Mars: Landing Site Geology, Mineralogy and Geochemistry 335
strong evidence for surface processes involving water to de-
termine the aqueous, climatic, and geologic history of sites
where conditions may have been favorable to the preserva-
tion of prebiotic or biotic processes. The site selected for
theSpiritrover was within Gusev crater, an ancient 160 km
diameter impact crater at the edge of the cratered high-
lands in the eastern hemisphere. The southern rim of Gusev
is breached by Ma’adim Vallis, an 800 km long branching
valley network that drains the ancient cratered highlands
to the south (Fig. 3). The smooth flat floor of Gusev was
interpreted as sediments deposited in a crater lake, so that
the rover could analyze fluvial sediments deposited in a
lacustrine environment (Fig. 4). The site selected for the
FIGURE 3 Regional color mosaic of Ma’adim Valles and Gusev
crater. The 800 km long Ma’adim Valles, one of the largest
branching valley networks on Mars, drains the heavily cratered
terrain to the south and breaches the southern rim of Gusev
crater. Gusev crater, which formed much earlier, is 150 km in
diameter, and the smooth flat floor strongly suggests it was a
crater lake that filled with water and sediments.Spirithas yet to
identify sediments associated with Ma’adim Valles. The cratered
plains are underlain by basalt flows and so represent a late
volcanic cover. Rocks in the Columbia Hills have been altered by
water, but cannot be related to deposition in a lake associated
with Ma’adim discharge.
FIGURE 4 Mosaic of Gusev crater showing the landing ellipse,
landing location for theSpiritlander, and the extensive data sets
that were obtained to evaluate the Mars Exploration Rover
landing sites. Ma’adim Valles breaches the southern rim, and
hills immediately downstream have been interpreted as delta
deposits. Blue ellipse is the final targeted ellipse and the red X is
the landing location. Background of mosaic isViking230 m/pixel
mosaic, overlain by MOLA elevations in color. Thin image strips
mostly oriented to the north-northwest are MOC high-resolution
images typically at 3 m/pixel. Wider image strips mostly oriented
to the north-northeast areMars Odysseyvisible images at
18 m/pixel. Mosaic includes 13◦S–16◦S latitude and
174 ◦E–177◦E longitude; solid black lines are 0.5◦(∼30 km), and
dashed black grid is 0.1◦(∼6 km).Opportunityrover is in Meridiani Planum in which ther-
mal spectra from orbit indicated an abundance (somewhat
unique) of a dark gray, coarse-grained mineral (hematite)
that typically forms in the presence of liquid water. Layers
associated with the hematite deposit in Meridiani Planum
suggested a sequence of sedimentary rocks that could be
interrogated by the rover. Meridiani Planum is a unique
portion of the ancient heavily cratered terrain in western
Arabia Terra that was downwarped and heavily eroded early
in Mars history and thus stands at a lower elevation than the
adjacent southern highlands (Fig. 5). The atmospheric and
surface characteristics inferred from the extensive remote
sensing data were correct for both, andSpiritandOppor-
tunitylanded safely. Geologic interpretations of materials
available for study were less successful (no fluvial or lake
sediments were found in Gusev crater) underscoring the
inherent ambiguity of understanding the geology and ma-
terials available for study from remotely sensed data.