine of the jupiter family comets in our study produce more co2 than co. the six dynamically new comets produce more co2 relative to co than the eight oort cloud comets that have made multiple passes through the inner solar system. this may be explained by long-term cosmic-ray processing of a comet nucleus's outer layers. we find (q co/q ${}_{{{\rm{h}}}_{2}{\rm{o}}}$ )median = 3 ± 1% and (q ${}_{{\mathrm{co}}_{2}}$ /q ${}_{{{\rm{h}}}_{2}{\rm{o}}}$ )median = 12 ± 2%. the inorganic volatile carbon budget was estimated to be (q co+q ${}_{{\mathrm{co}}_{2}}$ )/q ${}_{{{\rm{h}}}_{2}{\rm{o}}}$ ~ 18% for most comets. between 0.7 and 4.6 au, co2 outgassing appears to be more intimately tied to the water production in a way that the co is not. the volatile carbon/oxygen ratio for 18 comets is c/omedian ~ 13%, which is consistent with a comet formation environment that is well within the co snow line. | a survey of co, co2, and h2o in comets and centaurs |
we present visible and near-infrared (nir) photometric and spectroscopic observations of interstellar object (iso) 2i/borisov taken from 2019 september 10 to 2019 december 20 using the growth, the apache point observatory astrophysical research consortium 3.5 m, and the nasa infrared telescope facility 3.0 m combined with pre- and postdiscovery observations of 2i obtained by the zwicky transient facility from 2019 march 17 to 2019 may 5. comparison with imaging of distant solar system comets shows an object very similar to mildly active solar system comets with an outgassing rate of ∼1027 mol s-1. the photometry, taken in filters spanning the visible and nir range, shows a gradual brightening trend of ∼0.03 mag day-1 since 2019 september 10 utc for a reddish object becoming neutral in the nir. the light curve from recent and prediscovery data reveals a brightness trend suggesting the recent onset of significant h2o sublimation with the comet being active with super volatiles such as co at heliocentric distances >6 au consistent with its extended morphology. using the advanced capability to significantly reduce the scattered light from the coma enabled by high-resolution nir images from keck adaptive optics taken on 2019 october 4, we estimate a diameter for 2i's nucleus of ≲1.4 km. we use the size estimates of 1i/'oumuamua and 2i/borisov to roughly estimate the slope of the iso size distribution, resulting in a slope of ∼3.4 ± 1.2, similar to solar system comets and bodies produced from collisional equilibrium. | characterization of the nucleus, morphology, and activity of interstellar comet 2i/borisov by optical and near-infrared growth, apache point, irtf, ztf, and keck observations |
the european space agency's rosetta mission1 has acquired unprecedented measurements of the surface of the nucleus of comet 67p/churyumov-gerasimenko (hereafter, 67p), the composition of which, as determined by in situ and remote-sensing instruments, including the virtis instrument2, seems to be an assemblage of ices, minerals and organic material3. we performed a refined analysis of infrared observations of the nucleus of 67p carried out by the virtis-m hyperspectral imager. we find that the overall shape of the infrared spectrum of 67p is similar to that of other carbon-rich outer solar system objects, suggesting a possible genetic link with them. more importantly, we also confirm the complex spectral structure of the wide 2.8-3.6 µm absorption feature populated by fainter bands. among these, we unambiguously identify the presence of aliphatic organics by their ubiquitous 3.38 µm, 3.42 µm and 3.47 µm bands. this infrared detection of aliphatic species on a cometary surface has strong implications for the evolutionary history of the primordial solar system and is evidence that comets provide an evolutionary link between interstellar material and solar system bodies4. | infrared detection of aliphatic organics on a cometary nucleus |
infrared observations of the coma of 67p/churyumov-gerasimenko were carried out from 2015 july to september, i.e. around perihelion (2015 august 13), with the high-resolution channel of the visible and infrared thermal imaging spectrometer instrument onboard rosetta. we present the analysis of fluorescence emission lines of h2o, co2, 13co2, ocs, and ch4 detected in limb sounding with the field of view at 2.7-5 km from the comet centre. measurements are sampling outgassing from the illuminated southern hemisphere, as revealed by h2o and co2 raster maps, which show anisotropic distributions, aligned along the projected rotation axis. an abrupt increase of water production is observed 6 d after perihelion. in the meantime, co2, ch4, and ocs abundances relative to water increased by a factor of 2 to reach mean values of 32, 0.47, and 0.18 per cent, respectively, averaging post-perihelion data. we interpret these changes as resulting from the erosion of volatile-poor surface layers. sustained dust ablation due to the sublimation of water ice maintained volatile-rich layers near the surface until at least the end of the considered period, as expected for low thermal inertia surface layers. the large abundance measured for co2 should be representative of the 67p nucleus original composition, and indicates that 67p is a co2-rich comet. comparison with abundance ratios measured in the northern hemisphere shows that seasons play an important role in comet outgassing. the low co2/h2o values measured above the illuminated northern hemisphere are not original, but the result of the devolatilization of the uppermost layers. | evolution of co2, ch4, and ocs abundances relative to h2o in the coma of comet 67p around perihelion from rosetta/virtis-h observations |
we present a multi-wavelength study of comet c/2016 r2 (panstarrs). this comet was observed on 23 and 24 january 2018 with the iram 30 m telescope, and in january to march 2018 with the nançay radio telescope. visible spectroscopy was performed in december 2017 and february 2018 with small amateur telescopes. we report on measurements of co, ch3oh, h2co and hcn production rates, and on the determination of the n2/co abundance ratio. several other species, especially oh, were searched for but not detected. the inferred relative abundances, including upper limits for sulfur species, are compared to those measured in other comets at about the same heliocentric distance of 2.8 au. the coma composition of comet c/2016 r2 is very different from all other comets observed so far, being rich in n2 and co and dust poor. this suggests that this comet might belong to a very rare group of comets formed beyond the n2 ice line. alternatively, comet c/2016 r2 (panstarrs) could be the fragment of a large and differentiated transneptunian object, with properties characteristic of volatile-enriched layers. based on observations carried out with the iram 30 m telescope. iram is supported by insu/cnrs (france), mpg (germany), and ign (spain).the radio spectra are only available at the cds via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?j/a+a/619/a127 | the extraordinary composition of the blue comet c/2016 r2 (panstarrs) |
the classic accepted view of the heliosphere is a quiescent, comet-like shape aligned in the direction of the sun’s travel through the interstellar medium (ism) extending for thousands of astronomical units (aus). here, we show, based on magnetohydrodynamic (mhd) simulations, that the tension (hoop) force of the twisted magnetic field of the sun confines the solar wind plasma beyond the termination shock and drives jets to the north and south very much like astrophysical jets. these jets are deflected into the tail region by the motion of the sun through the ism similar to bent galactic jets moving through the intergalactic medium. the interstellar wind blows the two jets into the tail but is not strong enough to force the lobes into a single comet-like tail, as happens to some astrophysical jets. instead, the interstellar wind flows around the heliosphere and into the equatorial region between the two jets. as in some astrophysical jets that are kink unstable, we show here that the heliospheric jets are turbulent (due to large-scale mhd instabilities and reconnection) and strongly mix the solar wind with the ism beyond 400 au. the resulting turbulence has important implications for particle acceleration in the heliosphere. the two-lobe structure is consistent with the energetic neutral atom (ena) images of the heliotail from ibex where two lobes are visible in the north and south and the suggestion from the cassini enas that the heliosphere is “tailless.” | magnetized jets driven by the sun: the structure of the heliosphere revisited |
isotopic ratios provide a powerful tool for understanding the origins of materials, including the volatile and refractory matter within solar system bodies. recent high sensitivity observations of molecular isotopologues, in particular with alma, have brought us new information on isotopic ratios of hydrogen, carbon, nitrogen and oxygen in star and planet forming regions as well as the solar system objects. solar system exploration missions, such as rosetta and cassini, have given us further new insights. meanwhile, the recent development of sophisticated models for isotope chemistry including detailed gas-phase and grain surface reaction network has made it possible to discuss how isotope fractionation in star and planet forming regions is imprinted into the icy mantles of dust grains, preserving a record of the initial isotopic state of solar system materials. this chapter reviews recent progress in observations of molecular isotopologues in extra-solar planet forming regionsã¢â€â"prestellar/protostellar cores and protoplanetary disks—as well as objects in our solar system—comets, meteorites, and planetary/satellite atmospheres—and discusses their connection by means of isotope chemical models. | the isotopic links from planet forming regions to the solar system |
aims: the osiris camera on board the rosetta spacecraft has been acquiring images of the comet 67p/churyumov-gerasimenko (67p)'s nucleus since august 2014. starting in may 2015, the southern hemisphere gradually became illuminated and was imaged for the first time. here we present the regional morphology of the southern hemisphere, which serves as a companion to an earlier paper that presented the regional morphology of the northern hemisphere.methods: we used osiris images that were acquired at orbits ~45-125 km from the center of the comet (corresponding to spatial resolutions of ~0.8 to 2.3 m/pixel) coupled with the use of digital terrain models to define the different regions on the surface, and identify structural boundaries accurately.results: seven regions have been defined in the southern hemisphere bringing the total number of defined regions on the surface of the nucleus to 26. these classifications are mainly based on morphological and/or topographic boundaries. the southern hemisphere shows a remarkable dichotomy with its northern counterpart mainly because of the absence of wide-scale smooth terrains, dust coatings and large unambiguous depressions. as a result, the southern hemisphere closely resembles previously identified consolidated regions. an assessment of the overall morphology of comet 67p suggests that the comet's two lobes show surface heterogeneities manifested in different physical/mechanical characteristics, possibly extending to local (i.e., within a single region) scales. | regional surface morphology of comet 67p/churyumov-gerasimenko from rosetta/osiris images: the southern hemisphere |
the rosetta lander philae is part of the esa rosetta mission which reached comet 67p/churyumov-gerasimenko after a 10 year cruise in august 2014. since then, rosetta has been studying both its nucleus and coma with instruments aboard the orbiter. on november 12th, 2014 the lander, philae, was successfully delivered to the surface of the comet and operated for approximately 64 h after separation from the mother spacecraft. since the active cold gas system aboard the lander as well as the anchoring harpoons did not work, philae bounced after the first touch-down at the planned landing site "agilkia". at the final landing site, "abydos", a modified first scientific sequence was performed. due to the unexpectedly low illumination conditions and a lack of anchoring the sequence had to be adapted in order to minimize risk and maximize the scientific output. all ten instruments could be activated at least once, before philae went into hibernation. in june 2015, the lander contacted rosetta again having survived successfully a long hibernation phase. this paper describes the lander operations around separation, during descent and on the surface of the comet. we also address the partly successful attempts to re-establish contact with the lander in june/july, when the internal temperature & power received were sufficient for philae to become active again. | rosetta lander - landing and operations on comet 67p/churyumov-gerasimenko |
67p/churyumov-gerasimenko (67p) is a jupiter-family comet and the object of investigation of the european space agency mission rosetta. this report presents the first full 3d simulation results of 67p's neutral gas coma. in this study we include results from a direct simulation monte carlo method, a hydrodynamic code, and a purely geometric calculation which computes the total illuminated surface area on the nucleus. all models include the triangulated 3d shape model of 67p as well as realistic illumination and shadowing conditions. the basic concept is the assumption that these illumination conditions on the nucleus are the main driver for the gas activity of the comet. as a consequence, the total production rate of 67p varies as a function of solar insolation. the best agreement between the model and the data is achieved when gas fluxes on the night side are in the range of 7% to 10% of the maximum flux, accounting for contributions from the most volatile components. to validate the output of our numerical simulations we compare the results of all three models to in situ gas number density measurements from the rosina cops instrument. we are able to reproduce the overall features of these local neutral number density measurements of rosina cops for the time period between early august 2014 and january 1 2015 with all three models. some details in the measurements are not reproduced and warrant further investigation and refinement of the models. however, the overall assumption that illumination conditions on the nucleus are at least an important driver of the gas activity is validated by the models. according to our simulation results we find the total production rate of 67p to be constant between august and november 2014 with a value of about 1 × 1026 molecules s-1. | comparison of 3d kinetic and hydrodynamic models to rosina-cops measurements of the neutral coma of 67p/churyumov-gerasimenko |
this paper is the result of the international cometary workshop, held in toulouse, france in april 2014, where the participants came together to assess our knowledge of comets prior to the esa rosetta mission. in this paper, we look at the composition of the gas and dust from the comae of comets. with the gas, we cover the various taxonomic studies that have broken comets into groups and compare what is seen at all wavelengths. we also discuss what has been learned from mass spectrometers during flybys. a few caveats for our interpretation are discussed. with dust, much of our information comes from flybys. they include in situ analyses as well as samples returned to earth for laboratory measurements. remote sensing ir observations and polarimetry are also discussed. for both gas and dust, we discuss what instruments the rosetta spacecraft and philae lander will bring to bear to improve our understanding of comet 67p/churyumov-gerasimenko as "ground-truth" for our previous comprehensive studies. finally, we summarize some of the initial rosetta mission findings. | the composition of comets |
we study the development of activity in the incoming long-period comet c/2017 k2 over the heliocentric distance range 9 ≲ rh ≲ 16 au. the comet continues to be characterized by a coma of submillimeter-sized and larger particles ejected at low velocity. in a fixed co-moving volume around the nucleus we find that the scattering cross section of the coma, c, is related to the heliocentric distance by a power law, $c\propto {r}_{h}^{-s}$ <!-- --> ,with heliocentric index s = 1.14 ± 0.05. this dependence is significantly weaker than the rh-2 variation of the insolation as a result of two effects. these are, first, the heliocentric dependence of the dust velocity and, second, a lag effect due to very slow-moving particles ejected long before the observations were taken. a monte carlo model of the photometry shows that dust production beginning at rh ∼ 35 au is needed to match the measured heliocentric index, with only a slight dependence on the particle size distribution. mass-loss rates in dust at 10 au are of order 103 kg s-1, while loss rates in gas may be much smaller, depending on the unknown dust to gas ratio. consequently, the ratio of the nongravitational acceleration to the local solar gravity, α', may, depending on the nucleus size, attain values of ∼10-7 ≲ α' ≲ 10-5, comparable to values found in short-period comets at much smaller distances. nongravitational acceleration in c/2017 k2 and similarly distant comets, while presently unmeasured, may limit the accuracy with which we can infer the properties of the oort cloud from the orbits of long-period comets. | cometary activity begins at kuiper belt distances: evidence from c/2017 k2 |
