334 Encyclopedia of the Solar System
FIGURE 2 Regional color mosaic of Chryse
Planitia, Ares Vallis, and theMars Pathfinder
landing ellipse. Mosaic shows catastrophic outflow
channels cutting the heavily cratered (ancient)
terrain to the south and flowing to the lower
northern plains. Ares Vallis is about 100 km wide
and 2 km deep and by analogy with similar features
on Earth formed in about a 2 week period when
roughly the volume of water in the Great Lakes
carved the valley. Note streamlined islands
produced during the flooding. TheMars Pathfinder
landing ellipse shown is 200×100 km and lies
about 100 km north of the mouth of the channel
where it exits the highlands and thus was
interpreted to be a depositional plain composed of
materials deposited by the flood. Characterization
of the surface after landing supports this
interpretation.suggested that water and near-surface ice might have accu-
mulated there, possibly leading to organic molecules and
life.Viking 2was sent to the middle northern latitudes
where larger amounts of atmospheric water vapor existed,
thereby ostensibly improving the chance for life. Landing
was deferred forViking 2, and the site selection team an-
alyzed images and thermal observations, before landing in
the mid-northern plains, just west of the crater Mie (Fig. 1).
Although predictions of the surfaces and materials present
at theVikinglanding sites were incorrect (likely due to the
newness of the data and the coarse resolution of the orbital
images), the atmosphere was within specifications and both
landed successfully.
TheMars Pathfindersite selection effort involved little
new data since theVikingmission 20 years earlier, but there
was a much better understanding of how the twoViking
landing sites related to the remote sensing data. The site
selection effort took place over a 2^1 / 2 year period prior to
launch and included extensive analysis of all existing data as
well as the acquisition of Earth-based radar data. An Earth
analog in the Ephrata fan near the mouth of a catastrophic
outflow channel in the Channeled Scabland of western and
central Washington State was identified and studied as an
aid to understanding the surface characteristics of the se-
lected site on Mars. Important engineering constraints, in
addition to the required low elevation, were the narrow lati-
tudinal band 15◦N± 5 ◦for solar power and the large landing
ellipse 300×100 km, which required a relatively smooth flat
surface over a large area. This and the requirement to have
the landing area covered by high-resolutionViking Orbiter
images (<50 m/pixel) severely limited the number of possi-
ble sites to consider (∼7). The landing site selected forMars
Pathfinderwas near the mouth of the catastrophic outflow
channel, Ares Vallis, that drained into the Chryse Planitia
lowlands from the highlands to the southeast (Fig. 2). Ares
Vallis formed at an intermediate time in Mars history (after
the early warm and wet period) and involved outpourings
of huge volumes of water (roughly comparable to the water
in the Great Lakes) in a relatively short period of time (few
weeks). The surface appeared acceptably safe, and the site
offered the prospect of analyzing a variety of rock types from
the ancient cratered terrain and intermediate-aged ridged
plains. Surface and atmospheric predictions were correct,
andPathfinderlanded safely.
Landing site selection for the Mars Exploration Rovers
took place over a 2^1 / 2 year period involving an unprece-
dented profusion of new information from theMars Global
Surveyor(launched in 1996) andMars Odyssey(launched
in 2001) orbiters. These orbiters supplied targeted data
of the prospective sites that made them the best-imaged,
best-studied locations in the history of Mars exploration.
For comparison, most of the ellipses were covered by
∼3 m/pixel Mars Orbiter Camera (MOC) images, whereas
theMars Pathfinderellipse was covered by∼40 m/pixel
Vikingimages. All the major engineering constraints were
addressed by data and scientific analyses that indicated
the selected sites were safe. Important engineering re-
quirements for landing sites for these rovers included rel-
atively low elevation, a latitude band of 10◦Nto15◦S for
solar power, and ellipse sizes that were ultimately less than
100 km long and 15 km wide. Because of the smaller el-
lipse size compared toPathfinder,∼150 sites were initially
possible from which high science priority sites were se-
lected for further investigation. Both sites selected showed