348 Encyclopedia of the Solar System
of change inferred from theViking,Pathfinder, and Gusev
cratered plains landing sites, argues for a dry and desiccat-
ing climate similar to today’s for the past∼3.7 billion years.
Rocks in the Columbia Hills (Fig. 15) sampled by the
Spiritrover reveals an earlier period in which liquid water
was present. The Husband Hills appear to be older materi-
als that were either uplifted and/or eroded before deposi-
tion of the basalts responsible for the cratered plains. The
basalts of the cratered plains are intermediate in Mars his-
tory and so the Columbia Hills rocks are likely older than
roughly 3.7 billion years. These rocks record impact and
explosive volcanic processes in their deposition, but many
have been heavily altered and/or deeply weathered by wa-
ter. In contrast, soils in the Columbia Hills are similar to
basaltic soils elsewhere, suggesting these formed and were
deposited later in the cold and dry martian climate.
The geology and geomorphology of the Meridiani
Planum landing site explored by theOpportunityrover
shows clear evidence for an earlier wet and warm envi-
ronment followed by a drier period dominated by eolian
activity. The layered rocks examined byOpportunityare
older that 3.7 billion years based on the density of highly
eroded large craters observed in orbital images (Fig. 5).
These rocks are dirty evaporites composed of materials
that have precipitated from salty water and mobilized and
moved by the wind (Fig. 23) before being deposited and
altered by groundwater. On Earth, this sequence of events
and resulting rocks is common in hot and dry salt water
playa or sabkha environments such as the Persian Gulf, the
Gulf of California, and some inland enclosed basins. By
analogy, the environment on Mars was warm and wet when
these rocks were deposited prior to 3.7 billion years ago.
Because the evaporites are part of a sedimentary sequence
that outcrops throughout the broad Meridiani region, these
climatic conditions were operative over an area that was
at least 1000 km wide, arguing that the environment was
both warm and wet and the atmosphere was thicker. Latter
on in Mars history, the environment changed, and Merid-
iani Planum was dominated by eolian activity that eroded
and filled in impact craters and concentrated the hematite
spherules as a lag on the top of the layer of basaltic sand.
The presence of olivine in the basaltic sand suggests these
materials were not weathered by liquid water, and the salta-
tion of the sand appears to have efficiently eroded the weak
sulfates.
6.6 Implications for a Habitable World
The Meridiani Planum evaporites and Columbia Hills rocks
in Gusev crater indicate a warm and wet environment be-
fore about 3.7 billion years ago. This is consistent with a vari-
ety of coeval geomorphic indicators such as valley networks,
degraded and filled ancient craters, highly eroded terrain,
and layered sedimentary rocks that point to an early warm
and wet climate. The warm and wet environment would
also imply a thicker atmosphere capable of supporting liq-
uid water. In contrast, the surficial geology of the landing
sites younger than about 3.7 billion years old all indicate a
dry and desiccating environment in which liquid water was
not stable and eolian and impact processes dominate. This
further indicates a major climatic change occurred around
3.7 billion years ago.
A warm and wet environment before 3.7 billion years
suggests that Mars was habitable at a time when life started
on the Earth. However, the highly acidic nature of water at
some Mars landing sites may not have been conducive to
the appearance of early organisms. In any case, the earliest
chemical evidence for life on Earth is about 3.9 billion years
old, and the most important ingredient for life on Earth is
liquid water. If liquid water was stable on Mars when life
began on Earth, could a second genesis on Mars have oc-
curred? Is it possible that life actually started on Mars earlier
when it was more clement than Earth that was subject to
early giant possibly sterilizing impacts and was later trans-
ported to the Earth via meteorites ejected off the martian
surface? Will life form anywhere that liquid water is stable or
is it a rare occurrence? Are we alone in the universe? These
are the compelling questions that can be addressed by up-
coming landers and rovers in a Mars exploration program.Bibliography
Additional Reading
Golombek, M. P., et al. (1997), Overview of the Mars Pathfinder
mission and assessment of landing site predictions.Science 278 ,
1743–1748. And the next five papers inScience(pp. 1734–1774)
in which the scientific results of theMars Pathfindermission were
first reported.
Golombek, M. P., et al. (1999). Overview of theMars Pathfinder
mission: Launch through landing, surface operations, data sets,
and science results.J. Geophys. Res. 104 , 8523–8553. Special is-
sues of theJournal of Geophysical Research, Planets(volume 104,
pages 8521–9096, April 25, 1999; volume 105, pages 1719–1865,
January 25, 2000) also featured the scientific results of the mission.
Kieffer, H. H., Jakosky, B. M., Snyder, C. W., and Matthews,
M. S. (1992).‘‘MARS.’’Univ. Arizona Press, Tucson.
Squyres, S. W., et al. (2004). TheSpiritrover’s Athena science
investigation at Gusev crater, Mars.Science 305 (5685), 794–799,
DOI: 10.1126/science.1100194. And the next ten papers (pp.793–
845) in which the first results of theSpiritrover were reported.
Squyres, S. W., et al. (2004). TheOpportunityrover’s Athena
science investigation at Meridiani Planum, Mars.Science 306
(5702), 1698–1703, DOI: 10.1126/science.1106171. And the next
ten papers (pp. 1697–1756) in which the first results of theOp-
portunityrover were reported.
Golombek, M., et al. (2005). Assessment of Mars Explo-
ration Rover landing site predictions.Nature 436 , 44–48, DOI:
10.1038/nature03600. And the next five papers (pp. 42–70) in
which futher results from the Mars Exploration Rovers were re-
ported.