context. the study of large and representative samples of low-metallicity star-forming galaxies at different cosmic epochs is of great interest to the detailed understanding of the assembly history and evolution of low-mass galaxies.aims: we present a thorough characterization of a large sample of 183 extreme emission-line galaxies (eelgs) at redshift 0.11 ≤ z ≤ 0.93 selected from the 20k zcosmos bright survey because of their unusually large emission line equivalent widths.methods: we use multiwavelength cosmos photometry, hst-acs i-band imaging, and optical zcosmos spectroscopy to derive the main global properties of star-forming eelgs, such as sizes, stellar masses, star formation rates (sfr), and reliable oxygen abundances using both "direct" and "strong-line" methods.results: the eelgs are extremely compact (r50 ~ 1.3 kpc), low-mass (m∗ ~ 107-1010 m⊙) galaxies forming stars at unusually high specific star formation rates (ssfr ≡ sfr/m⋆ up to 10-7 yr-1) compared to main sequence star-forming galaxies of the same stellar mass and redshift. at rest-frame uv wavelengths, the eelgs are luminous and show high surface brightness and include strong lyα emitters, as revealed by galex spectroscopy. we show that zcosmos eelgs are high-ionization, low-metallicity systems, with median 12+log (o/h) = 8.16 ± 0.21 (0.2 z⊙) including a handful of extremely metal-deficient (<0.1 z⊙) eelgs. while ~80% of the eelgs show non-axisymmetric morphologies, including clumpy and cometary or tadpole galaxies, we find that ~29% of them show additional low-surface-brightness features, which strongly suggests recent or ongoing interactions. as star-forming dwarfs in the local universe, eelgs are most often found in relative isolation. while only very few eelgs belong to compact groups, almost one third of them are found in spectroscopically confirmed loose pairs or triplets.conclusions: the zcosmos eelgs are galaxies caught in a transient and probably early period of their evolution, where they are efficiently building up a significant fraction of their present-day stellar mass in an ongoing, galaxy-wide starburst. therefore, the eelgs constitute an ideal benchmark for comparison studies between low- and high-redshift low-mass star-forming galaxies. full tables 1 and 2 are only available at the cds via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?j/a+a/578/a105 | extreme emission-line galaxies out to z ~ 1 in zcosmos. i. sample and characterization of global properties |
comet c/2014 un271 (bernardinelli-bernstein), incoming from the oort cloud, is remarkable in having the brightest (and presumably largest) nucleus of any well-measured comet and having been discovered at the heliocentric distance rh ≈ 29 au, farther than any oort cloud comet. in this work, we describe the discovery process and observations and the properties that can be inferred from images recorded until the first reports of activity in 2021 june. the orbit has i = 95°, with a perihelion of 10.97 au to be reached in 2031 and a previous aphelion at 40,400 ± 260 au. backward integration of the orbit under a standard galactic tidal model and known stellar encounters suggests a perihelion of q ≈ 18 au on its previous perihelion passage 3.5 myr ago; hence, the current data could be the first ever obtained of a comet that has not been inside uranus's orbit in 4 gyr. the photometric data show an unresolved nucleus with absolute magnitude hr = 8.0, colors that are typical of comet nuclei or damocloids, and no secular trend as it traversed the range 34-23 au. for the r-band geometric albedo pr, this implies a diameter of $150{({p}_{r}/0.04)}^{-0.5}$ km. there is strong evidence of brightness fluctuations at the ±0.2 mag level, but no rotation period can be discerned. a coma, nominally consistent with a "stationary" 1/ρ surface brightness distribution, grew in scattering cross section at an exponential rate from afρ ≈ 1 to ≈150 m as the comet approached from 28 to 20 au. the activity rate is consistent with a very simple model of sublimation of a surface species in radiative equilibrium with the sun. the inferred enthalpy of sublimation matches those of co2 and nh3. more volatile species, such as n2, ch4, and co, must be far less abundant on the sublimating surfaces. | c/2014 un271 (bernardinelli-bernstein): the nearly spherical cow of comets |
based on new high-resolution multi-beam bathymetry and multichannel seismic reflection data, two new groups of numerous pockmarks and mud volcanoes were discovered in the northern zhongjiannan basin at water depths between 600 and 1400 m. individual pockmarks are circular, elliptical, crescent-shaped or elongated, with diameters ranging from several hundreds to thousands of meters and tens or hundreds of meters in depth, and they often form groups or strings. crescent pockmarks, approximately 500-1500 m wide in cross-section and 50-150 m deep, occur widely in the southern study area, both as individual features and in groups or curvilinear chains, and they are more widespread and unique in this area than anywhere else in the world. conical mud volcanoes, mostly with kilometer-wide diameters and ca. 100 m high, mainly develop in the northern study area as individual features or in groups. seismic data show that the observed pockmarks are associated with different kinds of fluid escape structures and conduits, such as gas chimneys, diapirs, zones of acoustic blanking, acoustic turbidity and enhanced reflections, inclined faults, small fractures and polygonal faults. the mapped mud volcanoes appear to be fed from deep diapirs along two main conduit types: the conventional conduits with downward tapering cones and another other conduit type with a narrow conduit in the lower half and emanative leakage passages in the upper half. various types of pockmarks are found and a comprehensive pockmark classification scheme is proposed, according to: (a) their shape in plan view, which includes circular, elliptical, crescent, comet-shape, elongated and irregular; (b) their magnitude, which includes small, normal, giant and mega-pockmarks; and (c) their composite pattern, which includes composite pockmarks, pockmark strings and pockmark groups. for the genesis of the crescent pockmark (strings), a 5-stage speculative formation model is proposed, implying possible controlling factors of gravity sliding/slumping, fluid escape activity and sandbodies intrusion. seismic data suggest that the mud volcanoes have likely undergone two episodes of evolution, which include a widespread drastic formation of mud volcanoes with dome-like structures, a wide range of seepage occurrences and the formation of complex sediments in the first stage, and the appearance of pockmarks, conical mud volcanoes (groups), and the formation of deformed, filled or broken buried complex sediments by continuous fluid flow processes during stage 2, in the pliocene and the quaternary. pockmark gullies are extensively found along the slope, due to the interaction of slope failure and fluid escape processes. the cenozoic sedimentary cover is thin in the northern zhongjiannan basin, which evolved from a late cretaceous or palaeogene-oligocene rift to the neogene-quaternary post-rift thermal subsidence, and therefore intense deep thermogenic fluid flow can easily affect the soft cenozoic sediments, produce complex sediment deformation, and form numerous fluid flow structures at the seafloor, in particular pockmarks and mud volcanoes that dominantly emerged during the period of post-rift thermal subsidence and neotectonic movement since ca. 5.5 ma. | morphologies, classification and genesis of pockmarks, mud volcanoes and associated fluid escape features in the northern zhongjiannan basin, south china sea |
context. during the period between 15 september 2014 and 4 february 2015, the rosetta spacecraft accomplished the circular orbit phase around the nucleus of comet 67p/churyumov-gerasimenko (67p). the grain impact analyzer and dust accumulator (giada) onboard rosetta monitored the 67p coma dust environment for the entire period.aims: we aim to describe the dust spatial distribution in the coma of comet 67p by means of in situ measurements. we determine dynamical and physical properties of cometary dust particles to support the study of the production process and dust environment modification.methods: we analyzed giada data with respect to the observation geometry and heliocentric distance to describe the coma dust spatial distribution of 67p, to monitor its activity, and to retrieve information on active areas present on its nucleus. we combined giada detection information with calibration activity to distinguish different types of particles that populate the coma of 67p: compact particles and fluffy porous aggregates. by means of particle dynamical parameters measured by giada, we studied the dust acceleration region.results: giada was able to distinguish different types of particles populating the coma of 67p: compact particles and fluffy porous aggregates. most of the compact particle detections occurred at latitudes and longitudes where the spacecraft was in view of the comet's neck region of the nucleus, the so-called hapi region. this resulted in an oscillation of the compact particle abundance with respect to the spacecraft position and a global increase as the comet moved from 3.36 to 2.43 au heliocentric distance. the speed of these particles, having masses from 10-10 to 10-7 kg, ranged from 0.3 to 12.2 m s-1. the variation of particle mass and speed distribution with respect to the distance from the nucleus gave indications of the dust acceleration region. the influence of solar radiation pressure on micron and submicron particles was studied. the integrated dust mass flux collected from the sun direction, that is, particles reflected by solar radiation pressure, was three times higher than the flux coming directly from the comet nucleus. the awakening 67p comet shows a strong dust flux anisotropy, confirming what was suggested by on-ground dust coma observations performed in 2008. | giada: shining a light on the monitoring of the comet dust production from the nucleus of 67p/churyumov-gerasimenko |
life on earth employs chiral amino acids in stereochemical l-form, but the cause of molecular symmetry breaking remains unknown. chiroptical properties of amino acids - expressed in circular dichroism (cd) - have been previously investigated in solid and solution phase. however, both environments distort the intrinsic charge distribution associated with cd transitions. here we report on cd and anisotropy spectra of amino acids recorded in the gas phase, where any asymmetry is solely determined by the genuine electromagnetic transition moments. using a pressure- and temperature-controlled gas cell coupled to a synchrotron radiation cd spectropolarimeter, we found cd active transitions and anisotropies in the 130-280 nm range, which are rationalized by ab initio calculation. as gas phase glycine was found in a cometary coma, our data may provide insights into gas phase asymmetric photochemical reactions in the life cycle of interstellar gas and dust, at the origin of the enantiomeric selection of life's l-amino acids. | amino acid gas phase circular dichroism and implications for the origin of biomolecular asymmetry |
a theoretical investigation has been presented to study the effect of polarization force on nonlinear dust acoustic (da) cnoidal waves in dusty plasma composed of negatively charged dust fluid, maxwellian electrons and superthermally distributed ions. the effect of polarization force is significantly modified due to the presence of the superthermal ions. in particular, an increase in superthermality index of ions leads to a decrease in polarization parameter. by employing reductive perturbation method, the nonlinear korteweg-de vries (kdv) equation is derived for the study of da cnoidal waves. further, the sagdeev potential approach is employed to find the solution of kdv equation to analyze the characteristics of da cnoidal waves. only negative potential da cnoidal waves are observed. furthermore, the combined effects of polarization force and superthermality of ions on the characteristics of negative potential da cnoidal waves have been studied in detail. it is emphasized that the real implementation of our present results is in laboratory experiments as well as in different regions of space and astrophysical environments especially in saturn's magnetosphere, comet tails, etc. | effect of polarization force on dust-acoustic cnoidal waves in dusty plasma |
we propose to identify the main sources of ionization of the plasma in the coma of comet 67p/churyumov-gerasimenko at different locations in the coma and to quantify their relative importance, for the first time, for close cometocentric distances (<20 km) and large heliocentric distances (>3 au). the ionospheric model proposed is used as an organizing element of a multi-instrument data set from the rosetta plasma consortium (rpc) plasma and particle sensors, from the rosetta orbiter spectrometer for ion and neutral analysis and from the microwave instrument on the rosetta orbiter, all on board the esa/rosetta spacecraft. the calculated ionospheric density driven by rosetta observations is compared to the rpc-langmuir probe and rpc-mutual impedance probe electron density. the main cometary plasma sources identified are photoionization of solar extreme ultraviolet (euv) radiation and energetic electron-impact ionization. over the northern, summer hemisphere, the solar euv radiation is found to drive the electron density - with occasional periods when energetic electrons are also significant. over the southern, winter hemisphere, photoionization alone cannot explain the observed electron density, which reaches sometimes higher values than over the summer hemisphere; electron-impact ionization has to be taken into account. the bulk of the electron population is warm with temperature of the order of 7-10 ev. for increased neutral densities, we show evidence of partial energy degradation of the hot electron energy tail and cooling of the full electron population. | ionospheric plasma of comet 67p probed by rosetta at 3 au from the sun |
the long duration of the rosetta mission allows us to study the evolution of the diamagnetic cavity at comet 67p/churyumov-gerasimenko in detail. from 2015 april to 2016 february 665 intervals could be identified where rosetta was located in a zero-magnetic-field region. we study the temporal and spatial distribution of this cavity and its boundary and conclude that the cavity properties depend on the long-term trend of the outgassing rate, but do not respond to transient events at the spacecraft location, such as outbursts or high neutral densities. using an empirical model of the outgassing rate, we find a functional relationship between the outgassing rate and the distance of the cavity to the nucleus. there is also no indication that this unexpectedly large distance is related to unusual solar wind conditions. because the deduced shape of the cavity boundary is roughly elliptical on small scales and the distances of the boundary from the nucleus are much larger than expected we conclude that the events observed by rosetta are due to a moving instability of the cavity boundary itself. | structure and evolution of the diamagnetic cavity at comet 67p/churyumov-gerasimenko |
comet c/2019 q4 (borisov) -later renamed 2i/borisov- was discovered on august 30, 2019 by g. borisov observing from crimea. the value of its orbital eccentricity was initially found to be over 3.6 that made this object only the second interstellar minor body ever found, after 1i/2017 u1 ('oumuamua). here, we present early photometric, spectroscopic, and numerical results about this object. observational results have been acquired using the 10.4 m gran telescopio canarias (gtc), located at the el roque de los muchachos observatory (la palma, canary islands, spain) with the osiris camera-spectrograph. our images confirm the cometary nature of 2i/borisov. we obtained a total of three low-resolution, visible spectra of 300 seconds in the 4900-9200 angstrom wavelength range at low solar elongation (43.6 degrees). the visible spectrum of c/2019 q4 resembles those of typical cometary nuclei with a value of the spectral slope roughly in the middle of the range of visible spectral slopes observed for cometary nuclei in the solar system. all the n-body simulations performed indicate that it came from interstellar space and it will return to it. its barycentric radiant appears projected close to open star cluster stock 2. | interstellar visitors: a physical characterization of comet c/2019 q4 (borisov) with osiris at the 10.4\u2009m gtc |
