**PRELIMINARY ENGINEERING CONSTRAINTS AND POTENTIAL LANDING SITES FOR
THE MARS EXPLORATION ROVERS.** M. Golombek^1^, T. Parker^1^, T.
Schofield^1^, D. Kass^1^, J. Crisp^1^, A. Haldemann^1^, P. Knocke^1^, R.
Roncoli^1^, W. Lee^1^, M. Adler^1^, N. Bridges^1^, S. Anderson^1^, J.
Grant^2^, and S. Squyres^3^, ^1^Jet Propulsion Laboratory, Caltech,
Pasadena, CA 91109, ^2^CEPS, Smithsonian Institution, Washington, D.C.
20560, ^3^Space Sciences, Cornell University, Ithaca, NY 14853.

**Introduction:** This abstract describes the preliminary definition of
engineering requirements on Mars Exploration Rover (MER) landing sites,
maps these requirements into remote sensing criteria, and uses these
criteria to identify potential landing sites. To first order, most of
the engineering requirements are the same as for the Mars Pathfinder
mission \[1\], because the landing system is the same.

Science objectives for the MERs relate to definition of the history of
water and climate on Mars at locations where conditions may have been
favorable for life. The goal of each MER is to determine the aqueous,
climatic, and geologic history of a site on Mars where conditions may
have been favorable to the preservation of evidence of possible
pre-biotic or biotic processes. Scientific criteria used in
identification and selection of landing sites include locations
possessing clear evidence for surface processes involving ancient water.
Hence, potential sites encompass a wide range of possible settings,
including former lacustrine or hydrothermal environments.

**Engineering Requirements:** Analysis of the entry, descent and landing
system and atmospheric profiles for the season and time of arrival
indicates that the MER spacecraft are capable of landing below --1.3 km,
with respect to the MOLA defined geoid \[2\]. This requirement stems
mostly from the need for an adequate atmospheric density column for the
parachute to bring the spacecraft to the correct terminal velocity.
Because the landing system has no means to reduce horizontal velocity,
low-altitude winds and wind shear together are major concerns and must
be below roughly \~20 m/s.

Preliminary analyses of power generation/usage and thermal cycling of
the rovers for the required 90 Sols restricts the landing sites to near
the subsolar latitude at arrival. This translates to 5°N to 15°S for
MER-A and 10°N to 10°S for MER-B, which arrives at Mars about 34 Sols
after MER-A. Operations and Deep Space Network coverage considerations
require the two landing sites to be separated by a minimum central angle
of 37° on the surface.

Because of the arrival geometry and prograde entry into the atmosphere,
landing ellipse size and orientation change significantly with latitude
and time of arrival. Preliminary analysis of the expected flight path
angle at atmospheric entry and dispersions produced by the atmosphere
for the opening of the launch period yield 3 sigma landing ellipses for
MER-A that vary linearly in length and azimuth from 77 km by 30 km,
oriented at 66° at 15°S to 219 km by 30 km, oriented at 88° at 5°N. For
MER-B, 3 sigma landing ellipses vary linearly in length and azimuth from
130 km by 30 km, oriented at 79° at 10°S to 338 km by 30 km, oriented at
99° at 10°N.

Surface slopes are an obvious concern for the landing system. Steep
slopes can spoof the radar altimeter and cause premature or late firing
of the solid rockets and airbag inflation. Small slopes over large
distances can lead to significant additional horizontal velocity and
prolonged bouncing by the lander within the inflated airbags.
Reconstruction of Mars Pathfinder landing indicates the lander traversed
a horizontal distance of about 1 km in more than 15 large bounces across
the surface \[3\], even though it landed at 3 AM local time when winds
should have been calm. Slopes can also affect the stability of the
lander, rover deployment and trafficability, and power generation. As a
result, surface slopes should generally be less than 15°. One relation
between measured radar RMS slopes and slope suggests surfaces with \<6°
RMS slopes have about 5% of their surfaces with slopes \>15° \[e.g.,
1\].

The airbags of the Mars Pathfinder landing system were qualified to
protect the lander from damage when landing on 0.5 m high rocks in any
orientation \[1\]. This required a landing site with less than 1% of the
surface covered by rocks greater than 0.5 m high. Model rock
size-frequency distributions based on Viking, Mars Pathfinder and rocky
locations on the Earth \[4\], generally suggest this requirement can be
satisfied at locations with total rock coverage of \<20% as derived from
thermal infrared measurements \[5\].

The surface must be radar reflective for the descent radar altimeter to
work properly, so radar reflectivity must be greater than \~0.05. The
surface must be load bearing for the rover and excessive dust would coat
rocks, which are of prime scientific interest (but which can impede
mobility), and could reduce surface lifetime by covering the solar
panels. Extremely high albedo and low thermal inertia regions should
therefore be avoided \[6\]. Areas with fine component thermal inertia of
less than 125-165 J m^-2^ s^-0.5^ K^-1^ or SI units should therefore be
avoided \[7, 8\].

