WASHINGTON, Jan. 8 (UPI) -- How exactly does one determine, from a distance of 110-million miles, whether the planet Mars has ever played host to the organic processes related to the evolution of life?
In three previous successful landings -- Vikings 1 and 2 in 1976 and Pathfinder/Sojourner in 1997 -- the answer has been: with great difficulty. None of the previous efforts produced even the slightest hint of Martian life.
Now, nearly seven years later, NASA is trying again with a pair of robotic craft called Mars Exploration Rovers -- named Spirit and Opportunity -- that are much more adept at determining whether life on Mars ever existed. The landers, and the dazzling array of instruments they carry, are designed to tease much more information out of the rocks lying on the red planet's surface than their predecessors were capable of doing.
"Rocks are time capsules, each of which contains its own little history," said Dave Des Marais, a planetary geologist with NASA's Ames Research Center at Moffett Field, Calif. So when engineers designed the MERs, they attempted to endow the robotic craft with the ability even to grind away the weathered outer layer of the rocks and study their pristine interiors.
In fact, the twin rovers will be looking for specific types of rocks. Spirit, which touched down on the Martian surface on Jan. 3 in a perfect landing, will troll across an area called Gusev Crater, an ancient impact basin south of the planet's equator that is thought to have contained a lake.
The rocks there could be sedimentary, meaning they were built up by years and years of deposits falling to the bottom of that lake. They would be sitting on the surface because sometime in the past they were thrust up by a smaller meteor impact. At least, that's what the MER mission team thinks because Gusev is pocked with smaller craters.
Spirit's instruments will reveal a sedimentary rock if they encounter one. The most obvious sign would be a structure that appears to be layered. Other characteristics are more subtle:
--Interaction with water tends to change the chemical composition of rock-forming material, and Spirit's trio of sensitive spectrometers can detect these changes.
--Some minerals are formed when dissolved salts are left behind as water evaporates. If the instruments detect them, it would be proof the basin once held a salty lake.
--Limestone and other carbonate minerals form when chemical reactions pull carbon dioxide out of the air. They usually are associated with hot springs, but wherever they are found, they almost always mean substantial water was present, and long enough to build up rocky material.
Carbonate would be the holy grail of Martian mineral deposits, Des Marais said. On Earth, over 90 percent of all fossils are contained in carbonates. But he also said he considers making such a find extremely unlikely, because the oldest Earth carbonate deposits are only 2.7 billion years old, far younger than the rocks expected to be found at the Gusev site.
"We don't expect to find carbonate platforms on Mars," Des Marais said.
Better luck might occur with another water-associated material, called hematite, which is the target of Opportunity, scheduled to land on Jan. 24 in an area called Meridiani Planum, also near the Martian equator, and coincidentally just about smack on the prime longitudinal meridian established arbitrarily for the planet years ago.
Orbital surveys of the planet have revealed a truly unusual surface feature: a large concentration of gray hematite, a type of iron oxide -- rust. On Earth, gray hematite usually forms in the presence of water. Nearly one-fifth of the Meridiani area appears to be covered with it, but Opportunity might find it difficult to study gray hematite in the rocks during its expected three-month expedition. That is because "Meridiani Planum is one of the smoothest, flattest, safest places on Mars," said Matt Golombek, a member of the MER team, who monitors mission activities from NASA's Jet Propulsion Laboratory in Pasadena, Calif. So it is possible the rover will not find many rocks lying loose on the surface.
Although the MERs will be trying primarily to find links to Martian water from the rocks, they will be looking for other clues to the planet's past as well.
"We have reason to think that Mars has been volcanically active throughout its history," said Bruce Jakosky, director of the University of Colorado's Center for Astrobiology in Boulder. Finding volcanic rocks might be less dramatic than discovering sediments, but the rovers will be able to transmit a lot of information about the Martian geologic past from studying them.
"Even if we don't find evidence of life," Des Marais said, "then maybe we can study the processes that inhibited its origins.
Next: More missions in the works
Phil Berardelli is UPI's Science & Technology Editor. E-mail firstname.lastname@example.org
(Editors: UPI photos WAS2004010752, WAS2004010753, WAS2004010651, WAS2004010652, WAS2004010653, WAS2004010655, WAS2004010401, WAS23003123106 and WAS23003123107 are available)