New Mexico State University planetary scientist Jim Murphy is among the scientists working to ascertain the safety of the landing sites for the two Mars Exploration Rovers that are scheduled to land in January 2004.

Using data obtained mainly from NASA's Mars Global Surveyor, which has been orbiting the planet for six years, Murphy is aiding in the analysis of two sites to predict what conditions the rovers might experience after they parachute and bounce down to the surface.

"The main concern is cold temperatures that can affect the instruments," he said. The rovers will touch down just south of Mars' equator during late southern winter and their expected 90-day mission lifespans will keep them operating until early autumn. Surface temperatures will likely range from zero to minus 100 Celsius, or roughly from freezing to 150 below zero on the Fahrenheit scale, he said.

The landing locations under scrutiny now were chosen because of strong signs that they may have held bodies of water at one time. One site, known as Gusev Crater, looks like a big lake bed with a winding riverbed feeding into it, Murphy said. The other site, Meridiani Planum, has the spectrographic signature of a mineral called gray hematite, which usually forms in the presence of liquid water. The two sites, both slightly south of the equator, are on opposite sides of the planet. By late October, officials at NASA Headquarters will have to commit to the rovers' landing sites, he said.

"A tremendous amount of effort has gone into evaluating possible landing sites in the past two years, to maximize the probability of mission success," Peter Theisinger, Mars Exploration Rover project manager at NASA's Jet Propulsion Laboratory in California, said in a NASA news release. Images and measurements from two NASA spacecraft orbiting Mars, Global Surveyor and Odyssey, have provided Murphy and other scientists and engineers evaluating potential landing sites with details of topography and geology.

"With data from the orbiters, we can use our modeling capabilities here to execute a model over a day-night time period and calculate the maximum and minimum temperatures," said Murphy, whose specialty is the atmosphere and the weather on Mars. "We know the reflexive characteristics of the surface materials, and the heating and cooling characteristics as the surface is subjected to sunlight and darkness."

Potential winds are another concern, he said, because the spacecraft will be landing during a season that is favorable to global dust storms on Mars.

The robotic rovers are carrying cameras, spectrometers and geology instruments to analyze their surroundings. "We should get some fabulous pictures," Murphy said.

Speculation about life on Mars has fascinated people since Percival Lowell, founder of the Lowell Observatory in Arizona near the turn of the 20th century, wrote about a system of canals on Mars and suggested they were a sign of civilization.

As astronomers have learned more about the planet, the conventional wisdom has gone through some rather wide swings, Murphy noted. "When the first spacecraft, Mariner 4, flew by Mars 38 years ago and took 21 pictures, it saw only 1 percent of the surface of the planet," he said. "It was heavily cratered like the moon, suggesting the planet was biologically dead."

But with subsequent missions, "we saw the entire planet, and we saw volcanoes, and we saw the Mariner Valley, and we saw that one hemisphere is heavily cratered but the other is not."

Under current conditions, water can exist as a vapor or as ice on the surface of Mars but it is not stable in liquid form because the planet's atmosphere is too thin. Whether liquid water ever existed on the surface, and whether it might exist now beneath the surface, are still unanswered questions.

"If these rovers can convincingly demonstrate that there was substantial liquid water at some point, that would be an important finding," Murphy said.

On the other hand, if the rovers find no evidence of water at these two sites, which were chosen because they seem to be compelling candidates for signs of water, "that would be interesting, too, and other theories will need to be developed," he said.

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