Mars: Landing Site Geology, Mineralogy and Geochemistry 347
be lost to space by either solar wind or impact erosion pro-
cesses.
The presence of significant amounts of sulfate and chlo-
ride in soils from all the landing sites further suggests that
acidic waters may have been common at one time in all
parts of Mars. Either evaporites were abundant and have
been redistributed as small particles throughout the planet’s
regolith, or they occur as cements formed by groundwater
leaching all over Mars. Results from the OMEGA spec-
trometer on theMars Expressorbiter support the finding
of abundant sulfates elsewhere on Mars.
6.3 Weathering on Mars
There is considerable controversy about the degree to
which Mars rocks are weathered. Weathering by acidic wa-
ter preferentially attacks olivine, and the surface layers of
rocks at the MER sites appear to be depleted in olivine.
However, remote sensing indicates that olivine is a com-
mon mineral in many places on Mars, and olivine appears
to be a ubiquitous constituent of martian soils and dust.
Perhaps weathering was more common in the distant past,
when acidic waters were abundant and produced outcrops
like those found byOpportunity. Then the acid waters dis-
appeared, and since that time the lavas that were erupted
and the soils that formed have only experienced limited
weathering.
OMEGA data indicate that clay minerals occur in some
localities in the ancient terrains of Mars, although clays have
not been found definitively at any of the landing sites. Nev-
ertheless, clays have been suggested to be present in some
rocks on Husband Hill (Gusev crater), based on aspects of
their chemistry. Clay minerals form by weathering, and they
clearly demonstrate that weathering occurred on Mars in
the distant past.
6.4 Eolian Processes
The remarkable uniformity of soil compositions at all the
landing sites, some separated by thousands of kilometers,
suggests an efficient homogenization process. Transport of
rock particles by the wind has apparently mixed these ma-
terials very efficiently, so that the soil everywhere on Mars
represents a globally distributed stratigraphic layer. A simi-
lar process must have occurred for the dust particles as well.
A dust cycle can be inferred from the omnipresent dusty at-
mosphere being supplied by dust devils and other processes
that occasionally lead to globe-encircling storms. Dust has
been observed to be deposited on most of the spacecraft
at a rate that is so high that it must be picked up at a sim-
ilar rate (or the surfaces would be quickly buried by thick
accumulations of dust). It may be that dust is deposited at
a higher rate overall in broad areas of the planet that have
very low thermal inertia and very high albedo. Sand-sized
particles created by impact and other processes have been
harnessed by the wind to form sand dunes and other eolian
bedforms observed. The consistent basaltic composition of
the soil and dust all over Mars further argues that Mars is
dominated by basaltic rocks and that the soil and dust forms
by physical weathering and minor oxidation without large
quantities of water. This further argues that these weather-
ing products have formed and been mobilized by the wind
in the current dry and desiccating environment.6.5 Geologic Evolution of the Landing Sites
and Climate
Study of the geology, geomorphology, and geochemistry of
the five landing sites in context with their regional geologic
setting allows constraints to be placed on the environmental
and climatic conditions on Mars through time. TheViking 1
landing site shows sedimentary drift and soil deposits over
angular, dark, presumably volcanic rocks with local outcrops
(Fig. 6). The location of this site on the ridged plains ter-
rain downstream from the mouth of Maja and Kasei Valles,
suggest that the site is on layered basalts (the preferred
interpretation of the ridged plains) with rocks, soils, and
drifts derived from impact ejecta, flood, and eolian pro-
cesses. The rocks at theViking 2landing site (Fig. 7), in
the mid-northern plains, are angular and pitted consistent
with their being volcanic rocks as part of the distal ejecta
from Mie crater. High-resolution orbiter images show the
surface has a small-scale hummocky character, and lander
images show small polygonal sediment-filled troughs, both
suggesting that the surface has been partially shaped by the
presence of ground ice. The density of craters observed
from orbit at both sites places them intermediate in Mars’
history (roughly 3.7–3.0 billion years ago), and constraints
on the geomorphologic development of the sites suggest
very little erosion or change of the surfaces.
Many characteristics of theMars Pathfinderlanding site
(Figs. 8 and 18) are consistent with its being a plain com-
posed of materials deposited by catastrophic floods as sug-
gested by its location near the mouth of the Ares Vallis
catastrophic outflow channel. Some of the rocks potentially
identified (conglomerate, pillow basalt) are suggestive of a
wetter past. However, given that the surface still appears
similar to that expected for a fresh depositional fan, any
erosional and/or depositional processes appear to have been
minimal since it formed around 3 billion years ago.
The cratered plains of Gusev thatSpirithas traversed
(exclusive of the Columbia Hills) are generally low-relief,
moderately rocky plains dominated by hollows, which ap-
pear to be craters filled with soil (Fig. 9). Rocks are gener-
ally angular basalt fragments in an unconsolidated regolith
greater than 10 m thick of likely impact origin (Fig. 10).
The observed gradation and deflation of ejected fines and
deposition in craters to form hollows thus provides a mea-
sure of the rate of erosion or redistribution since the plains
formed about 3.5 billion years ago. These rates of erosion
are so slow that they provide a broad indicator of a climate
that has been cold and dry. Taking together the slow rates