MARS

Keys to past life on Mars sought in Earth's dry lakes

SIGNS OF LIFE: Second of a three-day series

DEATH VALLEY - Clods of light brown dirt crunch beneath boot heels as scientists lug electronics gear across a dry lake bed stretching to dark volcanic peaks in the distance.

This portion of the vast sun-baked playa looks oddly like a plowed farmer's field ready for planting. The chunked earth shatters under foot like frozen slush.

``This whole basin here has bathtub rings of minerals that were left behind as the lake dried,'' Arizona State University geologist Jack Farmer said as he looked across the flats.

``We think there are similar settings on Mars,'' said Farmer, who heads Arizona State's NASA-funded astrobiology team.

Astrobiology is the study of life's origins and the search for life beyond our planet.

ASU researchers are helping the space agency identify places on Mars most likely to hold fossil evidence of ancient microbial life.

That effort takes Farmer, who specializes in microbial fossilization processes, to the Death Valley playas and to the hot springs of Yellowstone National Park.

Evaporating lakes and hot springs are among the best places to find fossil microbes.

In both settings, dissolved minerals are deposited quickly, trapping and preserving microbes such as insects stuck in amber.

Young Mars may have had lakes and hot springs. If so, the geologic deposits they left behind are the best places to look for traces of ancient life, Farmer said during a recent trip to Death Valley.

He stood on a lake bed coated with more than a dozen minerals called evaporites that crystallized when the water dried up.

``The key to finding past life on Mars is finding those kinds of minerals at playas or hot springs,'' Farmer said. ``You have to go to very specific environments where the mechanisms of preservation are operating.''

But finding ancient Martian lake beds and hot-springs minerals is tough.

Much of the arid planet is cloaked in fine reddish dust that piles into dunes and sometimes shrouds the surface in global dust storms.

Reaching ancient lake sediments could mean digging beneath 30 feet of dust in some places, NASA researchers wrote last year in ``Astrobiology Roadmap,'' a guide to the space agency's search for extraterrestrial life in the new millennium.

Digging through 30 feet of dust ``requires sampling capabilities beyond the current state-of-the-art,'' the report said.

``We're seeing a lot of areas that are covered by fields of dunes or just sheets of dust you can see sort of banking up against craters,'' said William K. Hartmann of Tucson's Planetary Science Institute.

Hartmann is a member of NASA's Mars Global Surveyor science team. That robotic spacecraft is orbiting the planet and mapping its entire surface.

``So we're flying around, kind of looking for places where the dust has been removed,'' he said.

In some places, the Martian wind has scoured the surface down to bare rock.

Global Surveyor researchers are searching those sites for water-formed minerals, using a heat-sensitive device designed at Arizona State University.

The thermal emission spectrometer, known as TES, was designed by Philip R. Christensen's research team. It identifies minerals by measuring their color in the infrared portion of the spectrum.

The spectrometer has been mapping Mars since Global Surveyor's main mission began in 1998.

So far, TES has seen no sign of salts or carbonates like limestone - minerals considered the best indicators of ancient lakes or oceans.

And TES has seen no trace of quartz or calcite, two minerals commonly formed in hot springs.

But the infrared instrument did find an outcropping of the mineral hematite about half the size of Arizona, Christensen said.

Hematite is an iron-rich mineral that usually forms in water.

On Earth, vast deposits of hematite around the Great Lakes were formed in an ocean several billion years ago, creating the primary source of iron in the United States.

The Martian hematite deposit found by TES is in a flat region of layered deposits that could be sediments from a lake or small sea, Christensen said.

The hematite region is NASA's primary landing site for a Mars probe scheduled for launch next year, he said.

The selection of landing sites is driven in part by the three big questions that underpin NASA's Mars exploration program, said Carl Pilcher, the agency's director of solar system exploration.

The key questions: Did life ever arise on Mars? What caused the planet to become a cold desert? Could Mars support humans?

The Mars program received $248.4 million in fiscal year 2000. President Clinton asked for $326.7 million for the coming fiscal year.

On Mars, answering the life question means finding the places that have or once held liquid water, since all known forms of life require it.

``Look for water, then life,'' is a search strategy that applies to Mars, to Jupiter's moon Europa, and to Earth-like planets that may be orbiting other stars.

``The idea is that it's more effective to look for the places that could support life, and then to look for life within those places, than to just try to go for the gold and just look for evidence of life out there,'' said David Des Marais, a biogeochemist at NASA's Ames Research Center in Mountain View, Calif.

Some scientists suspect there is liquid water below the surface of Mars today and - who knows - maybe microbes thrive there.

NASA is developing a robotic ``mole'' to bore hundreds of meters beneath the Mars surface in search of hidden reservoirs, but that kind of deep drilling is more than a decade away, said Sylvia Miller of NASA's Jet Propulsion Laboratory.

In the near future, unmanned probes will sample rocks and soil on, or within a few meters of, the surface.

Since there is no liquid water on Mars' surface today, and because it is bombarded by ultraviolet rays that break apart complex molecules, extant surface life is considered highly unlikely.

In 1976, two Viking landers looked for live microbes on the Mars surface and found none.

But four years before the Vikings arrived, the Mariner 9 probe sent back pictures of water-carved channels, suggesting that early Mars may have been much wetter and more Earth-like.

