Mars: Landing Site Geology, Mineralogy and Geochemistry 343
the resulting shape and characteristics of fresh craters. [See
Planetary Impacts.] Primary impact craters less than 1
km in diameter have well-understood bowl-shaped interi-
ors whose depth is about 0.2 times their diameters; they
also have uplifted rims and ejecta deposits (Fig. 10) that
get less rocky and thin with distance from the crater. As a
result, imaging impact craters provides clues to the geomor-
phologic changes that have occurred at the site such as the
amount of erosion and/or deposition.
5. Landing Site Mineralogy and Geochemistry
5.1 Rocks
Based on their appearance, rocks at theVikinglanding sites
(Figs. 6 and 7) were inferred to be basalts, but theViking
lander arms could not reach and collect small enough rocks
to analyze, so little is known about their composition. Rocks
at theMars Pathfinder,Spirit, andOdysseylanding sites
have been analyzed by a variety of rover-mounted instru-
ments, as described in Table 2.
Pathfinderrock chemical compositions were analyzed
by the APXS (Fig. 18), and partial mineralogy was inferred
from IMP spectra on the lander. The APXS analyzes only
the outer surface (generally just a few tens of micrometers)
of rocks. IMP images showed that the rocks were variably
coated with dust. Plots of different elements versus sulfur
yield straight lines, with soils plotting at the sulfur-rich end
best interpreted as mixing lines between the compositions
of rocks and soil. The composition of the dust-free rock in-
terior was inferred by extrapolating the rock composition
trends to zero sulfur. The dust-free rocks have concentra-
tions of alkalis and silica that would classify them as andesite
(two different calibrations of the APXS instrument data
are shown in Fig. 13), and it was inferred from the rocks’
FIGURE 18 Color mosaic ofSojournerwith APXS instrument
measuring the chemical composition of the rock Yogi. Note dusty
surface darkened by the rover wheels. Brighter toned soil in the
wheel tracks is cemented soil or duricrust. Note tabular rock on
the left horizon, called Couch, and other tabular and partially
rounded boulders as expected if deposited by catastrophic floods.
appearance that these were volcanic rocks. However, be-
cause the APXS analyzes only the rock surface, it is also
possible that this andesitic composition represents a silica-
rich weathering rind beneath the dust rather than the com-
position of the rock interiors. The IMP spectra indicate the
presence of iron oxides, but a more comprehensive spectral
interpretation is hampered by the dust coatings.
Rocks at theSpiritlanding site in Gusev crater were
analyzed using a greater variety of analytical instruments
(Table 2), aided by the RAT that can brush or grind the
outer rock surface. Rocks on the plains in the vicinity of
theSpiritlander are clearly basalts, in agreement with the
location of Gusev crater within an area mapped by TES
as Surface Type 1. Some of these rocks are vesicular—
pocked with holes that were once gas bubbles exsolved from
magma—and most rocks are coated with dust (Fig. 19).
Spectra from Pancam, Mini-TES, and MB of relatively dust-
free or RAT-abraded rocks provide a consistent picture of
the minerals that comprise these basalts—olivine, pyrox-
ene, and iron oxides. All the spectra from these instruments
are dominated by minerals containing iron and magnesium.
Chemical compositions of plains basalts measured by APXS
support the presence of olivine, pyroxene, and oxides, but
they also suggest abundant feldspar (plagioclase) and phos-
phate, which cannot be seen by other spectra. The APXS
analyses confirm that the rocks on the plains of Gusev
(Fig. 13) are basalts (Adirondack class) especially rich in
olivine (and hence lower in silica). Abundant dark crystals
interpreted to be olivine can be seen in MI images of RAT
holes in the rocks (Fig. 19). Surface alteration rinds and
veins cutting through the interiors of these rocks can also
be clearly seen in some MI images, suggesting limited in-
teractions of the rocks with water.
After analyzing rocks near the landing site, theSpirit
rover traversed about 3 km across the plains and climbedFIGURE 19 The Gusev cratered plains rock Humphrey studied
by theSpiritrover. (A) Pancam color image of rock after RAT
grinding showing darker interior and thus the presence of a
dusty and slightly weathered surface. (B) Microscopic image of
Humphrey RAT hole, illustrating dark grains thought to be
olivine crystals and holes likely to be vesicles, consistent with the
basaltic chemistry and mineralogy determined by the APXS and
MB.