the recent analysis of the composition of the frozen surface of comet 67p/churyumov-gerasimenko has revealed a significant number of complex organic molecules. methyl isocyanate (ch3nco) is one of the more abundant species detected on the comet surface. in this work we report extensive characterization of its rotational spectrum resulting in a list of 1269 confidently assigned laboratory lines and its detection in space towards the orion clouds where 399 lines of the molecule have been unambiguously identified. we find that the limited mm-wave laboratory data reported prior to our work require some revision. the abundance of ch3nco in orion is only a factor of ten below those of hnco and ch3cn. unlike the molecular abundances in the coma of comets, which correlate with those of warm molecular clouds, molecular abundances in the gas phase in orion are only weakly correlated with those measured on the comet surface. we also compare our abundances with those derived recently for this molecule towards sgr b2 (halfen et al. 2015, apj, 812, l5). a more accurate abundance of ch3nco is provided for this cloud based on our extensive laboratory work. this paper makes use of the following alma data: ads/jao.alma#2011.0.00009.sv. alma is a partnership of eso (representing its member states), nsf (usa), and nins (japan) with nrc (canada), nsc, and asiaa (taiwan), and kasi (republic of korea), in cooperation with the republic of chile. the joint alma observatory is operated by eso, aui/nrao, and naoj. this work was also based on observations carried out with the iram 30-meter telescope. iram is supported by insu/cnrs (france), mpg (germany), and ign (spain).full table a.6 is only available at the cds via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?j/a+a/587/l4 | a rigorous detection of interstellar ch3nco: an important missing species in astrochemical networks |
heat transport and ice sublimation in comets are interrelated processes reflecting properties acquired at the time of formation and during subsequent evolution. the microwave instrument on the rosetta orbiter (miro) acquired maps of the subsurface temperature of comet 67p/churyumov-gerasimenko, at 1.6 mm and 0.5 mm wavelengths, and spectra of water vapor. the total h2o production rate varied from 0.3 kg s-1 in early june 2014 to 1.2 kg s-1 in late august and showed periodic variations related to nucleus rotation and shape. water outgassing was localized to the “neck” region of the comet. subsurface temperatures showed seasonal and diurnal variations, which indicated that the submillimeter radiation originated at depths comparable to the diurnal thermal skin depth. a low thermal inertia (~10 to 50 j k-1 m-2 s-0.5), consistent with a thermally insulating powdered surface, is inferred. | subsurface properties and early activity of comet 67p/churyumov-gerasimenko |
the cometary secondary ion mass analyser instrument on board esa's rosetta mission has collected dust particles in the coma of comet 67p/churyumov-gerasimenko. during the early-orbit phase of the rosetta mission, particles and particle agglomerates have been imaged and analyzed in the inner coma at distances between 100 km and 10 km off the cometary nucleus and at more than 3 au from the sun. we identified 585 particles of more than 14 μm in size. the particles are collected at low impact speeds and constitute a sample of the dust particles in the inner coma impacting and fragmenting on the targets. the sizes of the particles range from 14 μm up to sub-millimeter sizes and the differential dust flux size distribution is fitted with a power law exponent of -3.1. after impact, the larger particles tend to stick together, spread out or consist of single or a group of clumps, and the flocculent morphology of the fragmented particles is revealed. the elemental composition of the dust particles is heterogeneous and the particles could contain typical silicates like olivine and pyroxenes, as well as iron sulfides. the sodium to iron elemental ratio is enriched with regard to abundances in ci carbonaceous chondrites by a factor from ∼1.5 to ∼15. no clear evidence for organic matter has been identified. the composition and morphology of the collected dust particles appear to be similar to that of interplanetary dust particles. | comet 67p/churyumov-gerasimenko: close-up on dust particle fragments |
as the spin axis of comet 67p/churyumov-gerasimenko (67p) is not normal to the orbital plane, 67p has strong seasonal changes in the illumination conditions on the nucleus' surface, with a short and intense summer in the southern hemisphere. we have been monitoring these seasonal variations in the gas coma with the rosetta orbiter spectrometer for ion and neutral analysis (rosina) instrument suite aboard the esa's rosetta spacecraft. rosetta followed 67p from its rendezvous in 2014 august, from a distance of almost 3.5 au to sun, through perihelion at 1.24 au, and away from sun again. in this study, we present the change of outgassing pattern during the 2016 march equinox based on measurements acquired with the rosina instruments: while h2o, o2 and nh3 abundances rapidly decreased during this period, co2, co, h2s, ch4 and hcn abundances decreased much less and showed a strong south-north heterogeneity for the whole period, thus not following sun. sublimation temperatures of the pure ices are found to be uncorrelated with the slope of the decrease for the minor species. this can be interpreted as a consequence of two different ice phases, water ice and co2 ice, in which the minor species are embedded in different relative abundances. | change of outgassing pattern of 67p/churyumov-gerasimenko during the march 2016 equinox as seen by rosina |
we present high spectral resolution optical spectra obtained with the arces instrument at apache point observatory showing detection of the [o i] 6300 å line in interstellar comet 2i/borisov. we employ the observed flux in this line to derive an h2o production rate of (6.3 ± 1.5) × 1026 mol s-1. comparing to previously reported observations of cn, this implies a cn/h2o ratio of ∼0.3%-0.6%. the lower end of this range is consistent with the average value in comets, while the upper end is higher than the average value for solar system comets, but still within the range of observed values. c2/h2o is depleted, with a value likely less than 0.1%. the dust-to-gas ratio is consistent with the normal value for solar system comets. using a simple sublimation model we estimate an h2o active area of 1.7 km2, which for current estimates for the size of borisov suggests active fractions between 1% and 150%, consistent with values measured in solar system comets. more detailed characterization of 2i/borisov, including compositional information and properties of the nucleus, is needed to fully interpret the observed h2o production rate. | detection of a water tracer in interstellar comet 2i/borisov |
aims: the two stable isotopes of nitrogen, 14n and 15n, exhibit a range of abundance ratios both inside and outside the solar system. the elemental ratio in the solar neighborhood is 440. recent alma observations showed hcn/hc15n ratios from 83 to 156 in six t tauri and herbig disks and a cn/c15n ratio of 323 ± 30 in one t tauri star. we aim to determine the dominant mechanism responsible for these enhancements of 15n: low-temperature exchange reactions or isotope-selective photodissociation of n2.methods: using the thermochemical code dali, we model the nitrogen isotope chemistry in circumstellar disks with a 2d axisymmetric geometry. our chemical network is the first to include both fractionation mechanisms for nitrogen. the model produces abundance profiles and isotope ratios for several key n-bearing species. we study how these isotope ratios depend on various disk parameters.results: the formation of cn and hcn is closely coupled to the vibrational excitation of h2 in the uv-irradiated surface layers of the disk. isotope fractionation is completely dominated by isotope-selective photodissociation of n2. the column density ratio of hcn over hc15n in the disk's inner 100 au does not depend strongly on the disk mass, the flaring angle or the stellar spectrum, but it is sensitive to the grain size distribution. for larger grains, self-shielding of n2 becomes more important relative to dust extinction, leading to stronger isotope fractionation. between disk radii of 50 and 200 au, the models predict hcn/hc15n and cn/c15n abundance ratios consistent with observations of disks and comets. the hcn/hc15n and cn/c15n column density ratios in the models are a factor of 2-3 higher than those inferred from the alma observations. | nitrogen isotope fractionation in protoplanetary disks |
context. the origin of the particular shape of comet 67p/churyumov-gerasimenko (67p) is a topic of active research. how and when it acquired its peculiar characteristics has distinct implications on the origin of the solar system and its dynamics.aims: we investigate how shapes such as that of comet 67p can result from a new type of low-energy, sub-catastrophic impact involving elongated, rotating bodies. we focus on parameters potentially leading to bi-lobed structures. we also estimate the probability of such structures surviving subsequent impacts.methods: we used a smooth particle hydrodynamics (sph) shock physics code to model the impacts, the subsequent re-accumulation of material and the reconfiguration into a stable final shape. the energy increase as well as the degree of compaction of the resulting bodies were tracked in the simulations.results: our modelling results suggest that the formation of bi-lobed structures like 67p is a natural outcome of the low-energy, sub-catastrophic collisions considered here.conclusions: sub-catastrophic impacts have the potential to alter the shape of a small body significantly, without leading to major heating or compaction. the currently observed shapes of cometary nuclei, such as 67p, may be a result of such a major shape forming impact. | formation of bi-lobed shapes by sub-catastrophic collisions. a late origin of comet 67p's structure |
the origin of terrestrial volatiles remains one of the most puzzling questions in planetary sciences. the timing and composition of chondritic and cometary deliveries to earth has remained enigmatic due to the paucity of reliable measurements of cometary material. this work uses recently measured volatile elemental ratios and noble gas isotope data from comet 67p/churyumov-gerasimenko (67p/c-g), in combination with chondritic data from the literature, to reconstruct the composition of earth's ancient atmosphere. comets are found to have contributed ~20% of atmospheric heavy noble gases (i.e., kr and xe) but limited amounts of other volatile elements (water, halogens and likely organic materials) to earth. these cometary noble gases were likely mixed with chondritic - and not solar - sources to form the atmosphere. we show that an ancient atmosphere composed of chondritic and cometary volatiles is more enriched in xe relative to the modern atmosphere, requiring that 8-12 times the present-day inventory of xe was lost to space. this potentially resolves the long-standing mystery of earth's "missing xenon", with regards to both xe elemental depletion and isotopic fractionation in the atmosphere. the inferred kr/h2o and xe/h2o of the initial atmosphere suggest that earth's surface volatiles might not have been fully delivered by the late accretion of volatile-rich carbonaceous chondrites. instead, "dry" materials akin to enstatite chondrites potentially constituted a significant source of chondritic volatiles now residing on the earth's surface. we outline the working hypotheses, implications and limitations of this model in the last section of this contribution. | the origin and fate of volatile elements on earth revisited in light of noble gas data obtained from comet 67p/churyumov-gerasimenko |
context. sulfur is a biogenic element used as a tracer of the evolution of interstellar clouds to stellar systems. however, most of the expected sulfur in molecular clouds remains undetected. sulfur disappears from the gas phase in two steps. the first depletion occurs during the translucent phase, reducing the gas-phase sulfur by 7-40 times, while the following freeze-out step occurs in molecular clouds, reducing it by another order of magnitude. this long-standing question awaits an explanation.aims: the aim of this study is to understand under what form the missing sulfur is hiding in molecular clouds. the possibility that sulfur is depleted onto dust grains is considered.methods: experimental simulations mimicking h2s ice uv photoprocessing in molecular clouds were conducted at 8 k under ultra-high vacuum. the ice was subsequently warmed up to room temperature. the ice was monitored using infrared spectroscopy, and the desorbing molecules were measured by quadrupole mass spectrometry in the gas phase. theoretical monte carlo simulations were performed for interpretation of the experimental results and extrapolation to the astrophysical and planetary conditions.results: h2s2 formation was observed during irradiation at 8 k. molecules h2sx with x > 2 were also identified and found to desorb during warm-up, along with s2 to s4 species. larger sx molecules up to s8 are refractory at room temperature and remained on the substrate forming a residue. monte carlo simulations were able to reproduce the molecules desorbing during warming up, and found that residues are chains of sulfur consisting of 6-7 atoms.conclusions: based on the interpretation of the experimental results using our theoretical model, it is proposed that s+ in translucent clouds contributes notoriously to s depletion in denser regions by forming long s chains on dust grains in a few times 104 yr. we suggest that the s2 to s4 molecules observed in comets are not produced by fragmentation of these large chains. instead, they probably come either from uv photoprocessing of h2s-bearing ice produced in molecular clouds or from short s chains formed during the translucent cloud phase. | photoprocessing of h2s on dust grains. building s chains in translucent clouds and comets |
models of the solar system's evolution show that almost all the primitive material leftover from the formation of the planets was ejected to the interstellar space as a result of dynamical instabilities1. accordingly, minor bodies should also be ejected from other planetary systems and should be abundant in the interstellar space2, giving hope for their direct detection and detailed characterization as they penetrate through the solar system3,4. these expectations materialized on 19 october 2017 ut with the panoramic survey telescope and rapid response system's discovery of 1i/`oumuamua5. here, we report homogeneous photometric observations of this body from gemini north, which densely cover a total of 8.06 h over two nights. a combined ultra-deep image of 1i/`oumuamua shows no signs of cometary activity, confirming the results from other, less sensitive searches6-9. our data also show an enormous range of rotational brightness variations of 2.6 ± 0.2 mag, larger than ever observed in the population of small solar system objects, suggesting a very elongated shape of the body. most significantly, the light curve does not repeat exactly from one rotation cycle to another and its double-peaked periodicity of 7.56 ± 0.01 h from our data is inconsistent with earlier determinations6,7,10-12. these are clear signs of a tumbling motion, a remarkable characteristic of 1i/`oumuamua's rotation that is consistent with a collision in the distant past. bearing marks of a violent history, this first-known interstellar visitor tells us that collisional evolution of minor body populations in other planetary systems might be common. | tumbling motion of 1i/`oumuamua and its implications for the body's distant past |
ablation of interplanetary dust supplies oxygen to the upper atmospheres of jupiter, saturn, uranus, and neptune. using recent dynamical model predictions for the dust influx rates to the giant planets (poppe et al., 2016), we calculate the ablation profiles and investigate the subsequent coupled oxygen-hydrocarbon neutral photochemistry in the stratospheres of these planets. we find that dust grains from the edgeworth-kuiper belt, jupiter-family comets, and oort-cloud comets supply an effective oxygen influx rate of 1.0-0.7+2.2 ×107 o atoms cm-2-1 to jupiter, 7.4-5.1+16 ×104 cm-2 s-1 to saturn, 8.9-6.1+19 ×104 cm-2 s-1 to uranus, and 7.5-5.1+16 ×105 cm-2 s-1 to neptune. the fate of the ablated oxygen depends in part on the molecular/atomic form of the initially delivered products, and on the altitude at which it was deposited. the dominant stratospheric products are co, h2o, and co2, which are relatively stable photochemically. model-data comparisons suggest that interplanetary dust grains deliver an important component of the external oxygen to jupiter and uranus but fall far short of the amount needed to explain the co abundance currently seen in the middle stratospheres of saturn and neptune. our results are consistent with the theory that all of the giant planets have experienced large cometary impacts within the last few hundred years. our results also suggest that the low background h2o abundance in jupiter's stratosphere is indicative of effective conversion of meteoric oxygen to co during or immediately after the ablation process - photochemistry alone cannot efficiently convert the h2o into co on the giant planets. | dust ablation on the giant planets: consequences for stratospheric photochemistry |
