WASHINGTON, Jan. 6 (UPI) -- In July and September 1976, the United States achieved a feat as momentous as sending astronauts to the moon. Two pool-table sized landers, called Viking 1 and 2, set down successfully on the surface of Mars.
The twin Viking spacecraft -- each husbanded by its own orbiting surveillance satellite -- treated earthbound observers to the first close-up views of the Martian surface. They also made the first attempts to see if the red-colored soil harbored any living organisms. Mission scientists had devised mechanical arms that would scoop up handfuls of soil and dump them into miniature onboard laboratories for analysis. The idea was if anything was alive in the Martian dust, the Vikings' instruments could detect it.
Hopes for such a landmark discovery were short-lived. Neither lander found the slightest trace of organic activity. Nor did Sojourner, the microwave-oven-sized rover that explored Mars along with its parent, the Pathfinder lander, in July 1997.
Now, in the new year, NASA again is sending spacecraft to the Martian surface. The first, Spirit, settled to a perfect landing on Jan. 3. But the mission of Spirit, and its twin, Opportunity -- which is scheduled to touch down on Jan. 25 -- has shifted somewhat. Instead of looking for organic life directly, the two new rovers will attempt to prove that water once flowed on the surface of Mars.
Water is the key to life. Despite fictional lore about beings whose biochemistry is based on exotic substances, modern science knows of no other substance that can promote the growth of long, complex molecules essential for organic processes. Many worlds receive sunlight and many worlds harbor carbon and oxygen -- two other life necessities -- but very few offer the life-sustaining environment provided by water.
That is why the mission scientists for the Mars Exploration Rovers, or MERs, as they are called, have spent the past several years painstakingly researching 155 potential landing sites with the best possible chances for finding evidence of water. That effort resulted in Spirit's landing in Gusev Crater, a 3.9-billion-year-old impact basin that most likely once was the site of a large lake. Later this month, Opportunity is supposed to land on the Meridiani Planum, a flat lowland near the Martian equator that holds the largest deposit of hematite on the planet, an iron compound almost always produced in water.
"These sites are the best-studied of anywhere on the Martian surface," said Matt Golombek, an MER mission scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "There is no place else on Mars that has been given this kind of imaging coverage. There are no places that we know better."
Although liquid water is unlikely to exist on Mars today -- even during the Martian summer, the mean surface temperature rarely climbs above 0 degrees Fahrenheit -- the evidence it once flowed across the red planet is overwhelming.
Nearly 30 years of photographic surveys of the planet -- beginning with the Viking orbiters in the late 1970s and continuing with NASA's Global Surveyor and Odyssey satellites, and the just-arrived European Mars Express orbiter -- have produced thousands of images of surface features that almost certain were produced by flowing or pooling water.
The patterns in the red surface are there. Rivers, streams, channels, lakes and even seas seem once to have proliferated the Martian landscape -- or at least part of it.
As Golombek explained, one-third of the Martian surface is a flat lowland that sits more than one mile lower than the average elevation, giving all the appearances of holding an ancient ocean -- big enough to cover the entire planet to a depth of about 300 feet.
Gusev sits at the edge of that ocean, a "classic crater lake," he said. Meridiani is located in a highland region, but in a part that is unusually low and denuded of sharp features, "as if it had been planed off," he added.
A sizable fraction of the ancient Martian seawater remains locked in the polar ice caps -- although the caps seem to be composed mostly of frozen carbon dioxide, also known as dry ice. Some probably is held as ice below the surface. The rest simply evaporated into space, the product of sunlight splitting water molecules into their constituent hydrogen and oxygen.
Whatever fate befell Martian water, Spirit and Opportunity were designed to determine whether liquid H2O ever dripped or coursed or crashed in waves on the Martian surface. In the coming weeks, that is the message the mission scientists will be eagerly awaiting.
Based on those assumptions, and given the care taken in selecting the landing sites, finding anything else would be disconcerting. "If we find only volcanic rocks," Golombek said, "that would be a big setback."
Next: Robotic landers as field astrobiologists
Phil Berardelli is UPI's Science & Technology Editor. E-mail firstname.lastname@example.org
(Editors: UPI photo WAS2004010651 is available)