NASA has chosen two scientifically compelling landing
sites for twin robotic rovers to explore on the surface of
Mars early next year. The two sites are a giant crater that
appears to have once held a lake, and a broad outcropping of
a mineral that usually forms in the presence of liquid
water.

Each Mars Exploration Rover (MER) will examine its landing
site for geological evidence of past liquid water activity
and past environmental conditions hospitable to life.

“Landing on Mars is very difficult, and it’s harder on some
parts of the planet than others,” said Dr. Ed Weiler, NASA
associate administrator for space science in Washington. “In
choosing where to go, we need to balance science value with
engineering safety considerations at the landing sites. The
sites we have chosen provide such balance.”

The first rover, scheduled for launch May 30, will be
targeted to land at Gusev Crater, 15 degrees south of Mars’
equator. The second, scheduled to launch June 25, will be
targeted to land at Meridiani Planum, an area with deposits
of an iron oxide mineral (gray hematite) about two degrees
south of the equator and halfway around the planet from
Gusev.

Which rover is targeted to a specific site is still
considered tentative, while further analyses and simulations
are conducted. NASA can change the order as late as
approximately one month after the launch of the first rover.
The first mission will parachute to an airbag-cushioned
landing on Jan. 4, 2004, and the second on Jan. 25, 2004.

“A tremendous amount of effort has gone into evaluating
possible landing sites in the past two years, to maximize
the probability of mission success” said Peter Theisinger,
MER project manager at NASA’s Jet Propulsion Laboratory
(JPL), Pasadena, Calif.

Images and measurements from two NASA spacecraft orbiting
Mars provided scientists and engineers, evaluating potential
landing sites, with details of candidate site topography,
composition, rockiness and geological context.

“Meridiani and Gusev both show powerful evidence of past
liquid water, but in very different ways,” said Dr. Steve
Squyres, principal investigator for the rovers’ science
toolkit and a geologist at Cornell University, Ithaca, N.Y.
“Meridiani has a chemical signature of past water. Gray
hematite is usually, but not always, produced in an
environment where there is liquid water. At Gusev, you’ve
got a big hole in the ground with a dry riverbed going right
into it. There had to have been a lake in Gusev Crater at
some point. They are fabulous sites, and they complement
each other because they’re so different,” he said.

MER site selection began with identifying all areas on Mars
that fit a set of engineering-driven requirements, said
JPL’s Dr. Matt Golombek, co-chair of a landing-site steering
committee. To qualify, candidate sites had to be near the
equator, low in elevation, not too steep, not too rocky and
not too dusty, among other criteria; 155 potential sites
were studied. A series of public meetings evaluated the
merits of potential landing sites. More than 100 Mars
scientists participated in the meetings.

“These two landing sites have been studied more than
anywhere else on Mars. Both sites have specific scientific
hypotheses that can be tested using the instruments on board
each rover. It should be a very busy and exciting time after
landing for the scientists analyzing the wealth of new data
from the ground,” said Dr. Cathy Weitz, MER program
scientist at NASA Headquarters, Washington.

“Clearly there is tremendous interest in the science
community in what these missions can accomplish and
eagerness to help see that the rovers go to the best
possible sites,” said National Air and Space Museum’s Dr.
John Grant, the steering committee’s other co-chair.

Once they reach their landing sites, each rover’s prime
mission will last at least 90 Martian days (92 Earth days).
The rovers are solar-powered, and in approximately 90 days,
dust accumulating on the solar arrays likely will be
diminishing the power supply.

The twin MER spacecraft are at NASA’s Kennedy Space Center,
Fla., in preparation for launch. JPL manages the MER Project
for NASA’s Office of Space Science, Washington. JPL is a
division of the California Institute of Technology in
Pasadena.

Information about the MER project is available online at:
http://mars.jpl.nasa.gov/mer/