context. the gas-driven dust activity of comets is still an unsolved question in cometary physics. homogeneous dust layers composed of micrometer-sized grains possess tensile strengths of ~1 kpa, which is far higher than typical gas pressures caused by the sublimation of the ices beneath the covering dust layer. this implies that the dust grains cannot be detached from the surface by the gas pressure of the sublimating ices. one possibility to avoid this problem is that the nucleus formed through the gravitational collapse of an ensemble of millimeter- to centimeter-sized aggregates. in this case, an aggregate layer with a tensile strength on the order of ~1 pa is formed on the surface of the nucleus, which allows for the release of the aggregates from the surface by the gas pressure build up at the ice-dust interface.aims: we use the gravitational instability formation scenario of cometesimals to derive the aggregate size that can be released by the gas pressure from the nucleus of comet 67p/churyumov-gerasimenko for different heliocentric distances and different volatile ices.methods: to derive the ejected aggregate sizes, we developed a model based on the assumption that the entire heat absorbed by the surface is consumed by the sublimation process of one volatile species. the calculations were performed for the three most prominent volatile materials in comets, namely, h2o ice, co2 ice, and co ice.results: we find that the size range of the dust aggregates able to escape from the nucleus into space widens when the comet approaches the sun and narrows with increasing heliocentric distance, because the tensile strength of the aggregates decreases with increasing aggregate size. the activity of co ice in comparison to h2o ice is capable to detach aggregates smaller by approximately one order of magnitude from the surface. as a result of the higher sublimation rate of co ice, larger aggregates are additionally able to escape from the gravity field of the nucleus.conclusions: our model can explain the large grains (ranging from 2 cm to 1 m in radius) in the inner coma of comet 67p/churyumov-gerasimenko that have been observed by the osiris camera at heliocentric distances between 3.4 au and 3.7 au. furthermore, the model predicts the release of decimeter-sized aggregates (trail particles) close to the heliocentric distance at which the gas-driven dust activity vanishes. however, the gas-driven dust activity cannot explain the presence of particles smaller than ~1 mm in the coma because the high tensile strength required to detach these particles from the surface cannot be provided by evaporation of volatile ices. these smaller particles can be produced for instance by spin-up and centrifugal mass loss of ejected larger aggregates. | what drives the dust activity of comet 67p/churyumov-gerasimenko? |
when a weakly outgassing comet is sufficiently close to the sun, the formation of an ionized coma results in solar wind mass loading and magnetic field draping around its nucleus. using a 3d fully kinetic approach, we distill the components of a generalized ohm's law and the effective electron equation of state directly from the self-consistently simulated electron dynamics and identify the driving physics in the various regions of the cometary plasma environment. using the example of space plasmas, in particular multispecies cometary plasmas, we show how the description for the complex kinetic electron dynamics can be simplified through a simple effective closure, and identify where an isotropic single-electron fluid ohm's law approximation can be used, and where it fails. | building a weakly outgassing comet from a generalized ohm's law |
an introduction to celestial mechanics for advanced undergraduates, graduate students, and researchers new to the fieldcelestial mechanics—the study of the movement of planets, satellites, and smaller bodies such as comets—is one of the oldest subjects in the physical sciences. since the mid-twentieth century, the field has experienced a renaissance due to advances in space flight, digital computing, numerical mathematics, nonlinear dynamics, and chaos theory, and the discovery of exoplanets. this modern, authoritative introduction to planetary system dynamics reflects these recent developments and discoveries and is suitable for advanced undergraduate and graduate students as well as researchers. the book treats both traditional subjects, such as the two-body and three-body problems, lunar theory, and hamiltonian perturbation theory, as well as a diverse range of other topics, including chaos in the solar system, comet dynamics, extrasolar planets, planetesimal dynamics, resonances, tidal friction and disruption, and more. the book provides readers with all the core concepts, tools, and methods needed to conduct research in the subject.<li>provides an authoritative introduction that reflects recent advances in the field</li><li>topics treated include andoyer variables, co-orbital satellites and quasi-satellites, hill's problem, the milankovich equations, colombo's top and cassini states, the yarkovsky and yorp effects, orbit determination for extrasolar planets, and more</li><li>more than 100 end-of-book problems elaborate on concepts not fully covered in the main text</li><li>appendixes summarize the necessary background material</li><li>suitable for advanced undergraduates and graduate students; some knowledge of hamiltonian mechanics and methods of mathematical physics (vectors, matrices, special functions, etc.) required</li><li>solutions manual available on request for instructors who adopt the book for a course</li>]]> | dynamics of planetary systems |
formamide (nh$_2$cho) has been identified as a potential precursor of a wide variety of organic compounds essential to life, and many biochemical studies propose it likely played a crucial role in the context of the origin of life on our planet. the detection of formamide in comets, which are believed to have --at least partially-- inherited their current chemical composition during the birth of the solar system, raises the question whether a non-negligible amount of formamide may have been exogenously delivered onto a very young earth about four billion years ago. a crucial part of the effort to answer this question involves searching for formamide in regions where stars and planets are forming today in our galaxy, as this can shed light on its formation, survival, and chemical re-processing along the different evolutionary phases leading to a star and planetary system like our own. the present review primarily addresses the chemistry of formamide in the interstellar medium, from the point of view of (i) astronomical observations, (ii) experiments, and (iii) theoretical calculations. while focusing on just one molecule, this review also more generally reflects the importance of joining efforts across multiple scientific disciplines in order to make progress in the highly interdisciplinary science of astrochemistry. | interstellar formamide (nh2cho), a key prebiotic precursor |
we study the solution $h_\varepsilon$ of the kardar-parisi-zhang (kpz) equation for $d \geq 3$: $$ \frac{\partial}{\partial t} h_{\varepsilon} = \frac12 \delta h_{\varepsilon} + \bigg[\frac12 |\nabla h_\varepsilon |^2 - c_\varepsilon\bigg]+ \beta \varepsilon^{\frac{d-2}2} \xi_{\varepsilon} $$ with $h_\varepsilon(0,x)=0$. here $\xi_\varepsilon=\xi\star \phi_\varepsilon$ is a spatially smoothened (at scale $\varepsilon$) gaussian space-time white noise and $c_\varepsilon$ is a divergent constant as $\varepsilon\to 0$. when the disorder $\beta$ is sufficiently small and $\varepsilon\to 0$, $h_\varepsilon(t,x)- \mathfrak h^{\mathrm{st}}_{\varepsilon}(t,x)\to 0$ in probability where $\mathfrak h^{\mathrm{st}}_{\varepsilon}(t,x)$ is the {\emph stationary solution} of the kpz equation - more precisely, $\mathfrak h^{\mathrm{st}}_{\varepsilon}$solves the above equation with a random initial condition (that is independent of the driving noise $\xi$) and its law is constant in $(\varepsilon,t,x)$. in the present article we quantify the rate of the above convergence in this regime and show that the fluctuation {\emph about} the stationary solution $$ (\varepsilon^{1-\frac d2} [h_\varepsilon(t,x) - \mathfrak h^{\mathrm{st}}_{\varepsilon}(t,x)])_{x,t} $$ converges pointwise (with finite dimensional distributions in space and time) to a gaussian free field (gff) evolved by the deterministic heat equation. we also identify the fluctuations {\it of} the stationary solution itself and show that the rescaled averages $\int_{\mathbb r^d} {\mathrm d} x \varphi(x) \varepsilon^{1-\frac d2} [\mathfrak h^{\mathrm{st}}_{\varepsilon}(t,x)- \mathbb e(\mathfrak h^{\mathrm{st}}_{\varepsilon}(t,x))]$ converge to that of the {\emph stationary solution} of the stochastic heat equation with additive noise, but with (random) {\emph gff marginals} (instead of flat initial condition). | space-time fluctuation of the kardar-parisi-zhang equation in $d\\geq 3$ and the gaussian free field |
from august to november 2014 the rosetta orbiter has performed an extensive observation campaign aimed at the characterization of 67p/cg nucleus properties and to the selection of the philae landing site. the campaign led to the production of a global map of the illuminated portion of 67p/cg nucleus. during this prelanding phase the comet's heliocentric distance decreased from 3.62 to 2.93 au while rosetta was orbiting around the nucleus at distances between 100 to 10 km. virtis-m, the visible and infrared thermal imaging spectrometer - mapping channel (coradini et al., [2007] space sci. rev., 128, 529-559) onboard the orbiter, has acquired 0.25-5.1 μm hyperspectral data of the entire illuminated surface, e.g. the north hemisphere and the equatorial regions, with spatial resolution between 2.5 and 25 m/pixel. i/f spectra have been corrected for thermal emission removal in the 3.5-5.1 μm range and for surface's photometric response. the resulting reflectance spectra have been used to compute several cometary spectral indicators (csi): single scattering albedo at 0.55 μm, 0.5-0.8 μm and 1.0-2.5 μm spectral slopes, 3.2 μm organic material and 2.0 μm water ice band parameters (center, depth) with the aim to map their spatial distribution on the surface and to study their temporal variability as the nucleus moved towards the sun. indeed, throughout the investigated period, the nucleus surface shows a significant increase of the single scattering albedo along with a decrease of the 0.5-0.8 and 1.0-2.5 μm spectral slopes, indicating a flattening of the reflectance. we attribute the origin of this effect to the partial removal of the dust layer caused by the increased contribution of water sublimation to the gaseous activity as comet crossed the frost-line. the regions more active at the time of these observations, like hapi in the neck/north pole area, appear brighter, bluer and richer in organic material than the rest of the large and small lobe of the nucleus. the parallel coordinates method (inselberg [1985] vis. comput., 1, 69-91) has been used to identify associations between average values of the spectral indicators and the properties of the geomorphological units as defined by (thomas et al., [2015] science, 347, 6220) and (el-maarr et al., [2015] astron. astrophys., 583, a26). three classes have been identified (smooth/active areas, dust covered areas and depressions), which can be clustered on the basis of the 3.2 μm organic material's band depth, while consolidated terrains show a high variability of the spectral properties resulting being distributed across all three classes. these results show how the spectral variability of the nucleus surface is more variegated than the morphological classes and that 67p/cg surface properties are dynamical, changing with the heliocentric distance and with activity processes. | the global surface composition of 67p/cg nucleus by rosetta/virtis. (i) prelanding mission phase |
deuterated methanol is one of the most robust windows astrochemists have on the individual chemical reactions forming deuterium-bearing molecules and the physicochemical history of the regions where they reside. the first-time detection of mono- and di-deuterated methanol in a cometary coma is presented for comet 67p/churyumov-gerasimenko using rosetta-rosina data. d-methanol (ch3od and ch2doh combined) and d2-methanol (ch2dod and chd2oh combined) have an abundance of 5.5 ± 0.46 and 0.00069 ± 0.00014 per cent relative to normal methanol. the data span a methanol deuteration fraction (d/h ratio) in the 0.71-6.6 per cent range, accounting for statistical corrections for the location of d in the molecule and including statistical error propagation in the rosina measurements. it is argued that cometary ch2doh forms from co hydrogenation to ch3oh and subsequent h-d substitution reactions in ch3-r. chd2oh is likely produced from deuterated formaldehyde. meanwhile, ch3od and ch2dod could form via h-d exchange reactions in oh-r in the presence of deuterated water ice. methanol formation and deuteration is argued to occur at the same epoch as d2o formation from hdo, with formation of mono-deuterated water, hydrogen sulphide, and ammonia occurring prior to that. the cometary d-methanol/methanol ratio is demonstrated to agree most closely with that in prestellar cores and low-mass protostellar regions. the results suggest that cometary methanol stems from the innate cold (10-20 k) prestellar core that birthed our solar system. cometary volatiles individually reflect the evolutionary phases of star formation from cloud to core to protostar. | prestellar grain-surface origins of deuterated methanol in comet 67p/churyumov-gerasimenko |
we present 2-5 μm spectroscopic observations of the dust coma of 67p/churyumov-gerasimenko obtained with the virtis-h instrument onboard rosetta during two outbursts that occurred on 2015, 13 september 13.6 h ut and 14 september 18.8 h ut at 1.3 au from the sun. scattering and thermal properties measured before the outburst are in the mean of values measured for moderately active comets. the colour temperature excess (or superheat factor) can be attributed to submicrometre-sized particles composed of absorbing material or to porous fractal-like aggregates such as those collected by the rosetta in situ dust instruments. the power-law index of the dust size distribution is in the range 2-3. the sudden increase of infrared emission associated with the outbursts is correlated with a large increase of the colour temperature (from 300 to 630 k) and a change of the dust colour at 2-2.5 μm from red to blue colours, revealing the presence of very small grains (≤100 nm) in the outburst material. in addition, the measured large bolometric albedos (∼0.7) indicate bright grains in the ejecta, which could either be silicatic grains, implying the thermal degradation of the carbonaceous material, or icy grains. the 3 μm absorption band from water ice is not detected in the spectra acquired during the outbursts, whereas signatures of organic compounds near 3.4 μm are observed in emission. the h2o 2.7 μm and co2 4.3 μm vibrational bands do not show any enhancement during the outbursts. | comet 67p outbursts and quiescent coma at 1.3 au from the sun: dust properties from rosetta/virtis-h observations |
the european space agency rosetta/philae mission to comet 67p/churyumov-gerasimenko in 2014-2016 is the most complete and diverse investigation of a comet carried out thus far. yet, many physical and chemical properties of the comet remain uncertain or unknown, and cometary activity is still not a well-understood phenomenon. we here attempt to place constraints on the nucleus abundances and sublimation front depths of h2o and co2 ice, and to reconstruct how the nucleus evolved throughout the perihelion passage. we employ the thermophysical modelling code 'numerical icy minor body evolution simulator', or nimbus, to search for conditions under which the observed h2o and co2 production rates are simultaneously reproduced before and after perihelion. we find that the refractories to water-ice mass ratio of relatively pristine nucleus material is μ ≈ 1, that airfall material has μ ≈ 2, and that the molar abundance of co2 relative h2o is near 30 per cent. the dust mantle thickness is typically $\lesssim 2\, \mathrm{cm}$. the average co2 sublimation front depths near aphelion were $\sim 3.8\, \mathrm{m}$ and $\sim 1.9\, \mathrm{m}$ on the northern and southern hemispheres, respectively, but varied substantially with time. we propose that airfall material is subjected to substantial fragmentation and pulverization due to thermal fatigue during the aphelion passage. sub-surface compaction of material due to co2 activity near perihelion seems to have reduced the diffusivity in a measurable way. | modelling the water and carbon dioxide production rates of comet 67p/churyumov-gerasimenko |
