336 Encyclopedia of the Solar System
FIGURE 5 Regional setting of Meridiani Planum in MOLA
shaded relief map (∼850 km wide). Note smooth, lightly
cratered plains on whichOpportunitylanded (cross), which bury
underlying heavily cratered (ancient) terrain with valley networks
to the south. The large degraded craters in the smooth plains
indicate that the sulfate rocks below the basaltic sand surface are
very old (>3.7 billion years). In contrast, the lightly cratered
basaltic sand surface thatOpportunityhas traverse is young.
3. Mars Landing Sites in Remotely
Sensed Data
3.1 Surface Characteristics
Understanding the relationship between orbital remote
sensing data and the surface is essential for safely landing
spacecraft and for correctly interpreting the surfaces and
kinds of materials globally present on Mars. Safely land-
ing spacecraft on the surface of Mars is obviously critically
important for future landed missions. Understanding the
surfaces and kinds of materials globally present on Mars is
also fundamentally important to deciphering the erosional,
weathering, and depositional processes that create and af-
fect the Martian surface layer. This surface layer or regolith,
composed of rocks and soils, although likely relatively thin,
represents the key record of geologic processes that have
shaped it, including the interaction of the surface and at-
mosphere through time via various alteration (weathering)
and eolian (wind-driven) processes.
Remote sensing data available for selecting landing sites
has varied for each of the landers, but most used visible
images of the surface as well as thermal inertia and albedo.
Thermal inertia is a measure of the resistance of surface
materials to a change in temperature and can be related
to particle size, bulk density, and cohesion. A surface com-
posed of mostly rocks will change temperature more slowly,
remaining warmer in the evening and night, than a surface
composed of fine-grained loose material that will change
temperature rapidly, thereby achieving higher and lower
surface temperatures during the warmest part of the day
and the coldest part of the night, respectively. As a result,
surfaces with high thermal inertia will be composed of more
rocks or cohesive material than surfaces with low thermal
inertia. Thermal inertia can be determined by measuring
the surface temperature using a spectrometer that measures
the thermal emission (temperature) at multiple times of the
day or by fitting a thermal model to a single temperature
measurement. Thermal observations of Mars have been
made by many orbiters, including theMariners,Viking,
Mars Global Surveyor, andMars Odyssey, with increas-
ingly high spatial resolution by the last three. In addition,
measurement of different thermal wavelength emissions
from the surface has been used to separate the area of the
surface covered by high inertia materials or rocks from the
area covered by lower inertia materials or soil. The albedo
is a measure of the brightness of the material in which the
viewing geometry has been taken into account.
Global thermal inertia and albedo data show that the
surface of Mars exhibits particular combinations that cover
most of the surface. One has high albedo and low thermal
inertia and is likely dominated by substantial thicknesses (a
meter or more) of bright red dust that is likely neither load-
bearing nor trafficable. These areas have very few rocks
and have been eliminated for landing solar-powered space-
craft, and they will likely be eliminated for rover missions
interested in studying rocks or outcrop. Moderate to high
thermal inertia and low albedo regions are likely relatively
dust free and composed of dark eolian sand and/or rock.
Moderate to high thermal inertia and intermediate to high
albedo regions are likely dominated by cemented crusty,
cloddy, and blocky soil units that have been referred to as
duricrust with some dust and various abundances of rocks.
Coarse resolution global abundance of rocks on Mars, de-
rived by thermal differencing techniques that remove the
high inertia (rocky) component, shows that the first type of
surface has almost no rocks and the latter two types of sur-
faces have rock abundances that vary from about 8% (the
global mode of rock abundance of Mars) to a maximum of
about 35% of the surface covered by rocks.
The five landing sites sample the latter two types of sur-
faces in the thermal inertia and albedo combinations that
cover most of Mars. Along with variations in their rock abun-
dance, they sample the majority of likely safe surfaces that
exist and are available for landing spacecraft on Mars. The
Vikinglanding sites both have relatively high albedo and
high rock abundance (∼17%), in addition to intermediate
thermal inertia. On the surface, these sites are consistent
with these characteristics, with both being rocky and some-
what dusty plains with a variety of soils, some of which are
cohesive and cemented (Figs. 6 and 7). Prior to landing, the
Mars Pathfindersite was expected to be a rocky plain com-
posed of materials deposited by the Ares Vallis catastrophic
flood that was safe for landing and roving and was less dusty
than theVikinglanding sites based on the intermediate to