Some scientists suspect the northern lowlands of Mars once held an ocean.

Traces of past Mars life - if there ever was any - may be locked inside surface rocks formed in those long-gone waters.

If you could find those rocks, what kind of life might they contain?

Earth and the other planets formed about 4.5 billion years ago, and for more than 2 billion years bacteria were our world's sole inhabitants.

Higher life forms emerged only after photosynthetic cyanobacteria, also known as blue-green algae, created an oxygen-rich atmosphere.

If life got started early on Mars - say 3 to 4 billion years ago - but disappeared after 1 or 2 billion years, it may not have had a chance to evolve beyond single-celled microorganisms like bacteria.

So the search for signs of life on the surface of Mars probably means looking for fossil microbes in water-formed rocks.

But that's a tricky task, too.

On Earth, bacteria and other microorganisms are ubiquitous, but their fossilized remains are harder to come by.

``There's life teeming out there, but I defy you to find a microbial fossil out there,'' Farmer said as he looked at the looming mountains from the Death Valley playa.

Only special types of environments - such as playas and hot springs - preserve microbial remains.

In fact, until the last half of the 20th century, most scientists doubted the very existence of fossil microbes.

Charles Darwin knew fossils as the remains or impressions of plants and animals preserved in rock.

In Darwin's time, the fossil record extended back to a period that has since been dated at 600 million years before the present.

Older rocks contained no plant and animal fossils. That blank-slate period later became known as the Precambrian Era.

But in the 1950s, researchers reported the discovery of fossil microorganisms in 1- to 2-billion-year-old sedimentary rocks called stromatolites.

It was the first convincing evidence that the earliest life forms on Earth were microorganisms.

More recently, microbial fossils have been found in Australian stromatolites that date to 3.5 billion years ago. Chemical hints of microbial life have even been seen in 3.87 billion-year-old rocks from an island off Greenland.

If those tantalizing Greenland traces can be believed, life on Earth probably sprang up a few hundred million years, or less, after the planet formed.

But researchers don't agree on what constitutes unassailable evidence for microbial life in rocks, whether the rocks are from Greenland or Mars.

The problem was highlighted in 1996 when scientists from NASA and several U.S. universities claimed they'd uncovered ``evidence for primitive life on early Mars'' in a meteorite blown off the surface of the red planet and recovered in Antarctica.

The potato-sized Allan Hills 84001 meteorite contains microscopic bacteria-like tubes and organic compounds that the researchers said may have formed biologically.

The announcement sparked a brouhaha that still smolders.

The researchers, headed by David S. McKay of NASA's Johnson Space Center, stand by their original claims and have made similar assertions about another Mars rock.

But other scientists say everything in the Allan Hills meteorite can be explained by non-biological processes.

``What we learned from Allan Hills was that we don't know how to detect life,'' said oceanographer and microbiologist Kenneth H. Nealson, who heads the astrobiology program at NASA's Jet Propulsion Laboratory in Pasadena, Calif.

``We don't know what life is,'' Nealson said. ``We haven't defined it in a way to make it easy to detect or easy to deny that it's there.''

NASA hopes to return 50 to 60 pencil-sized rock cores from the surface of Mars in 2008, though last year's failures of the Mars Climate Orbiter and Mars Polar Lander missions could delay the plan.

In the wake of the recent failures, the space agency is re-evaluating its entire Mars robotic exploration program.

An independent assessment team is expected to present its findings to NASA Administrator Daniel S. Goldin in mid-March.

If Mars rocks parachute into the Utah desert as planned in 2008, the last thing NASA needs is Allan Hills, Round Two.

To head off another endless debate, the space agency in October formed a task force of geochemists, microbiologists, paleontologists, mineralogists, planetary scientists and other researchers.

The 18-member panel is headed by isotope geochemist John F. Kerridge of the University of California, San Diego, and includes Arizona State's Farmer.

The panel will advise the space agency on what needs to be done in the next few years to develop a reliable set of ``biomarkers'' - definitive chemical and morphological criteria for stating that a rock contains extant life or fossilized life - before the Mars rocks are returned.

``There's been quite a lot of controversy over fossil evidence for life on Mars, and what we want to do is to try to reduce it,'' Kerridge said.

``We want to get the arguing out of the way now, so that when the samples come back we're all in agreement as to what is evidence for life and what isn't,'' he said.

Development of biomarkers is also under way at several of NASA's 11 astrobiology centers.

Nealson's JPL team is working on the problem, and so are Arizona State researchers and the same Johnson Space Center team that made the Mars meteorite announcement.

A key long-term goal for Nealson's team is to identify a set of life indicators that are ``non-Earth-centric.''

On our planet, DNA, RNA, proteins and lipids denote life. But there is no reason to believe that extraterrestrial life forms - if they exist - use those chemicals, Nealson said.

``We haven't divorced ourselves from believing that life should be like Earthly life,'' said Nealson, lead scientist for the Mars sample-return mission.

``If that's the way you feel about it, and if life turns out to be different on Mars or Europa or anyplace, you'll probably miss it.''

Photo by James S. Wood, The Arizona Daily Star: NASA researcher Jeffrey Moersch uses a thermal emission spectrometer to identify minerals by measuring their color.

NASA photo: a 370-mile-long water-carved channel snakes across Mars' Ma'adim Vallis region.

Map by the Arizona Daily Star