this study aimed at comparative examining of the interactions between conventionally used clay and carbon nanomaterials and human lung adenocarcinoma cells (a549 cells). the following platy and tubular nanomaterials were tested: carbon nanoparticles, i.e. multi-walled carbon nanotubes (mwcnts) and graphene oxide nanosheets (go) as well as nanoclays, i.e. halloysite nanotubes (hnts) and kaolinite nanosheets (kaol). nanoparticle physicochemical properties and their internalization into cells were examined using dynamic light scattering as well as atomic force, 3d laser scanning confocal and darkfield hyperspectral microscopies. biological aspects of the nanomaterial-cell interaction included assessment of cellular toxicity, dna damage, metabolic activity, and physical parameters of the cells. regardless of a shape, carbon nanomaterials demonstrated cell surface adsorption, but negligible penetration into cells compared to nanoclays. however, carbon nanomaterials were found to be the most toxic for cells as probed by the mts assay. they also turned out to be the most genotoxic for cells compared to nanoclays as revealed by the dna-comet assay. go significantly increased the fraction of apoptotic cells and was the most cytotoxic and genotoxic nanomaterial. comparison of flow cytometry and mts data indicated that a cytotoxic effect of mwcnts was not associated with increased cell death, but was rather due to a decrease in cell metabolic activity and/or proliferation. finally, no significant effect of the shape of the tested nanomaterials on their internalization and cytotoxicity was revealed. | comparative cytotoxicity of kaolinite, halloysite, multiwalled carbon nanotubes and graphene oxide |
this present investigation has been instigated to examine the impact of polarization force on modulational instability of dust acoustic (da) waves and transition of rogue wave triplets to super rogue waves in dusty plasma composed of negative dust as fluid, boltzmannian electrons, and superthermal ions. the presence of superthermal ions has remarkably altered the impact of polarization force. an increment in ion superthermality index restricts the polarization parameter toward smaller values. by adopting the reductive perturbation technique, the nonlinear schrödinger equation (nlse) is procured that determines the modulational instability of the dust acoustic waves. it is observed that the effect of polarization force is constricted by the wavenumber domain in the advent of the instability region. the rational solution of nlse describes the evolution of dust acoustic rogue wave triplets, which further transform into the super rogue waves by means of superposition of triplets. it is remarked that the amalgamation of polarization force and superthermal ions have an explicit impact on the characteristics of different kinds of dust acoustic rogue waves. it is intensified that our present theoretical pronouncements may shed light on the salient features of different kinds of da rogue waves in laboratory experiments and space/astrophysical regions, especially in saturn's magnetosphere, planetary rings, and comet tails, etc. | the evolution of rogue wave triplets and super rogue waves in superthermal polarized space dusty plasma |
context. outgassing from cometary nuclei involves complex surface and subsurface processes that need to be understood to investigate the composition of cometary ices from coma observations.aims: we investigate the production of water, carbon dioxide, and carbon monoxide from the nucleus of comet 67p/churyumov-gerasimenko (67p). these species have different volatility and are key species of cometary ices.methods: using the high spectral-resolution channel of the visible infrared thermal imaging spectrometer (virtis-h), we observed the ν3 vibrational bands of h2o and co2 at 2.67 and 4.27 μm, respectively, from 24 november 2014 to 24 january 2015, when comet 67p was between 2.91 and 2.47 au from the sun. observations were undertaken in limb-viewing geometry at distances from the surface of 0 to 1.5 km and with various line-of-sight (los) orientations in the body-fixed frame. a geometry tool was used to characterize the position of the los with respect to geomorphologic regions and the illumination properties of these regions.results: the water production of 67p did not increase much from 2.9 to 2.5 au. high water column densities are observed for los above the neck regions, suggesting they are the most productive in water vapor. while water production is weak in regions with low solar illumination, co2 is outgassing from both illuminated and non-illuminated regions, which indicates that co2 sublimates at a depth that is below the diurnal skin depth. the co2/h2o column density ratio varies from 2 to 60%. for regions that are in sunlight, mean values between 2 and 7% are measured. the lower bound value is likely representative of the co2/h2o production rate ratio from the neck regions. for carbon monoxide, we derive column density ratios co/h2o < 1.9% and co/co2< 80%. an illumination-driven model, with a uniformly active surface releasing water at a mean rate of 8 × 1025 s-1, provides an overall agreement to virtis-h data, although some mismatches show local surface inhomogeneities in water production. rotational temperatures of 90-100 k are derived from h2o and co2 averaged spectra. | first observations of h2o and co2 vapor in comet 67p/churyumov-gerasimenko made by virtis onboard rosetta |
isotopic ratios are keys to understanding the origin and early evolution of the solar system in the context of galactic nucleosynthesis. the large range of measured 14n/15n isotopic ratios in the solar system reflects distinct reservoirs of nitrogen whose origins remain to be determined. we have directly measured a c14n/c15n abundance ratio of 323 ± 30 in the disk orbiting the nearby young star tw hya. this value, which is in good agreement with nitrogen isotopic ratios measured for prestellar cores, likely reflects the primary present-day reservoir of nitrogen in the solar neighbourhood. these results support models invoking novae as primary 15n sources as well as outward migration of the sun over its lifetime, and suggest that comets sampled a secondary, 15n-rich reservoir during solar system formation. | direct evidence of multiple reservoirs of volatile nitrogen in a protosolar nebula analogue |
we present an x-ray and radio study of the famous `el gordo', a massive and distant (z = 0.87) galaxy cluster. in the deep (340 ks) chandra observation, the cluster appears with an elongated and cometary morphology, a sign of its current merging state. the gmrt radio observations at 610 mhz confirm the presence of a radio halo, which remarkably overlaps the x-ray cluster emission and connects a couple of radio relics. we detect a strong shock (m≳3) in the nw periphery of the cluster, co-spatially located with the radio relic. this is the most distant (z = 0.87) and one of the strongest shocks detected in a galaxy cluster. this work supports the relic-shock connection and allows us to investigate the origin of these radio sources in an uncommon regime of m≳3. for this particular case we found that shock acceleration from the thermal pool is still a viable possibility. | a m ≳ 3 shock in `el gordo' cluster and the origin of the radio relic |
the rosetta spacecraft spent ~2 years orbiting comet 67p/churyumov-gerasimenko, most of it at distances that allowed surface characterization and monitoring at submeter scales. from december 2014 to june 2016, numerous localized changes were observed, which we attribute to cometary-specific weathering, erosion, and transient events driven by exposure to sunlight and other processes. while the localized changes suggest compositional or physical heterogeneity, their scale has not resulted in substantial alterations to the comet’s landscape. this suggests that most of the major landforms were created early in the comet’s current orbital configuration. they may even date from earlier if the comet had a larger volatile inventory, particularly of co or co2 ices, or contained amorphous ice, which could have triggered activity at greater distances from the sun. | surface changes on comet 67p/churyumov-gerasimenko suggest a more active past |
comets contain abundant amounts of organic and inorganic species. many of the volatile molecules in comets have also been observed in the interstellar medium and some of them even with similar relative abundances, indicating formation under similar conditions or even sharing a common chemical pathway. there is a growing amount of evidence that suggests comets inherit and preserve substantial fractions of materials inherited from previous evolutionary phases, potentially indicating that commonplace processes occurred throughout comet-forming regions. through impacts, part of this material has also been transported to the inner planetary system, including the terrestrial planets. while comets have been ruled out as a major contributor to terrestrial ocean water, substantial delivery of volatile species to the earth's atmosphere, and as a consequence also organic molecules to its biomass, appears more likely. comets contain many species of pre-biotic relevance and molecules that are related to biological processes on earth, and have hence been proposed as potential indicators for the presence of biological processes in the search of extraterrestrial life. while the delivery of cometary material to earth may have played a crucial role in the emergence of life, the presence of such alleged biosignature molecules in the abiotical environment of comets complicates the detection of life elsewhere in the universe. | volatile species in comet 67p/churyumov-gerasimenko: investigating the link from the ism to the terrestrial planets |
context. the rosetta mission and its exquisite measurements have revived the debate on whether comets are pristine planetesimals or collisionally evolved objects.aims: we investigate the collisional evolution experienced by the precursors of current comet nuclei during the early stages of the solar system in the context of the so-called nice model.methods: we considered two environments for the collisional evolution: (1) the transplanetary planetesimal disk, from the time of gas removal until the disk was dispersed by the migration of the ice giants; and (2) the dispersing disk during the time that the scattered disk was formed. we performed simulations using different methods in the two cases to determine the number of destructive collisions typically experienced by a comet nucleus of 2 km radius.results: in the widely accepted scenario, where the dispersal of the planetesimal disk occurred at the time of the late heavy bombardment about 4 gy ago, comet-sized planetesimals have a very low probability of surviving destructive collisions in the disk. on the extreme assumption that the disk was dispersed directly upon gas removal, a significant fraction of the planetesimals might have remained intact. however, these survivors would still bear the marks of many nondestructive impacts.conclusions: the nice model of solar system evolution predicts that typical km-sized comet nuclei are predominantly fragments resulting from collisions experienced by larger parent bodies. an important goal for future research is to investigate whether the observed properties of comet nuclei are compatible with such a collisional origin. | comets as collisional fragments of a primordial planetesimal disk |
molecular oxygen has been confirmed as the fourth most abundant molecule in cometary material (o2/h2o ∼ 4 per cent) and is thought to have a primordial nature, i.e. coming from the interstellar cloud from which our solar system was formed. however, interstellar o2 gas is notoriously difficult to detect and has only been observed in one potential precursor of a solar-like system. here, the chemical and physical origin of o2 in comets is investigated using sophisticated astrochemical models. three origins are considered: (i) in dark clouds; (ii) during forming protostellar discs; and (iii) during luminosity outbursts in discs. the dark cloud models show that reproduction of the observed abundance of o2 and related species in comet 67p/c-g requires a low h/o ratio facilitated by a high total density (≥105 cm-3), and a moderate cosmic ray ionization rate (≤10-16 s-1) while a temperature of 20 k, slightly higher than the typical temperatures found in dark clouds, also enhances the production of o2. disc models show that o2 can only be formed in the gas phase in intermediate disc layers, and cannot explain the strong correlation between o2 and h2o in comet 67p/c-g together with the weak correlation between other volatiles and h2o. however, primordial o2 ice can survive transport into the comet-forming regions of discs. taken together, these models favour a dark cloud (or 'primordial') origin for o2 in comets, albeit for dark clouds which are warmer and denser than those usually considered as solar system progenitors. | a primordial origin for molecular oxygen in comets: a chemical kinetics study of the formation and survival of o2 ice from clouds to discs |
context. the scattering properties of the dust originating from debris discs are still poorly known. the analysis of scattered light is however a powerful remote-sensing tool to understand the physical properties of dust particles orbiting other stars. scattered light is indeed widely used to characterise the properties of cometary dust in the solar system.aims: we aim to measure the morphology and scattering properties of the dust from the debris ring around hr 4796 a in polarised optical light.methods: we obtained high-contrast polarimetric images of hr 4796 a in the wavelength range 600-900 nm with the sphere/zimpol instrument on the very large telescope.results: we measured for the first time the polarised phase function of the dust in a debris system over a wide range of scattering angles in the optical. we confirm that it is incompatible with dust particles being compact spheres under the assumption of the mie theory, and propose alternative scenarios compatible with the observations, such as particles with irregular surface roughness or aggregate particles. the reduced images (fits files) presented in fig. 1 are only available at the cds via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?j/a+a/626/a54 | optical polarised phase function of the hr 4796a dust ring |
although there is evidence for periodic geological perturbations driven by regular or semi-regular extra-terrestrial bombardment, the production of earth's continental crust is generally regarded as a function of planetary differentiation driven by internal processes. we report time series analysis of the hf isotopic composition of zircon grains from the north atlantic and pilbara cratons, the archetypes of archean plate tectonic and non-plate tectonic settings, respectively. an ~170-200 m.y. frequency is recognized in both cratons that matches the transit of the solar system through the galactic spiral arms, where the density of stars is high. an increase in stellar density is consistent with an enhanced rate of earth bombardment by comets, the larger of which would have initiated crustal nuclei production via impact-driven decompression melting of the mantle. hence, the production and preservation of continental crust on the early earth may have been fundamentally influenced by exogenous processes. a test of this model using oxygen isotopes in zircon from the pilbara craton reveals correlations between crust with anomalously light isotopic signatures and exit from the perseus spiral arm and entry into the norma spiral arm, the latter of which matches the known age of terrestrial spherule beds. our data support bolide impact, which promoted the growth of crustal nuclei, on solar system transit into and out of the galactic spiral arms. | did transit through the galactic spiral arms seed crust production on the early earth? |
context. the rosetta spacecraft is escorting comet 67p/churyumov-gerasimenko from a heliocentric distance of >3.6 au, where the comet activity was low, until perihelion at 1.24 au. initially, the solar wind permeates the thin comet atmosphere formed from sublimation.aims: using the rosetta plasma consortium ion composition analyzer (rpc-ica), we study the gradual evolution of the comet ion environment, from the first detectable traces of water ions to the stage where cometary water ions accelerated to about 1 kev energy are abundant. we compare ion fluxes of solar wind and cometary origin.methods: rpc-ica is an ion mass spectrometer measuring ions of solar wind and cometary origins in the 10 ev-40 kev energy range.results: we show how the flux of accelerated water ions with energies above 120 ev increases between 3.6 and 2.0 au. the 24 h average increases by 4 orders of magnitude, mainly because high-flux periods become more common. the water ion energy spectra also become broader with time. this may indicate a larger and more uniform source region. at 2.0 au the accelerated water ion flux is frequently of the same order as the solar wind proton flux. water ions of 120 ev-few kev energy may thus constitute a significant part of the ions sputtering the nucleus surface. the ion density and mass in the comet vicinity is dominated by ions of cometary origin. the solar wind is deflected and the energy spectra broadened compared to an undisturbed solar wind.conclusions: the flux of accelerated water ions moving from the upstream direction back toward the nucleus is a strongly nonlinear function of the heliocentric distance. | evolution of the ion environment of comet 67p/churyumov-gerasimenko. observations between 3.6 and 2.0 au |
