Mars: Landing Site Geology, Mineralogy and Geochemistry 341
subrounded clasts that range in size from several meters
diameter down to small pebbles that are a centimeter or
less in diameter. Most appear as float, or individual rocks
not associated with a continuous outcrop or a body of rock.
Many appear dust covered and there is evidence at Gusev
for some surface chemical alteration as is common on Earth
(see next section), where rocks exposed to the atmosphere
develop an outer rind of weathered material. Although the
composition of rocks could not be measured at theViking
1 and 2 landing sites (Figs. 5, 6 and 7), their dark angu-
lar and occasionally pitted appearance is consistent with a
common igneous rock known as basalt. Rocks making up the
cratered plains on whichSpiritlanded and traversed (Figs.
9 and 10) for the first few kilometers are clearly made up
of basalts (see next section). The distribution and shape of
many of the rocks at theViking 1and 2 landing sites and the
Gusev cratered plains are all consistent with a surface that
has experienced impact cratering with the rocks constitut-
ing the ejected fragments. Many subrounded rocks at the
Mars Pathfinder(Fig. 8) andViking 1landing site have been
attributed to deposition in the catastrophic floods in which
motion in the water partially rounded the clasts. Some rocks
at the Pathfinder site had textures that looked like layers
(perhaps sedimentary or volcanic), one resembled a pillow
basalt in which hot lava cools rapidly in the presence of wa-
ter, and several rocks resembled conglomerates, in which
rounded pebbles and cobbles were embedded in a rock,
in which the cobbles were rounded by running water and
later cemented in a finer grained matrix. At most of the
landing sites, some rocks known as ventifacts, appear pol-
ished, fluted, and grooved as a result of sand-sized grains,
entrained by the wind, that impact and erode the rocks.
4.3 Outcrop
Continuous expanses of rocks typically referred to as out-
crop (or bedrock) have been observed at three of the landing
sites. An area of continuous jointed rocks has been observed
at theViking 1landing site, but little else is known about
it (Fig. 6). Outcrop has been discovered in the Columbia
Hills bySpiritwhere there may be coherent stratigraphic
layers in and nearby the Cumberland Ridge on the flank
of Husband Hill (Fig. 15). These rocks, described in the
next section, appear to be layers of ejecta or explosive vol-
canics deposited early in Mars history. In places, the rocks
are finely layered, and in other places they appear massive.
At Meridiani Planum light-toned outcrops are exposed in
crater walls and areas where the covering dark, basaltic sand
sheet is thin (Fig. 11). These outcrops appear to be thinly
laminated evaporites that formed via evaporation of sub-
areal salt water (see next section) early in Mars history. The
layers are composed of sand-sized grains of fairly uniform
composition that appear to have been reworked by the wind
in sand dunes before being diagenetically altered by acid
groundwater of differing compositions (see next section).
FIGURE 15 Color mosaic of the northeast flank of Husband
Hill showing layered strata called Methuselah dipping to the
northwest. These rocks are clastic rocks consistent with impact
ejecta that have been highly altered by liquid water. Hills in the
background are the rim of the 20 km diameter Thira crater near
the eastern end of the landing ellipse shown in Fig. 4.4.4 Soils
All the landing sites have soils composed of generally small
fragments of granules, sand, and finer materials. Except
where they have been sorted into bedforms by the wind,
they have a variety of grain sizes and cohesion, even though
their composition appears remarkably similar at all the land-
ing sites (dominantly basaltic). Crusty to cloddy and blocky
soils are also present at most of the landing sites and are
distinguished as more cohesive and cemented materials.
These materials appear to be the duricrust inferred to be
present over much of Mars based on higher thermal in-
ertia, but generally low rock abundance. Strong cemented
light-toned duricrust was uncovered at theMars Pathfinder
site bySojournerand may contribute to the higher thermal
inertia at this site than at the others. Some bright soil de-
posits outside the reach of the arm at theViking 1landing
site (Fig. 6a) show layers and hints of coarse particles that
could be fluvial materials deposited by the Maja or Kasei
Valles floods.4.5 Eolian Deposits
Most of the landing sites have examples of eolian bedforms,
or materials that have been transported and typically sorted
by the wind. Sand-sized particles that are several hundred
micrometers in diameter can be moved by saltation in which
they are picked up by the wind and hop in parabolic arcs
across the surface. Because these particles can be preferen-
tially moved by the wind, they are effectively sorted by the
wind into bedforms. Sand dunes form when sand-sized par-
ticles are sorted into a large enough pile to move across the
surface. Sand dunes take a variety of forms such as barchan
or crescent-shaped (horns pointing downwind), star-shaped
from reversing winds, transverse to the wind, and longitudi-
nal or parallel to the wind, but they are generally diagnostic