340 Encyclopedia of the Solar System
FIGURE 13 Alkalis (Na 2 O+K 2 O) versus silica (SiO 2 ) diagram,
commonly used to classify volcanic rocks. Several estimates of
the compositions of Surface Type 1 and Surface Type 2 materials
are shown, along with the measured compositions of martian
meteorites and APXS analyses of rocks from theMars Pathfinder
andSpiritlanding sites (Gusev). See text for discussion of rock
types and classes.
usually formed by interaction with water. The Meridiani
Planum region has the highest concentration of hematite
measured from orbit, which as discussed earlier led to its
selection as a landing site for theOpportunityrover.
The global distribution of these spectrally identified units
on Mars is distinctive (Fig. 14). The southern hemisphere of
Mars is heavily pocked with impact craters, indicating that
it is very ancient. This material is mapped mostly as Surface
Type 1. In contrast, much of the northern hemisphere is
topographically lower than the terrain to the south, and it is
extremely smooth and relatively uncratered. The surface of
the northern lowlands is inferred to be much younger than
FIGURE 14 Global map showing the distributions of Surface
Type 1 (green) and Surface Type 2 (red) materials, based on
thermal emission spectroscopy fromMars Global Surveyor.
Dust-covered areas where this technique cannot distinguish rock
units are shown in blue. The preponderance of red spectra of
Surface Type 2 in the northern lowlands is consistent with these
materials being slightly weathered basalts.
the southern highlands, although the basement beneath this
surface layer is also old. Within this northern basin are lo-
cated most of the Surface Type 2 materials. The distribution
of global geochemical units is illustrated in Fig. 14. About
half of the surface of Mars is covered with a layer of dust,
which precludes the thermal emission spectrometers from
mapping the compositions of the rocks that underlie the
dust. Unfortunately, most of the spacecraft landing sites
on Mars are located near the equator (this constraint maxi-
mizes the solar energy received by landers or rovers), which
is also where most dust is concentrated. Consequently, it is
difficult to compare interpretations of orbital spectra with
rocks actually on the ground. The two MER landing sites
are exceptions—Spiritlanded in a region mapped as Sur-
face Type 1, and theMars Odysseysite in Meridiani was
selected because of its hematite spectral signature.4. Landing Site Geology
4.1 Introduction
The geology of the five landing sites has been investigated
from color, stereo, panoramic imaging that provides infor-
mation on the morphology of the landing sites, on the lithol-
ogy, texture, distribution, and shape of rocks and eolian and
soil deposits and on local geologic features that are present.
All landing sites that have been investigated on Mars are
composed of rocks, outcrops, eolian bedforms and soils,
many of which are cemented. Craters and eroded crater
forms are also observed at almost all of the landing sites, and
other hills have been observed at some of the landing sites.
Our knowledge of how the surfaces at the different landing
sites developed and the important geological processes that
have acted on them is directly related to the mobility of the
lander (arm) or rover and the ability of the lander or rover to
make basic field geologic observations. The lack of mobility
of the twoVikinglanders and the inability to analyze rocks
at these sites hampered our ability to constrain their geo-
logic evolution. In contrast, even the limited mobility of the
Sojournerrover and its ability to make basic field observa-
tions over a couple of hundred square meter area resulted in
a much better understanding of the geology and the events
that shaped the Ares Vallis surface. The two Mars Explo-
ration Rovers that traversed over 6 km each have collected
a robust collection of geologic observations over a wide area
that have resulted in a much better knowledge of the geo-
logic evolution of the rocks and surfaces investigated. This
section will review the basic geological materials found at
the five landing sites and discuss the landforms present.4.2 Rocks
Rocks are common at all of the landing sites (except Merid-
iani). At most sites, they are distinct dark, angular to