bilobate comets—small icy bodies with two distinct lobes—are a common configuration among comets, but the factors shaping these bodies are largely unknown. cometary nuclei, the solid centres of comets, erode by ice sublimation when they are sufficiently close to the sun, but the importance of a comet's internal structure on its erosion is unclear. here we present three-dimensional analyses of images from the rosetta mission to illuminate the process that shaped the jupiter-family bilobate comet 67p/churyumov-gerasimenko over billions of years. we show that the comet's surface and interior exhibit shear-fracture and fault networks, on spatial scales of tens to hundreds of metres. fractures propagate up to 500 m below the surface through a mechanically homogeneous material. through fracture network analysis and stress modelling, we show that shear deformation generates fracture networks that control mechanical surface erosion, particularly in the strongly marked neck trough of 67p/churyumov-gerasimenko, exposing its interior. we conclude that shear deformation shapes and structures the surface and interior of bilobate comets, particularly in the outer solar system where water ice sublimation is negligible. | bilobate comet morphology and internal structure controlled by shear deformation |
the fractal dust particles observed by rosetta cannot form in the physical conditions observed today in comet 67p/churyumov-gerasimenko (67p hereinafter), being instead consistent with models of the pristine dust aggregates coagulated in the solar nebula. since bouncing collisions in the protoplanetary disc restructure fractals into compact aggregates (pebbles), the only way to preserve fractals in a comet is the gentle gravitational collapse of a mixture of pebbles and fractals, which must occur before their mutual collision speeds overcome ≈1 m s-1. this condition fixes the pebble radius to ≲1 cm, as confirmed by comet nucleus infrared and visible analyser onboard philae. here, we show that the flux of fractal particles measured by rosetta constrains the 67p nucleus in a random packing of cm-sized pebbles, with all the voids among them filled by fractal particles. this structure is inconsistent with any catastrophic collision, which would have compacted or dispersed most fractals, thus leaving empty most voids in the reassembled nucleus. comets are less numerous than current estimates, as confirmed by lacking small craters on pluto and charon. bilobate comets accreted at speeds <1 m s-1 from cometesimals born in the same disc stream. | fractal dust constrains the collisional history of comets |
mid-infrared spectra of amorphous and crystalline acetone are presented along with measurements of the refractive index and density for both forms of the compound. infrared band strengths are reported for the first time for amorphous and crystalline acetone, along with ir optical constants. vapor pressures and a sublimation enthalpy for crystalline acetone also are reported. positions of 13c-labeled acetone are measured. band strengths are compared to gas-phase values and to the results of a density-functional calculation. a 73% error in previous work is identified and corrected. | ir spectra and properties of solid acetone, an interstellar and cometary molecule |
the radiation data collected by the standard radiation environment monitor (srem) aboard esa missions integral (international gamma-ray astrophysics laboratory), rosetta, herschel, planck and proba-1, and by the high-energy neutron detector (hend) instrument aboard mars odyssey, are analysed with an emphasis on characterising galactic cosmic rays (gcrs) in the inner heliosphere. a cross calibration between all sensors was performed for this study, which can also be used in subsequent works. we investigate the stability of the srem detectors over long-term periods. the radiation data are compared qualitatively and quantitatively with the corresponding solar activity. based on integral and rosetta srem data, a gcr helioradial gradient of 2.96 % au-1 is found between 1 and 4.5 au. in addition, the data during the last phase of the rosetta mission around comet 67p/churyumov-gerasimenko were studied in more detail. an unexpected yet unexplained 8 % reduction of the galactic comic ray flux measured by rosetta srem in the vicinity of the comet is noted. | multi-point galactic cosmic ray measurements between 1 and 4.5 au over a full solar cycle |
soon after the arrival of the rosetta spacecraft at comet 67/p churyumov-gerasimenko the onboard instrument cosima ("cometary secondary ion mass analyzer") collected a large number of cometary dust particles on targets from gold black of thickness between 10 and 30 μm. inspection by its camera subsystem revealed that many of them consist of smaller units of typically some tens of micrometers in size. the collection process left the smaller dust particles in an essentially unaltered state whereas most particles larger than about 100 μm got fragmented into smaller pieces. using the observed fragment size distributions, the present paper includes a first assessment of the strength for those dust particles that were disrupted upon impact. | a first assessment of the strength of cometary particles collected in-situ by the cosima instrument onboard rosetta |
the surfaces of many objects in the solar system comprise substantial quantities of water ice sometimes mixed with minerals and/or organic molecules. the sublimation of the ice changes the structural and optical properties of these objects. we present laboratory data on the evolution of the structure and the visible and near-infrared spectral reflectance of icy surface analogues of cometary ices, made of water ice, complex organic matter (tholins) and silicates, as they undergo sublimation under low temperature (<-70 °c) and pressure (10-5 mbar) conditions inside the sciteas simulation chamber. as the water ice sublimated, we observed in situ the formation of a porous sublimation lag deposit, or sublimation mantle, at the top of the ice. this mantle is a network of filaments made of the non-volatile particles. organics or phyllosilicates grains, able to interact via stronger inter-particulate forces than olivine grains, can form a foam-like structure having internal cohesiveness, holding olivine grains together. as this mantle builds-up, the band depths of the sub-surface water ice are attenuated until complete extinction under only few millimeters of mantle. optically thick sublimation mantles are mainly featureless in the near infrared. the absorption bands of the minerals present in the mantle are weak, or even totally absent if minerals are mixed with organics which largely dominate the vis-nir reflectance spectrum. during sublimation, ejections of large fragments of mantle, triggered by the gas flow, expose ice particles to the surface. the contrast of brightness between mantled and ice-exposed areas depends on the wavelength range and the dust/ice ratio considered. we describe how the chemical nature of the non-volatiles, the size of their particles, the way they are mixed with the ice and the dust/ice mass ratio influence the texture, activity and spectro-photometric properties of the sublimation mantles. these data provide useful references for interpreting remote-sensing observations of comets and also icy satellites or trans-neptunian objects. | sublimation of water ice mixed with silicates and tholins: evolution of surface texture and reflectance spectra, with implications for comets |
the geophysical evolution of many icy moons and dwarf planets seems to have provided opportunities for interaction between liquid water and rock (silicate and organic solids). here, we explore two ways by which water-rock interaction can feed back on geophysical evolution: the production or consumption of antifreeze compounds, which affect the persistence and abundance of cold liquid; and the potential leaching into the fluid of lithophile radionuclides, affecting the distribution of a long-term heat source. we compile, validate, and use a numerical model, implemented with the phreeqc code, of the interaction of chondritic rock with pure water and with c, n, s-bearing cometary fluid, thought to be the materials initially accreted by icy worlds, and describe the resulting equilibrium fluid and rock assemblages at temperatures, pressures, and water-to-rock ratios of 0-200 ° c, 1-1000 bar, and 0.1-10 by mass, respectively. our findings suggest that water-rock interaction can strongly alter the nature and amount of antifreezes, resulting in solutions rich in reduced nitrogen and carbon, and sometimes dissolved h2, with additional sodium, calcium, chlorine, and/or oxidized carbon. such fluids can remain partially liquid down to 176 k if nh3 is present. the prominence of cl in solution seems to hinge on its primordial supply in ices, which is unconstrained by the meteoritical record. equilibrium assemblages, rich in serpentine and saponite clays, retain thorium and uranium radionuclides unless u-cl or u-hco3 complexing, which was not modeled, significantly enhances u solubility. however, the radionuclide 40 k can be leached at high water:rock ratio and/or low temperature at which k is exchanged with ammonium in minerals. we recommend the inclusion of these effects in future models of the geophysical evolution of ocean-bearing icy worlds. our simulation products match observations of chloride salts on europa and enceladus; ci chondrites mineralogies; the observation of serpentines, nh4-phyllosilicates, and carbonates on ceres' surface; and of na and nh4-carbonate and chloride in ceres' bright spots. they also match results from previous modeling studies with similar assumptions, and systematically expand these results to heretofore unexplored physico-chemical conditions. this work involved the compilation and careful validation of a comprehensive phreeqc database, which combines the advantages of the default databases phreeqc.dat (carefully vetted data, molar volumes) and llnl.dat (large diversity of species), and should be of broad use to anyone seeking to model aqueous geochemistry at pressures that differ from 1 bar with phreeqc. | aqueous geochemistry in icy world interiors: equilibrium fluid, rock, and gas compositions, and fate of antifreezes and radionuclides |
the european rosetta mission has been following comet 67p/churyumov-gerasimenko for 2 years, studying the nucleus and coma in great detail. for most of these 2 years the rosetta orbiter sensor for ion and neutral analysis (rosina) has analysed the volatile part of the coma. with its high mass resolution and sensitivity it was able to not only detect deuterated water hdo, but also doubly deuterated water, d2o and deuterated hydrogen sulfide hds. the ratios for [hdo]/[h2o], [d2o]/[hdo] and [hds]/[h2s] derived from our measurements are (1.05 ± 0.14) × 10-3, (1.80 ± 0.9) × 10-2 and (1.2 ± 0.3) × 10-3, respectively. these results yield a very high ratio of 17 for [d2o]/[hdo] relative to [hdo]/[h2o]. statistically one would expect just 1/4. such a high value can be explained by cometary water coming unprocessed from the presolar cloud, where water is formed on grains, leading to high deuterium fractionation. the high [hds]/[h2s] ratio is compatible with upper limits determined in low-mass star-forming regions and also points to a direct correlation of cometary h2s with presolar grain surface chemistry. this article is part of the themed issue 'cometary science after rosetta'. | d2o and hds in the coma of 67p/churyumov-gerasimenko |
the gj 436 planetary system is an extraordinary system. the neptune-sized planet that orbits the m3 dwarf revealed in the ly α line an extended neutral hydrogen atmosphere. this material fills a comet-like tail that obscures the stellar disc for more than 10 h after the planetary transit. here, we carry out a series of 3d radiation hydrodynamic simulations to model the interaction of the stellar wind with the escaping planetary atmosphere. with these models, we seek to reproduce the ${\sim}56{{\ \rm per\ cent}}$ absorption found in ly α transits, simultaneously with the lack of absorption in h α transit. varying the stellar wind strength and the euv stellar luminosity, we search for a set of parameters that best fit the observational data. based on ly α observations, we found a stellar wind velocity at the position of the planet to be around [250-460] km s-1 with a temperature of [3-4] × 105 k. the stellar and planetary mass-loss rates are found to be 2 × 10-15 m⊙ yr-1 and ∼[6-10] × 109 g s-1, respectively, for a stellar euv luminosity of [0.8-1.6] × 1027 erg s-1. for the parameters explored in our simulations, none of our models present any significant absorption in the h α line in agreement with the observations. | gj 436b and the stellar wind interaction: simulations constraints using ly α and h α transits |
the structure of the kreutz system of sungrazing comets is shown to be much more complex than formerly believed. marsden's (1989) division into three subgroups (i, ii, iia) is now greatly expanded, as new evidence is being offered on nine populations of fragments -- i, ia, ii, iia, iii, pre-i, pe (side branch of i), iiia, and iv, incorporating carefully screened data sets from a collection of gravitational orbits for 1500 soho/stereo dwarf kreutz comets. tight correlations between the nominal perihelion latitude and nominal longitude of the ascending node are the result of ignored effects of an outgassing-driven acceleration on the orbital motion. the average width of a gap between adjacent populations in the corrected nodal longitude is near 9 deg; the overall range equals 66 deg. a self-consistent model postulates (i) an initial breakup, in general proximity of aphelion, of a contact-binary parent (progenitor) into its two lobes and the neck (originally linking the lobes), giving birth to, respectively, population i (lobe i; the main residual mass c/1843 d1), population ii (lobe ii; c/1882 r1), and population ia (the neck); followed by (ii) progressive fragmentation of the lobes (primarily lobe ii), mostly (but not exclusively) far from perihelion, giving successively rise to the other populations and clusters of naked-eye kreutz sungrazers and their debris. the separation velocities were a few meters per second. massive fragments of populations pre-i, iiia, and iv are yet to be discovered. relations among the products of cascading fragmentation are depicted in a pedigree chart. the age of the kreutz system is estimated at two millennia and a mean orbital period of lobe i and its main residual mass at ~740 yr. the status is reviewed of the possible historical kreutz comets seen in ad 1106, ad 363, and 372 bc. | new model for the kreutz sungrazer system: contact-binary parent and upgraded classification of discrete fragment populations |
given its abundant coal mining activities, the upper silesian coal basin (uscb) in southern poland is one of the largest sources of anthropogenic methane (ch4) emissions in europe. here, we report on ch4 emission estimates for coal mine ventilation facilities in the uscb. our estimates are driven by pairwise upwind-downwind observations of the column-average dry-air mole fractions of ch4 (xch4) by a network of four portable, ground-based, sun-viewing fourier transform spectrometers of the type em27/sun operated during the comet campaign in may-june 2018. the em27/sun instruments were deployed in the four cardinal directions around the uscb approximately 50 km from the center of the basin. we report on six case studies for which we inferred emissions by evaluating the mismatch between the observed downwind enhancements and simulations based on trajectory calculations releasing particles out of the ventilation shafts using the lagrangian particle dispersion model flexpart. the latter was driven by wind fields calculated by wrf (weather research and forecasting model) under assimilation of vertical wind profile measurements of three co-deployed wind lidars. for emission estimation, we use a phillips-tikhonov regularization scheme with the l-curve criterion. diagnosed by the emissions averaging kernels, we find that, depending on the catchment area of the downwind measurements, our ad hoc network can resolve individual facilities or groups of ventilation facilities but that inspecting the emissions averaging kernels is essential to detect correlated estimates. generally, our instantaneous emission estimates range between 80 and 133 kt ch4 a-1 for the southeastern part of the uscb and between 414 and 790 kt ch4 a-1 for various larger parts of the basin, suggesting higher emissions than expected from the annual emissions reported by the e-prtr (european pollutant release and transfer register). uncertainties range between 23 % and 36 %, dominated by the error contribution from uncertain wind fields. | observational constraints on methane emissions from polish coal mines using a ground-based remote sensing network |