**Potential Landing Sites:** MOLA elevations were plotted within the 25°
latitude band from 10°N to 15°S. Because the southern hemisphere of Mars
is dominantly heavily cratered highlands, little area is actually below
--1.3 km in elevation for the MER-A (5°N to 15°S). The largest region
below this elevation is in southern Elysium and Amazonis Planitiae.
Unfortunately, most of this area (150°W to 200°W) is dominated by
extremely low thermal inertia, with fine component thermal inertias
below 125 and so is excluded. For the latitude band of MER-B (10°N to
10°S), more area is below --1.3 km elevation. Nevertheless, most of the
area between 135°W and 190°W is excluded on thermal inertia grounds.
Areas available to seek landing sites are thus reduced to southern
Isidis and Elysium Planitiae in the eastern hemisphere and western
Arabia Terra, Terra Meridiani, Xanthe Terra, Chryse Planitia, and the
bottom of Valles Marineris in the western hemisphere.

Landing ellipses were placed in all locations that are below --1.3 km in
elevation, have acceptable fine component thermal inertia values, and
are free of obvious hazards in the MDIMs (Mars Digital Image Mosaics).
This is exactly the same procedure employed to initially identify
potential landing sites for Mars Pathfinder. Only sites that appear
smooth and flat in the MDIM without scarps, large hills, depressions or
large fresh craters (\>5 km) were acceptable.

Nearly 200 potential landing sites meet these criteria: 100 sites for
MER-A and 85 for MER-B. Even though the area available to land north of
the equator is at least twice as great as south of the equator, the
smaller ellipse size towards the south compensates. Geologic units
accessible are diverse and range from Noachian Plateau dissected, hilly,
cratered, and subdued cratered units to Hesperian ridged plains, channel
materials, and the Vastitas Borealis Formation to Amazonian smooth
plains, channel materials, volcanics, knobby materials, and the Medusae
Fossae Formation. A complete listing of all of the sites can be viewed
at <http://webgis.wr.usgs.gov/mer> and at
http://marsoweb.nas.nasa.gov/landingsites/mer2003. For comparison, the
landing ellipse for Mars Pathfinder (300 km by 100 km) and the 10°
latitude band reduced available sites to just 10 \[1\].

**Future Plans:** Landing site selection activities include a first
landing site workshop scheduled for January 2001. Abstracts were
submitted and presentations on various aspects of landing site selection
relevant for the MER missions have been scheduled. High-priority science
sites in these abstracts and those identified by the community are being
targeted by the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC)
during the initial period of the extended mission. Roughly 30 sites were
identified, with remarkable consensus on one site, the Hematite site
centered near 2°S, 2°W \[9\]. This site is the subject of about 10 talks
in the workshop and was recommended by 14 participants. Next highest
requested sites were crater lakes, with 5 recommendations for Gale, 3
for Gusev, 4 for other crater lakes, and 2 for a similar fluvial outflow
region. Next highest were sites in Valles Marineris, with 4
recommendations for canyon sites, 4 for Valles Marineris outflow sites,
and one for the Ganges sand sheet. Beyond these sites that received
multiple recommendations are sites in Elysium, Isidis and the highlands.
Note that if one lander were targeted to the Hematite site, the other
lander could land no closer to the first than Valles Marineris due to
the 37° operations exclusion zone.

Because the number of sites that meet the basic engineering constraints
is so large, this process uses the science objectives of the mission as
the main discriminator of sites to be investigated in detail. This
contrasts markedly with previous site selection efforts for the Viking
and Mars Pathfinder landers, in which the sites studied in detail were
dominated by engineering considerations \[1, 10\].

Following this meeting a prioritized list of prospective landing sites
will be targeted by MOC and other instruments. Following acquisition of
new MOC images and landing site evaluations, the top sites for each
rover will be selected sometime between June and October 2001. These
sites would undergo further scrutiny and additional new MOC images would
be acquired throughout 2001 and early 2002, when Mars Odyssey mission
begins acquiring THEMIS (Thermal Emission Imaging System) data. Another
open landing site workshop will be scheduled around April 2002 that
focuses in detail on the highest priority sites. The project will select
the highest 2 priority locations about 1 year before launch, when
guidance algorithms are needed for the Delta launch vehicle engineers.
These locations are boxes 10° in latitude by 15° in longitude. Landing
sites within these boxes will continue to be evaluated until launch in
June 2003, with final targeting of the specific landing ellipses at the
first trajectory correction maneuver after launch.

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Golombek, M. P. et al. (1999) *JGR 104,* 8523-8553. \[4\] Golombek, M.,
& Rapp, D. (1997) *JGR* 4117-4129. Golombek, M. P. et al. (1999) *JGR
104,* 8585-8594. \[5\] Christensen, P. R. (1986) *Icarus, 68*, 217-238.
\[6\] Christensen, P. R. & H. J. Moore (1992) in [MARS]{.underline}, U.
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9985-9998; (1986) *JGR 91*, 3533-3545. \[8\] Mellon, M. T. et al. (2000)
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