Mars: Landing Site Geology, Mineralogy and Geochemistry 339
FIGURE 11 False color mosaic of theOpportunitylanding site
showing dark, basaltic sand plain and the rim of Eagle crater in
the foreground (brighter). Note light toned pavement outcrop
near the rim, which is slightly brighter and dustier than the
plains. Parachute and 1 m high backshell thatOpportunityused
to land are 450 m away and demonstrate the exceptionally
smooth, flat surface as expected from orbital data. The dust-free
surface of the plains is in agreement with their very low albedo
from orbital data. The ridge on the horizon to the left is the rim of
Endurance crater about 800 m away thatOpportunitytraversed
to and drove into to study the stratigraphic section. Even though
dust has rapidly fallen on the solar panels, the basalt surface is
relatively dust-free, indicating that the dust is being swept off the
surface at a rate that roughly equals its deposition rate.
engineering parameters important for safely landing space-
craft such as elevation, atmospheric profile, bulk density,
rock distribution and slope can be adequately constrained
using available and targeted remote sensing data.
3.2 Global Geochemical Units
The compositions of surface materials on Mars can be
inferred from measurements of heat emitted from the
planet’s surface. Thermal emission spectrometers on the
Mars Global SurveyorandMars Odysseyorbiting space-
craft reveal two distinct kinds of spectra (thermal energy
emitted as a function of wavelength). Based on spectral
similarity to rocks measured in the laboratory on Earth, Sur-
face Type 1 material is interpreted as basaltic rock and/or
sand derived from basalt (Fig. 12). Basalt consists mostly
of silicate minerals—pyroxene, feldspar (plagioclase), and
olivine—and forms by partial melting of the upper mantle
producing a mafic (magnesium and iron rich) magma that
erupts on the surface as a dark lava flow (or shallow intru-
sion). Basalt is the most abundant type of lava on Earth,
comprising the floors of the oceans and significant flooded
areas of the continents, and it is no surprise that it is com-
mon on Mars as well. The giant shield volcanoes of Olympus
Mons and the Tharsis Montes are likely composed of basalts
based on their similar morphology to shield volcanoes as
well as the many plains that resemble basalt plains on Earth.
Surface Type 2 material is variously interpreted as either
andesite or partly weathered basalt; the spectrum is con-
sistent with either possibility (Fig. 12). Andesite is another
FIGURE 12 Examples of Surface Type 1 and Surface Type 2
thermal emission spectra, from theMars Global Surveyor
spacecraft. Surface Type 1 spectra match laboratory spectra of
basalt. Surface Type 2 spectra could be either andesite, a more
silica rich volcanic rock, or slightly weathered basalt.common lava type on Earth, occurring primarily at sub-
duction zones. Andesite contains pyroxene (or amphibole)
and feldspar. Andesite can form when mafic crystals form in
cooling basaltic magma and are extracted from the liquid,
leaving an andesitic liquid behind. The spectra of Surface
Type 2 can also be explained as a mixture of basaltic minerals
plus clays, which commonly form when basalt is weathered
by interaction with water.
The thermal emission spectrometers have fairly large
footprints (one is about 5 km/pixel and the other is
100 m/pixel), so they cover big regions. Mars surface spec-
tra (Fig. 12) represent mixtures of spectra for the individual
minerals that comprise the rocks and soil. The spectrum
can be unmixed (“deconvolved”) into the spectra for con-
stituent minerals, allowing not only their identification but
also an estimate of their proportions. Because we know the
chemical compositions of the minerals in the spectral li-
brary and the proportions needed to produce the measured
spectra, it is possible to calculate the chemical composition
of the mixture. That is important because volcanic rocks
are usually classified based on their chemistry rather than
their mineralogy (minerals in volcanic rocks are small and
hard to identify, and quickly solidified magmas often form
glass rather than crystalline minerals). The commonly used
chemical classification for volcanic rocks, based on the mea-
sured abundances of the alkali elements (sodium and potas-
sium, expressed as oxides) versus silica (silicon dioxide), is
shown in Fig. 13. The estimated chemical compositions of
Surface Type 1 and Surface Type 2 are illustrated in this
figure.
In addition to these major units, a few areas on Mars
show the distinctive thermal spectra of hematite, iron oxide