aims: we search for signs of falling evaporating bodies (febs, also known as exocomets) in photometric time series obtained for β pictoris after fitting and removing its δ scuti-type pulsation frequencies.methods: using photometric data obtained by the tess satellite we determined the pulsational properties of the exoplanet host star β pictoris through frequency analysis. we then pre-whitened the 54 identified δ scuti p-modes and investigated the residual photometric time series for the presence of febs.results: we identify three distinct dipping events in the light curve of β pictoris over a 105-day period. these dips have depths from 0.5 to 2 millimagnitudes and durations of up to 2 days for the largest dip. these dips are asymmetric in nature and are consistent with a model of an evaporating comet with an extended tail crossing the disc of the starconclusions: we present the first broadband detections of exocomets crossing the disc of β pictoris, complementing the predictions made 20 years earlier by lecavelier des etangs et al. (1999, a&a, 343, 916). no periodic transits are seen in this time series. these observations confirm the spectroscopic detection of exocomets in calcium h and k lines that have been seen in high resolution spectroscopy. the reduced lightcurves for the events are only available at the cds via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?j/a+a/625/l13 | transiting exocomets detected in broadband light by tess in the β pictoris system |
we use measurements from the rosetta plasma consortium langmuir probe and mutual impedance probe to study the spatial distribution of low-energy plasma in the near-nucleus coma of comet 67p/churyumov-gerasimenko. the spatial distribution is highly structured with the highest density in the summer hemisphere and above the region connecting the two main lobes of the comet, i.e., the neck region. there is a clear correlation with the neutral density and the plasma to neutral density ratio is found to be ∼1-2·10-6, at a cometocentric distance of 10 km and at 3.1 au from the sun. a clear 6.2 h modulation of the plasma is seen as the neck is exposed twice per rotation. the electron density of the collisionless plasma within 260 km from the nucleus falls off with radial distance as ∼1/r. the spatial structure indicates that local ionization of neutral gas is the dominant source of low-energy plasma around the comet. | spatial distribution of low-energy plasma around comet 67p/cg from rosetta measurements |
the basic properties of nonlinear ion-acoustic (ia) waves (iaws), particularly finite amplitude ia rogue waves (iarws) in a plasma medium (containing pair ions, iso-thermal positrons, and non-thermal electrons) are theoretically investigated by deriving the nonlinear schrödinger equation (nlse). the criteria for the modulational instability of iaws, and the basic features of finite amplitude iarws are identified. the modulationally stable and unstable regions are determined by the sign of the ratio of the dispersive coefficient to the nonlinear coefficient of nlse. the latter is analyzed to obtain the region for the existence of the iarws, which corresponds to the unstable region. the shape of the profile of the rogue waves depends on the non-thermal parameter α and the ratio of electron temperature to positron temperature. it is found that the increase in the value of the non-thermal parameter enhances both the amplitude and width of iarws, and that the enhancement of electron (positron) temperature reduces (enhances) the amplitude and width of iarws. it is worth to mention that our present investigation may be useful for understanding the salient features of iarws in space (viz., upper region of titan's atmosphere, cometary comae, and earth's ionosphere, etc.) and laboratory (viz., plasma processing reactor and neutral beam sources, etc.) plasmas. | ion-acoustic rogue waves in multi-ion plasmas |
29p/schwassmann-wachmann is an unusual solar system object. originally classified as a short-period comet, it is now known as a centaur that recently transferred to its current orbit, and may become a jupiter family comet. it has exhibited a dust coma for over 90 yr, and regularly undergoes significant dust outbursts. carbon monoxide is routinely detected in high amounts and is typically assumed to play a large role in generating the quiescent dust coma and outbursts. to test this hypothesis, we completed two three-month-long observing campaigns of the co j = 2-1 rotational line using the arizona radio observatory 10 m submillimeter telescope during 2016 and 2018-2019, and compared the results to visible magnitudes obtained at the same time. as the centaur approached its 2019 perihelion, the quiescent dust coma grew ∼45% in brightness, while it is unclear whether the quiescent co production rate also increased. a doubling of the co production rate on 2016 february 28.6 ut did not trigger an outburst nor a rise in dust production for at least 10 days. similarly, two dust outbursts occurred in 2018 while co production continued at quiescent rates. two other dust outbursts may show gas involvement. the data indicate that co and dust outbursts are not always well correlated. this may be explained if co is not always substantially incorporated with the dust component in the nucleus, or if co is primarily released through a porous material. additionally, other minor volatiles or physical processes may help generate dust outbursts. | co gas and dust outbursts from centaur 29p/schwassmann-wachmann |
the oort cloud and the scattered disk are the two primary reservoirs for long-period and short-period comets, respectively. in this review, we assess the known observational constraints on these reservoirs' properties and their formation. in addition, we discuss how the early orbital evolution of the giant planets generated the modern scattered disk from the early, massive planetesimal disk and how $\sim$5\% of this material was captured into the oort cloud. we review how the sun's birth environment and dynamical history within the milky way alters the formation and modern structure of the oort cloud. finally, we assess how the coming decade's anticipated observing campaigns may provide new insights into the formation and properties of the oort cloud and scattered disk. | dynamical population of comet reservoirs |
kuiper belt objects, such as arrokoth, the probable progenitors of short-period comets, formed and evolved at large heliocentric distances, where the ambient temperatures appear to be sufficiently low for preserving volatile ices. by detailed numerical simulations, we follow the long-term evolution of small bodies, composed of amorphous water ice, dust, and ices of other volatile species that are commonly observed in comets. the heat sources are solar radiation and the decay of short-lived radionuclides. the bodies are highly porous and gases released in the interior flow through the porous medium. the most volatile ices, co and ch4, are found to be depleted down to the centre over a time-scale of the order of 100 myr. sublimation fronts advance from the surface inward, and when the temperature in the inner part rises sufficiently, bulk sublimation throughout the interior reduces gradually the volatile ices content until they are completely lost. all the other ices survive, which is compatible with data collected by new horizons on arrokoth, showing the presence of methanol, and possibly, h2o, co2, nh3, and c2h6, but no hypervolatiles. the effect of short-lived radionuclides is to increase the sublimation equilibrium temperatures and reduce volatile depletion times. we consider the effect of the bulk density, abundance ratios, and heliocentric distance. at 100 au, co is depleted, but ch4 survives to present times, except for a thin outer layer. since, co is abundantly detected in comets, we conclude that the source of highly volatile species in active comets must be gas trapped in amorphous ice. | sublimation of ices during the early evolution of kuiper belt objects |
observations of the nucleus of 67p/churyumov-gerasimenko in the millimeter-wave continuum have been obtained by the microwave instrument for the rosetta orbiter (miro). we present data obtained at wavelengths of 0.5 mm and 1.6 mm during september 2014 when the nucleus was at heliocentric distances between 3.45 and 3.27 au. the data are fit to simple models of the nucleus thermal emission in order to characterize the observed behavior and make quantitative estimates of important physical parameters, including thermal inertia and absorption properties at the miro wavelengths. miro brightness temperatures on the irregular surface of 67p are strongly affected by the local solar illumination conditions, and there is a strong latitudinal dependence of the mean brightness temperature as a result of the seasonal orientation of the comet's rotation axis with respect to the sun. the miro emission exhibits strong diurnal variations, which indicate that it arises from within the thermally varying layer in the upper centimeters of the surface. the data are quantitatively consistent with very low thermal inertia values, between 10-30 j k-1 m-2 s-1/2, with the 0.5 mm emission arising from 1 cm beneath the surface and the 1.6 mm emission from a depth of 4 cm. although the data are generally consistent with simple, homogeneous models, it is difficult to match all of its features, suggesting that there may be some vertical structure within the upper few centimeters of the surface. the miro brightness temperatures at high northern latitudes are consistent with sublimation of ice playing an important role in setting the temperatures of these regions where, based on observations of gas and dust production, ice is known to be sublimating. | miro observations of subsurface temperatures of the nucleus of 67p/churyumov-gerasimenko |
context. strong electron cooling on the neutral gas in cometary comae has been predicted for a long time, but actual measurements of low electron temperature are scarce.aims: our aim is to demonstrate the existence of cold electrons in the inner coma of comet 67p/churyumov-gerasimenko and show filamentation of this plasma.methods: in situ measurements of plasma density, electron temperature and spacecraft potential were carried out by the rosetta langmuir probe instrument, lap. we also performed analytical modelling of the expanding two-temperature electron gas.results: lap data acquired within a few hundred km from the nucleus are dominated by a warm component with electron temperature typically 5-10 ev at all heliocentric distances covered (1.25 to 3.83 au). a cold component, with temperature no higher than about 0.1 ev, appears in the data as short (few to few tens of seconds) pulses of high probe current, indicating local enhancement of plasma density as well as a decrease in electron temperature. these pulses first appeared around 3 au and were seen for longer periods close to perihelion. the general pattern of pulse appearance follows that of neutral gas and plasma density. we have not identified any periods with only cold electrons present. the electron flux to rosetta was always dominated by higher energies, driving the spacecraft potential to order - 10 v.conclusions: the warm (5-10 ev) electron population observed throughout the mission is interpreted as electrons retaining the energy they obtained when released in the ionisation process. the sometimes observed cold populations with electron temperatures below 0.1 ev verify collisional cooling in the coma. the cold electrons were only observed together with the warm population. the general appearance of the cold population appears to be consistent with a haser-like model, implicitly supporting also the coupling of ions to the neutral gas. the expanding cold plasma is unstable, forming filaments that we observe as pulses. | cold and warm electrons at comet 67p/churyumov-gerasimenko |
aims: the alice far-ultraviolet spectrograph onboard rosetta is designed to observe emissions from various atomic and molecular species from within the coma of comet 67p/ churyumov-gerasimenko and to determine their spatial distribution and evolution with time and heliocentric distance.methods: following orbit insertion in august 2014, alice made observations of the inner coma above the limbs of the nucleus of the comet from cometocentric distances varying between 10 and 80 km. depending on the position and orientation of the slit relative to the nucleus, emissions of atomic hydrogen and oxygen were initially detected. these emissions are spatially localized close to the nucleus and spatially variable with a strong enhancement above the comet's neck at northern latitudes. weaker emission from atomic carbon and co were subsequently detected.results: analysis of the relative line intensities suggests photoelectron impact dissociation of h2o vapor as the source of the observed h i and o i emissions. the electrons are produced by photoionization of h2o. the observed c i emissions are also attributed to electron impact dissociation, of co2, and their relative brightness to h i reflects the variation of co2 to h2o column abundance in the coma. | measurements of the near-nucleus coma of comet 67p/churyumov-gerasimenko with the alice far-ultraviolet spectrograph on rosetta |
lyman α transits have been detected from several nearby exoplanets and are one of our best insights into the atmospheric escape process. however, due to ism absorption, we typically only observe the transit signature in the blue-wing, making them challenging to interpret. this challenge has been recently highlighted by non-detections from planets thought to be undergoing vigorous escape. pioneering 3d simulations have shown that escaping hydrogen is shaped into a cometary tail receding from the planet. motivated by this work, we develop a simple model to interpret lyman α transits. using this framework, we show that the lyman α transit depth is primarily controlled by the properties of the stellar tidal field rather than details of the escape process. instead, the transit duration provides a direct measurement of the velocity of the planetary outflow. this result arises because the underlying physics is the distance a neutral hydrogen atom can travel before it is photoionized in the outflow. thus, higher irradiation levels, expected to drive more powerful outflows, produce weaker, shorter lyman α transits because the outflowing gas is ionized more quickly. our framework suggests that the generation of energetic neutral atoms may dominate the transit signature early, but the acceleration of planetary material produces long tails. thus, lyman α transits do not primarily probe the mass-loss rates. instead, they inform us about the velocity at which the escape mechanism is ejecting material from the planet, providing a clean test of predictions from atmospheric escape models. | the fundamentals of lyman α exoplanet transits |
we have studied four complex organic molecules (coms), the oxygen-bearing methyl formate (ch3ocho) and dimethyl ether (ch3och3) as well as the nitrogen-bearing formamide (nh2cho) and ethyl cyanide (c2h5cn), towards a large sample of 39 high-mass star-forming regions representing different evolutionary stages, from early to evolved phases. we aim to identify potential correlations and chemical links between the molecules and to trace their evolutionary sequence through the star formation process. we analysed spectra obtained at 3, 2, and 0.9 mm with the iram-30m telescope. we derived the main physical parameters for each species by fitting the molecular lines. we compared them and evaluated their evolution while also taking several other interstellar environments into account. we report detections in 20 sources, revealing a clear dust absorption effect on column densities. derived abundances range between ~ 10-10-10-7 for ch3ocho and ch3och3, ~ 10-12-10-10 for nh2cho, and ~ 10-11-10-9 for c2h5cn. the abundances of ch3ocho, ch3och3, and c2h5cn are very strongly correlated (r ≥ 0.92) across ~ 4 orders of magnitude. we note that ch3ocho and ch3och3 show the strongest correlations in most parameters, and a nearly constant ratio (~ 1) over a remarkable ~ 9 orders of magnitude in luminosity for the following wide variety of sources: pre-stellar to evolved cores, low- to high-mass objects, shocks, galactic clouds, and comets. this indicates that coms chemistry is likely early developed and then preserved through evolved phases. moreover, the molecular abundances clearly increase with evolution, covering ~ 6 orders of magnitude in the luminosity/mass ratio. we consider ch3ocho and ch3och3 to be most likely chemically linked. they could, for example, share a common precursor, or be formed one from the other. based on correlations, ratios, and the evolutionary trend, we propose a general scenario for all coms, involving a formation in the cold, earliest phases of star formation and a following increasing desorption with the progressive thermal and shock-induced heating of the evolving core. iram source spectra are only available at the cds via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/j/a+a/641/a54 | evolutionary study of complex organic molecules in high-mass star-forming regions |
