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In the spring of 2003, NASA will launch two spacecraft on 35-million-mile journeys to Mars. Each craft will carry a small rover, equipped with a set of robotic geologic tools designed to search out mineral evidence within Martian rocks that water was once present on Earth’s neighboring planet.

The Mars Explorer Rovers, as they are tentatively dubbed, are larger and more-advanced descendants of the Sojourner rover that roamed the Martian surface in July 1997. Both MERs will arrive at the Red Planet early in 2004, but they will go to different, widely separated locations.

The question NASA is trying to answer now is where to send them.

JPL scientist Matt Golombek, who was the project scientist for the Pathfinder/Sojourner mission, is in charge of the landing-site-selection process for the MER missions. According to Golombek, the landing sites under consideration are “all focused on aqueous minerals, places where liquid water may have been stable, where you have a chance using the instruments on the rover to investigate a liquid water environment in the past on Mars.”

To begin the winnowing process, NASA recently held the First Landing Site Workshop for the 2003 Mars Explorer Rovers, which took p lace in January at Ames Research Center in Mountain View, CA. Over 100 scientists came together to offer suggestions, debate the merits of the different proposals and decide which candidates to pursue most vigorously.

There are hundreds of places on Mars that are scientifically interesting. But many of them are ruled out by the constraints placed on the mission by its engineers. NASA wants to make sure these rovers land safely. The trick is to find sites that both meet the stringent engineering constraints and have scientific value.

The landing mechanism that will be used for the MERs is the same as that used for Pathfinder/Sojourner. A small capsule drops out of the Martian sky. The capsule’s leading edge is an aeroshell a heat shield that uses the friction of the atmosphere to slow it down. After it has slowed considerably, the aeroshell is discarded and a parachute deploys. Just before it reaches the ground, rockets fire to slow it to nearly zero vertical velocity and a set of massive airbags inflates. When the airbag assemblage hits the ground, it bounces along the surface for a while before finally coming to a halt. Then the airbags deflate and retract, a set of petals opens and the rover rolls out.

As the capsule falls through the planet’s atmosphere, it can be destablilized by air friction. This can affect the precise direction in which it falls and thus the precise location where the spacecraft ultimately lands. One of NASA’s primary constraints in picking a site, therefore, is that it needs to be relatively smooth and boulder-free over the entire potential landing area, which can be as much as 200 kilometers (124 miles) long.

The site not only has to be smooth, it has to be level. If the rover encounters too severe a slope the limit is a 15-degree incline it could go into a runaway roll, like a car parked on a steep hill without the parking brake set.

Nor can the rovers land at a site too high in altitude. If the site were too high up, the Martian atmosphere would be too thin for the parachute to slow the airbag sufficiently during its descent. Instead of bouncing, the airbag would likely explode on impact.

Moreover, because the rovers are solar-powered, each rover must land within a prescribed range of latitudes near the equator to ensure that sufficient sunlight will be available for the duration of the mission. And the list goes on.

Despite these restrictions, dozens of sites were presented at the workshop for consideration.

One site favored most broadly by workshop attendees is known as “the hematite site.” It’s actually a collection of several different possible landing sites, all within close proximity and all part of a massive deposit of hematite.

Hematite is an iron oxide, gray to black in color, that on Earth is typically found in sedimentary deposits, where water has interacted with iron-bearing rocks. The hematite site is popular among scientists because it is one of the few places on Mars identified as containing aqueous minerals.

It was proposed by Phil Christensen of Arizona State University (ASU). Christensen, a member of the NASA Astrobiology Institute, is the principal investigator for the Thermal Emission Spectrometer. TES is one of the instruments aboard the Mars Global Surveyor (MGS), which has been orbiting Mars for the past three years. By taking temperature readings of the Martian surface from orbit, TES can identify many different types of minerals, including hematite.

Other sites that sparked interest at the workshop included two large crater lakes, Gale and Gusev. These sites were presented at the meeting by several researchers, among them Nathalie Cabrol, an NAI member from Ames Research Center. Both lakes contain large areas of what appear to be layers of ancient sedimentary deposits, the result of flowing or standing water.

Cabrol currently favors Gale Crater as a landing site because engineering constraints may preclude getting to the most interesting part of Gusev. Going to Gale would be valuable, Cabrol says, because it would help answer questions that can’t be answered by looking at photographs taken from orbit by the Mars Orbiter Camera, another of the instruments aboard the MGS.

“At MOC resolution,” Cabrol explains, “you cannot tell the difference between two meters of one layer of volcanic deposit and a succession of very thin lacustrine varves (annual sedimentary deposits) that total two meters, but which in fact are subdivided into multiple sub-millimeter layers.” If the camera on-board a MER rover saw these subdivided layers, says Cabrol, “you would know that they were deposited by a lake and not by a volcano.”

Perhaps the most spectacular site proposed was one in Melas Chasma, in Valles Marineris. From a scientific standpoint, Melas Chasma is interesting because it is one of the areas on Mars where MOC images reveal layered terrain that appears to have been deposited by water.

But a rover that landed in Melas Chasma would send back to Earth images not only of that layered terrain, but of perhaps the most spectacular scenery in the solar system. Valles Marineris is a massive canyon system some 4000 kilometers (2500 miles) long and up to 7 kilometers (4.3 miles) deep. The Grand Canyon, by comparison, is a mere squiggle, only 446 kilometers (277 miles) long and 1.8 kilometers (1.1 miles) deep.

Also of interest was the suggestion put forward by Jack Farmer of Arizona State University that the MERs might actually be able to detect signs of fossilized life in Martian rocks. The MERs’ scientific instruments were designed to look for specific minerals, not for life signs. Nevertheless, believes Farmer, who is the head of the NAI’s Mars Focus Group, the Microscopic Imagers on the MERs essentially powerful magnifying glasses may be able to detect distinctive patterns, characteristic of past life, in the rocks it observes.

What Next

In all, some 15 sites were selected by the workshop as “high-priority” sites. These sites will be targeted as locations at which the MOC will be scheduled capture additional high-resolution images. These MOC images will enable scientists to examine the proposed sites in greater detail and to clarify which ones meet the established safety requirements.

A second workshop in early 2002 will further narrow the selection.