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submerged underwater, while others are profoundly altered when hot water runs
through them, leaving behind residues. Up until now, it has been very difficult to get to
know the minerals in martian rocks because we have not had the tools to unravel their
mineralogies. By understanding Mars' rocks in a more complete manner, scientists
can gain a better view into the history of liquid water on the planet. Like their prede-
cessor mission, Mars Pathfinder, the Mars Exploration Rovers will pursue this goal by
placing robotic geologists on the planet's surface -- ideally suited to "reading the rocks"
to understand the still mysterious history of water, and even of life-friendly ancient envi-
Myths and Reality
Mars caught public fancy in the late 1870s, when Italian astronomer Giovanni
Schiaparelli reported using a telescope to observe "canali," or channels, on Mars. A
possible mistranslation of this word as "canals" may have fired the imagination of
Percival Lowell, an American businessman with an interest in astronomy. Lowell
founded an observatory in Arizona, where his observations of the Red Planet con-
vinced him that the canals were dug by intelligent beings -- a view that he energetically
promoted for many years.
By the turn of the last century, popular songs envisioned sending messages between
worlds by way of huge signal mirrors. On the dark side, H.G. Wells' 1898 novel "The
War of the Worlds" portrayed an invasion of Earth by technologically superior Martians
desperate for water. In the early 1900s novelist Edgar Rice Burroughs, known for the
"Tarzan" series, also entertained young readers with tales of adventures among the
exotic inhabitants of Mars, which he called Barsoom.
Fact began to turn against such imaginings when the first robotic spacecraft were sent
to Mars in the 1960s. Pictures from the 1965 flyby of Mariner 4 and the 1969 flybys of
Mariner 6 and 7 showed a desolate world, pocked with impact craters similar to those
seen on Earth's Moon. Mariner 9 arrived in 1971 to orbit Mars for the first time, but
showed up just as an enormous dust storm was engulfing the entire planet. When the
storm died down, Mariner 9 revealed a world that, while partly crater-pocked like
Earth's Moon, was much more geologically complex, complete with gigantic canyons,
volcanoes, dune fields and polar ice caps. This first wave of Mars exploration culmi-
nated in the Viking mission, which sent two orbiters and two landers to the planet in
1975. The landers included a suite of experiments that conducted chemical tests in
direct search of life. Most scientists interpreted the results of these tests as negative,
deflating hopes of identifying another world on where life might be or have been wide-
spread. However, Viking left a huge legacy of information about Mars that fed a hungry
science community for two decades.
The science community had many other reasons for being interested in Mars, apart
from the direct search for life; the next mission on the drawing boards concentrated on
a study of the planet's geology and climate using advanced orbital reconnaissance.
Over the next 20 years, however, new findings in laboratories on Earth came to
change the way that scientists thought about life and Mars.
One was the 1996 announcement by a team from Stanford University and NASA's
Johnson Space Center that a meteorite believed to have originated on Mars contained
what might be the fossils of ancient bacteria. This rock and other likely Mars mete-
orites discovered on several continents on Earth are believed to have been blasted off
the Red Planet by asteroid or comet impacts. They are presently believed to have
come from Mars because of gases trapped in them that unmistakably match the com-
position of Mars' atmosphere as measured by the Viking landers. Many scientists
questioned the conclusions of the team announcing the discovery of possible life in
one martian meteorite, but if nothing else the mere presence of organic compounds in
the meteorites increases the odds of life forming at an earlier time on a far wetter
Another development that shaped scientists' thinking was spectacular new findings on
how and where life thrives on Earth. The fundamental requirements for life as we
know it today are liquid water, organic compounds and an energy source for synthesiz-
ing complex organic molecules. Beyond these basics, we do not yet understand the
environmental and chemical evolution that leads to the origin of terrestrial life. But in
recent years, it has become increasingly clear that life can thrive in settings much dif-
ferent -- and more harsh -- from a tropical soup rich in organic nutrients.
for surviving in extreme environments -- niches that by turn are extraordinarily hot, or
cold, or dry, or under immense pressures -- that would be completely inhospitable to
humans or complex animals. Some scientists even concluded that life may have
begun on Earth in heat vents far under the ocean's surface.
This in turn had its effect on how scientists thought about Mars. Martian life might not
be so widespread that it would be readily found at the foot of a lander spacecraft, but it
may have thrived billions of years ago in an underground thermal spring or other hos-
pitable environment. Or it might still exist in some form in niches below the currently
frigid, dry, windswept surface, perhaps entombed in ice or in liquid water aquifers.
After years of studying pictures from the Viking orbiters, scientists gradually came to
conclude that many features they saw suggested that Mars may have been warm and
wet in an earlier era. And two currently operating orbiters -- Mars Global Surveyor and
Mars Odyssey -- are giving scientists yet new insights into the planet. Global
Surveyor's camera detected possible evidence for recent liquid water in a large num-
ber of settings, while Odyssey's camera system has found large amounts of ice mixed
in with Mars surface materials at high latitudes, as well as potential evidence of ancient
The Three Ages of Mars
Based on what they have learned from spacecraft missions, scientists view Mars as
the "in-between" planet of the inner solar system. Small rocky planets such as
Mercury and Earth's Moon apparently did not have enough internal heat to power vol-
canoes or to drive the motion of tectonic plates, so their crusts grew cold and static rel-
atively soon after they formed when the solar system condensed into planets about 4.6
billion years ago. Devoid of atmospheres, they are riddled with craters that are relics
of impacts during a period of bombardment when the inner planets were sweeping up
remnants of small rocky bodies that failed to "make it as planets" in the solar system's
early times.
Earth and Venus, by contrast, are larger planets with substantial internal heat sources
and significant atmospheres. Earth's surface is continually reshaped by tectonic plates
sliding under and against each other and materials spouting forth from active volca-
noes where plates are ripped apart. Both Earth and Venus have been paved over so
recently that both lack any discernible record of cratering from the era of bombardment
in the early solar system.
Mars appears to stand between those sets of worlds, on the basis of current yet evolv-
ing knowledge. Like Earth and Venus, it possesses a myriad of volcanoes, although
they probably did not remain active as long as counterparts on Earth and Venus. On
Earth, a single "hot spot" or plume might form a chain of middling-sized islands such
as the Hawaiian Islands as a tectonic plate slowly slides over it. On Mars there are
apparently no such tectonic plates, at least as far as we know today, so when volca-
noes formed in place they had the time to become much more enormous than the
rapidly moving volcanoes on Earth. Overall Mars appears to be neither as dead as
Mercury and our Moon, nor as active as Earth and Venus. As one scientist quips,
"Mars is a warm corpse if not a fire-breathing dragon." Thanks to the ongoing obser-