comet 2i/borisov, the first unambiguous interstellar comet ever found, was discovered in 2019 august at ∼3 au from the sun on its inbound leg. no pre-discovery detection beyond 3 au has yet been reported, mostly due to the comet's proximity to the sun as seen from the earth. here we present a search for pre-discovery detections of comet borisov using images taken by the catalina sky survey, pan-starrs, and the zwicky transient facility (ztf), with a further comprehensive follow-up campaign being presented in bolin et al. we identified comet borisov in ztf images taken in 2019 may and use these data to update its orbit. this allowed us to identify the comet in images acquired as far back as 2018 december, when it was 7.8 au from the sun. the comet was not detected in 2018 november when it was 8.6 au from the sun, possibly implying an onset of activity around this time. this suggests that the activity of the comet is either driven by a more volatile species other than h2o, such as co or co2, or by exothermic crystallization of amorphous ice. we derive the radius of the nucleus to be <7 km using the non-detection in 2018 november, and estimate an area of ∼0.5- $10\,{\mathrm{km}}^{2}$ has been active between 2018 december and 2019 september, though this number is model-dependent and is highly uncertain. the behavior of comet borisov during its inbound leg is observationally consistent with dynamically new comets observed in our solar system, suggesting some similarities between the two. | pre-discovery activity of new interstellar comet 2i/borisov beyond 5 au |
pulsar wind nebulae (pwn) are fascinating systems and archetypal sources for high-energy astrophysics in general. due to their vicinity, brightness, to the fact that they shine at multi-wavelengths, and especially to their long-living emission at gamma rays, modelling their properties is particularly important for the correct interpretation of the visible galaxy. a complication in this respect is the variety of properties and morphologies they show at different ages. here, we discuss the differences among the evolutionary phases of pwn, how they have been modeled in the past and what progresses have been recently made. we approach the discussion from a phenomenological, theoretical (especially numerical) and observational point of view, with particular attention to the most recent results and open questions about the physics of such intriguing sources. | the dawes review 11: from young to old: the evolutionary path of pulsar wind nebulae |
the solid, central part of a comet—its nucleus—is subject to destructive processes, which cause nuclei to split at a rate of about 0.01 per year per comet. these destructive events are due to a range of possible thermophysical effects; however, the geophysical expressions of these effects are unknown. separately, over two-thirds of comet nuclei that have been imaged at high resolution show bilobate shapes, including the nucleus of comet 67p/churyumov-gerasimenko (67p), visited by the rosetta spacecraft. analysis of the rosetta observations suggests that 67p’s components were brought together at low speed after their separate formation. here, we study the structure and dynamics of 67p’s nucleus. we find that sublimation torques have caused the nucleus to spin up in the past to form the large cracks observed on its neck. however, the chaotic evolution of its spin state has so far forestalled its splitting, although it should eventually reach a rapid enough spin rate to do so. once this occurs, the separated components will be unable to escape each other; they will orbit each other for a time, ultimately undergoing a low-speed merger that will result in a new bilobate configuration. the components of four other imaged bilobate nuclei have volume ratios that are consistent with a similar reconfiguration cycle, pointing to such cycles as a fundamental process in the evolution of short-period comet nuclei. it has been shown that comets were not strong contributors to the so-called late heavy bombardment about 4 billion years ago. the reconfiguration process suggested here would preferentially decimate comet nuclei during migration to the inner solar system, perhaps explaining this lack of a substantial cometary flux. | fission and reconfiguration of bilobate comets as revealed by 67p/churyumov-gerasimenko |
rosetta has followed comet 67p from low activity at more than 3.6 au heliocentric distance to high activity at perihelion (1.24 au) and then out again. we provide a general overview of the evolution of the dynamic ion environment using data from the rpc-ica ion spectrometer. we discuss where rosetta was located within the evolving comet magnetosphere. for the initial observations, the solar wind permeated all of the coma. in 2015 mid-april, the solar wind started to disappear from the observation region, to re-appear again in 2015 december. low-energy cometary ions were seen at first when rosetta was about 100 km from the nucleus at 3.6 au, and soon after consistently throughout the mission except during the excursions to farther distances from the comet. the observed flux of low-energy ions was relatively constant due to rosetta's orbit changing with comet activity. accelerated cometary ions, moving mainly in the antisunward direction gradually became more common as comet activity increased. these accelerated cometary ions kept being observed also after the solar wind disappeared from the location of rosetta, with somewhat higher fluxes further away from the nucleus. around perihelion, when rosetta was relatively deep within the comet magnetosphere, the fluxes of accelerated cometary ions decreased, as did their maximum energy. the disappearance of more energetic cometary ions at close distance during high activity is suggested to be due to a flow pattern where these ions flow around the obstacle of the denser coma or due to charge exchange losses. | evolution of the ion environment of comet 67p during the rosetta mission as seen by rpc-ica |
there are plenty of monitoring methods to quantify gas emission rates based on gas concentration measurements around the strong sources. however, there is a lack of quantitative models to evaluate methane emission rates from coal mines with less prior information. in this study, we develop a genetic algorithm-interior point penalty function (ga-ippf) model to calculate the emission rates of large point sources of ch4 based on concentration samples. this model can provide optimized dispersion parameters and self-calibration, thus lowering the requirements for auxiliary data accuracy. during the carbon dioxide and methane mission (comet) pre-campaign, we retrieve ch4-emission rates from a ventilation shaft in pniówek coal mine (silesia coal mining region, poland) based on the data collected by an unmanned aerial vehicle (uav)-based aircore system and a ga-ippf model. the concerned ch4-emission rates are variable even on a single day, ranging from 621.3 ± 19.8 to 1452.4 ± 60.5 kg h−1 on 18 august 2017 and from 348.4 ± 12.1 to 1478.4 ± 50.3 kg h−1 on 21 august 2017. results show that ch4 concentration data reconstructed by the retrieved parameters are highly consistent with the measured ones. meanwhile, we demonstrate the application of ga-ippf in three gas control release experiments, and the accuracies of retrieved gas emission rates are better than 95.0 %. this study indicates that the ga-ippf model can quantify the ch4-emission rates from strong point sources with high accuracy. | retrieving ch4-emission rates from coal mine ventilation shafts using uav-based aircore observations and the genetic algorithm-interior point penalty function (ga-ippf) model |
context. several sulfur-bearing molecules are observed in the interstellar medium and in comets, in strong contrast to protoplanetary disks where only cs, h2cs, and so have been detected so far.aims: we combine observations and chemical models to constrain the sulfur abundances and their sensitivity to physical and chemical conditions in the dm tau protoplanetary disk.methods: we obtained 0.5'' atacama large millimeter array observations of dm tau in bands 4 and 6 in lines of cs, so, so2, ocs, ccs, h2cs, and h2s, achieving a 5 mjy sensitivity. using the non-local thermodynamical equilibrium radiative transfer code radex and the forward-modeling tool diskfit, disk-averaged cs column densities and upper limits for the other species were derived.results: only cs was detected with a derived column density of 2-6 × 1012 cm-2. we report a first tentative detection of so2 in dm tau. the upper limits range between 1011 and 1014 cm-2 for the other s-bearing species. the best-fit chemical model matching these values requires a gas-phase c/o ratio of ≳1 at r ≳ 50-100 au. with chemical modeling we demonstrate that sulfur-bearing species could be robust tracers of the gas-phase c/o ratio, surface reaction rates, grain size and uv intensities.conclusions: the lack of detections of a variety of sulfur-bearing molecules in dm tau other than cs implies a dearth of reactive sulfur in the gas phase, either through efficient freeze-out or because most of the elemental sulfur is in other large species, as found in comets. the inferred high cs/so and cs/so2 ratios require a non-solar c/o gas-phase ratio of ≳1, consistent with the recent observations of hydrocarbon rings in dm tau. the stronger depletion of oxygen-bearing s-species compared to cs is likely linked to the low observed abundances of gaseous water in dm tau and points to a removal mechanism of oxygen from the gas. | chemistry in disks. xi. sulfur-bearing species as tracers of protoplanetary disk physics and chemistry: the dm tau case |
kuiper belt objects, or more generally trans-neptunian objects (tnos), are planetesimals found beyond the orbit of neptune. some tnos evolve onto neptune-crossing orbits and become centaurs. many centaurs, in turn, reach jupiter-crossing orbits and become jupiter-family comets (jfcs). tnos are the main source of the jfcs. tnos offer a different window than the jfcs, of more primordial bodies and over a different size and temperature range. it is in that context that this chapter is written. here we discuss the dynamical pathways taken from the trans-neptunian region to the jfcs, and the most important properties of tnos that relate to the jfc population, including considerations of their origins, compositions, morphologies, and size distributions. we relate these properties to the jfcs whenever possible. we reflect on a few key outstanding issues regarding our incomplete knowledge of tnos as they pertain to the centaurs and jfc populations. we finish with a short discussion of notable new and upcoming facilities and the impacts they will have regarding these outstanding questions. | the transition from the kuiper belt to the jupiter-family (comets) |
thermal and mechanical material properties determine comet evolution and even solar system formation because comets are considered remnant volatile-rich planetesimals. using data from the multipurpose sensors for surface and sub-surface science (mupus) instrument package gathered at the philae landing site abydos on comet 67p/churyumov-gerasimenko, we found the diurnal temperature to vary between 90 and 130 k. the surface emissivity was 0.97, and the local thermal inertia was 85 ± 35 j m-2 k-1s-1/2. the mupus thermal probe did not fully penetrate the near-surface layers, suggesting a local resistance of the ground to penetration of >4 megapascals, equivalent to >2 megapascal uniaxial compressive strength. a sintered near-surface microporous dust-ice layer with a porosity of 30 to 65% is consistent with the data. | thermal and mechanical properties of the near-surface layers of comet 67p/churyumov-gerasimenko |
we report new light curves and phase functions for nine jupiter-family comets (jfcs). they were observed in the period 2004-2015 with various ground telescopes as part of the survey of ensemble physical properties of cometary nuclei as well as during devoted observing campaigns. we add to this a review of the properties of 35 jfcs with previously published rotation properties. the photometric time series were obtained in bessel r, harris r and sdss r΄ filters and were absolutely calibrated using stars from the pan-starrs survey. this specially developed method allowed us to combine data sets taken at different epochs and instruments with absolute-calibration uncertainty down to 0.02 mag. we used the resulting time series to improve the rotation periods for comets 14p/wolf, 47p/ashbrook-jackson, 94p/russell and 110p/hartley 3 and to determine the rotation rates of comets 93p/lovas and 162p/siding spring for the first time. in addition to this, we determined the phase functions for seven of the examined comets and derived geometric albedos for eight of them. we confirm the known cut-off in bulk densities at ∼0.6 g cm-3 if jfcs are strengthless. using a model for prolate ellipsoids with typical density and elongations, we conclude that none of the known jfcs requires tensile strength larger than 10-25 pa to remain stable against rotational instabilities. we find evidence for an increasing linear phase function coefficient with increasing geometric albedo. the median linear phase function coefficient for jfcs is 0.046 mag deg-1 and the median geometric albedo is 4.2 per cent. | rotation of cometary nuclei: new light curves and an update of the ensemble properties of jupiter-family comets |
the metastable helium (he*) lines near 10830 å are ideal probes of atmospheric erosion-a common phenomenon of close-in exoplanet evolution. a handful of exoplanet observations yielded well-resolved he* absorption features in transits, yet they were mostly analyzed with 1d isothermal models prescribing mass-loss rates. this work devises 3d hydrodynamics coevolved with ray-tracing radiative transfer and nonequilibrium thermochemistry. starting from the observed stellar/planetary properties with reasonable assumptions about the host's high-energy irradiation, we predict from first principles the mass-loss rate, the temperature and ionization profiles, and 3d outflow kinematics. our simulations well reproduce the observed he* line profiles and light curves of wasp-69b. we further investigate the dependence of he* observables on simulation conditions and host radiation. the key findings are as follows: (1) simulations reveal a photoevaporative outflow (~0.55 m⊕ gyr-1 ≃ 1011 g s-1) for wasp-69b without a prominent comet-like tail, consistent with the symmetric transit shape. (2) 3d simulations are mandatory for hydrodynamic features, including coriolis force, advection, and kinematic line broadening. (3) euv (>13.6 ev) photons dominate photoevaporative outflows and populate he* via recombination; fuv is also detrimental by destroying he*; x-ray plays a secondary role. (4) k stars hit the sweet spot of euv/far-uv balance for he* line observation, while g and m stars are also worthy targets. (5) stellar flares create characteristic responses in the he* line profiles. | metastable helium absorptions with 3d hydrodynamics and self-consistent photochemistry. i. wasp-69b, dimensionality, x-ray and uv flux level, spectral types, and flares |
the composition of cometary ices provides key information on the thermal and chemical properties of the outer parts of the protoplanetary disk where they formed 4.6 gy ago. this chapter reviews our knowledge of composition of cometary comae based on remote spectroscopy and in-situ investigations techniques. cometary comae can be dominated by water vapour, co or co2. the abundances of several dozen of molecules, with a growing number of complex organics, have been measured in comets. many species that are not directly sublimating from the nucleus ices have also been observed and traced out into the coma in order to determine their production mechanisms. chemical diversity in the comet population and compositional heterogeneity of the coma are discussed. with the completion of the rosetta mission, isotopic ratios, which hold additional clues on the origin of cometary material, have been measured in several species. finally, important pending questions (e.g., the nitrogen deficiency in comets) and the need for further work in certain critical areas are discussed in order to answer questions and resolve discrepancies between techniques. | chemistry of comet atmospheres |
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.