Searching for Signs of a Wet Mars

When astronomer Percival Lowell gazed through his mountainside telescope at Mars a century ago, he saw cities, patches of vegetation and an intricate network of canals — features that seemed to indicate vast quantities of water.

Lowell, of course, was wrong. But the search for water continues today, driven by the belief that where there is water, there is the possibility of life. It is a quest that has thrilled — and disappointed — a vast constituency of sci-fi junkies, scientists and ordinary people who have gazed at the Red Planet.

“I would characterize Mars as being a bipolar place,” said Jonathan Lunine, a planetary scientist at the University of Arizona. “It creates great excitement and great depression.”

After centuries of speculation and decades of conflicting data, NASA scientists hope they will finally be able to answer what is still one of the most contentious questions in planetary science: whether Mars was once “warm and wet” and hospitable to life or has always been a frozen wasteland.

NASA is scheduled to launch on Sunday two massive robots armed with chemical analyzers, rock grinders and microscopic lenses able to detect the faintest evidence that rocks may have formed in a watery environment. While they will not directly search for life or water — a task that would likely require deep drilling and different instruments — the Mars Exploration Rovers will be able to look for evidence of what the past was like in the place where such ancient records are best kept: the rocks.

“If we don’t find evidence of persistent water then we are either very, very unlucky or we’re looking at a dry planet,” said Dan McCleese, an atmospheric physicist and chief scientist of the Jet Propulsion Laboratory’s Mars exploration program.

Mars has always been a distant tabula rasa: a blank slate to paint with imaginings born of what was known of life on Earth. Lowell dreamed of a dying civilization forced to build canals to survive the desiccation that surrounded them. Other astronomers thought they saw changing seasons and greenish tints from vegetation.

It was not until the mid-1960s, when the string of missions to Mars began, that science started to overwhelm imagination, raising entirely new questions with each swing past the planet. The debate has been a seesawing free-for-all ever since.

“Every time we send a mission to Mars,” said Bruce Jakosky, a planetary scientist who directs the Center for Astrobiology at the University of Colorado, “we seem to find a new planet.”

The first close-up images of Mars were sent back from Mariner 4 in 1965. Hazy as the images were, they made it clear there were no canals, no civilizations, no vegetation, no oceans, not even a puddle. The only water was frozen in the icecaps.

Hopes soared in 1971 after Mariner 9 sent back images of volcanoes, canyons and what looked to be vast river systems. Though dry, they appeared carved by ancient waters.

But in 1976, the Viking mission put the first robots on the surface of Mars to seek signs of life or organic material. The two landers found nothing of the sort, disappointing scientists and stalling exploration.

In 1996, NASA’s interest was reignited with the discovery of a Mars meteorite purported to contain traces of primitive life that had existed on Mars 3.6 billion years ago. The finding, however, was largely dismissed after further analysis. The real explosion of information about Mars has occurred only in the last four years with a flotilla of orbiters photographing and analyzing the chemistry of Mars in unprecedented detail.

Mars Global Surveyor has snapped more than 100,000 detailed pictures of the surface — the most exciting showing gullies that might have been cut by flowing water very recently.

Another orbiter, called Odyssey, found evidence of vast deposits of ice just beneath the Martian surface — far more ice and far closer to the frozen surface than scientists had expected.

The find provided a partial answer to the question of where all of that Martian water went. Another instrument detected hematite, a mineral that often forms in warm water, although it also has just found evidence of thick layers of the mineral olivine, which exists only in the driest of places because it decomposes in water.

The prodigious flow of new information, however, has only fueled more debate over water and life. In short, the geology says there was water. The chemistry and the physics still say no.

“We don’t know what’s going on,” said Mike Carr, a U.S. Geological Survey geologist who has participated in nearly every mission to Mars. “The more knowledge we get, the less we understand Mars.”

The biggest problem is that basic physics don’t easily allow for a warm Mars. The planet is about 50 million miles farther from the sun than Earth. And in the period when some scientists think Mars would have been warm — about 3 billion years ago — the sun was about 70% dimmer than it is today.

Computer models of the Martian atmosphere suggest it is nearly impossible to create the carbon dioxide, or “greenhouse,” atmosphere needed to warm the planet from today’s average surface temperature of 81 degrees below zero. And there is no evidence of huge carbonate rock deposits, like limestone, that would likely exist if such an atmosphere had once thickened the skies over Mars and then vanished.

But still, there are those nagging features all over the surface that look like river valleys, channels and streams. Some scientists even argue there was once a massive ocean.

In the last few years, a number of scientists have proposed several theories that could explain the features without requiring a warm Mars or even long-standing water.

In one scenario, proposed by Brian Toon and Teresa Segura of the University of Colorado, the features were cut by drastic heating and melting spawned by asteroid impacts.

These impacts generated so much heat that they melted ice caps, vaporized rock and sent steam into the atmosphere. The water vapor eventually rained back onto the surface, carving the features seen today.

“You can release enough water to carve these rivers several times over,” said Toon, who is among those who believe a carbon dioxide-warmed Mars goes against the planet’s known physics. “No matter what you do, you can’t get Mars within 20 degrees of the freezing point.”

Another theory is that the features were cut by the melting of ice and snow over eons as the Martian climate gradually warmed. This idea matches a computer simulation by Marc Kramer and his colleagues at NASA Ames Research Center to see how water might have once pooled and flowed on Mars’ surface. He thinks the flat and subdued surface was more likely carved by melting, rather than by the pelting rainstorms of a warm planet.

The spread of ice from the poles toward the equator and its melting could be explained by the shifting tilt of the planet’s axis, which would radically change surface temperatures.

The idea fits with a theory by Arizona State University geologist Phil Christensen that explains why freshly carved gullies usually exist on the shadiest and coldest sides of craters. They may have been cut in recent time by water trickling out of melting patches of ice and snow.

The Mars Exploration Rovers may be able to settle the issue of Mars’ past. Built to mimic human geologists, their main camera is at about human eye level. They can roam the length of 10 football fields during their trip, scan the horizon, grind open rocks, look at them under a magnifying glass and analyze their minerals and chemistry.

The brawny, 384-pound rovers, equipped with six-wheel drive, dwarf their predecessor, Pathfinder. That little rover, which reached Mars in 1996, weighed a mere 22 pounds and was mainly a testing mission to try out an air-bag landing.

The rovers will be launched from Cape Canaveral in Florida. They face a long journey and violent landing on Mars.

The first of the new rovers is scheduled to land in the massive Gusev Crater on Jan. 4, 2004, and will look at sediment layers to see if they match those that settle to the bottom of lakes or are merely wind-blown dust or volcanic ash.

The second rover will land three weeks later at Meridiani Planum, a vast field containing the mineral hematite that is linked to water. The rover should be able to determine if the hematite formed in a pool of long-standing water, or an ephemeral hot spring, or simply spewed out of a volcanic eruption.

Both rovers will also look for the missing carbonates that might prove that a greenhouse atmosphere once sheltered a balmier Mars.

If Mars turns out to have never been warm and wet, it will be a disappointment to many, including JPL’s McCleese. But he is determined to prove — or disprove — the idea with the next few missions to Mars.

But some scientists believe that a frigid Mars could still harbor life. On Earth, life exists in ice-covered lakes, long-lived meltwater pools and even in pores within ice sheets.

“We’re not talking about palm trees on beaches. We’re just talking about a little bit above freezing,” said Maria Zuber, an MIT geophysicist who has studied Mars since 1980. “The prospects for exploration could not possibly be more thrilling.”