The Search for Life

The table in Sarah Gavit's compact, sunny office at the Jet Propulsion Laboratory in Pasadena, Calif., is strewn with a dozen bruised and battered hunks of metal, most of them shaped like big bullets. "You're looking at a museum here," she says--a museum of one of the more audacious space experiments ever: two probes that will burrow into the surface of Mars at 400 miles per hour and sniff out hints of whether the planet could once have supported life. To design the contraptions, Gavit, already a veteran of unmanned missions to Venus and Saturn, began designing and testing prototypes in 1995. "We threw them out of Cessnas over the Mojave Desert at 10,000 feet," she recalls. "We ended up losing quite a few. Very hard to find these things." Then, when she needed more speed, she rented an air gun. Wearing hard hats and crouching in bunkers before every shot, Gavit and her team figured out which aeroshell shape could protect the instruments inside from a force of 60,000 G's. And this week, if Gavit's luck holds, two acorn-shape aeroshells will scream across the Martian sky and will each release a tiny probe that will punch into the soil like a meat thermometer into last week's turkey.

The stakes are somewhat higher than finding if a drumstick is cooked through, though slow roasting is on the Martian agenda. A little corkscrewlike drill will pop out of the four-inch probes, dig in, grab some soil and drop it into a tiny chamber to be cooked and analyzed. Gavit, hunched over her console at JPL, will be scrutinizing the data stream for hints of water ice, a sign that once, long ago, life might have gained a toehold on our sister planet. And if there is frozen water, there may be liquid water deep underground. "It may be reasonably warm under the surface of Mars," says Scott Hubbard, who heads NASA's new Institute of Astrobiology. "If there's still liquid water underground, like the ancient aquifers under Phoenix, then life could still, possibly, exist on Mars. That's a respected speculation within the scientific community."

It is spring in the southern hemisphere of Mars, and the south polar icecap is retreating like frost on a sun-drenched window. Just as the season stands for the renewal of life on Earth, so a number of scientists are hoping that the Martian spring will bring clues to the existence of life, past or present, on the Red Planet. Late this week, if all goes well, the $165 million Mars Polar Lander will touch down on Earth's closest neighbor after an 11-month journey. At 1,400 miles up, it will release Gavit's aeroshells, which it has been carrying on its exterior, like canteens strapped to a hiker's belt. Then it will parachute down to a landing target about 480 miles from the planet's south pole. This is where the carbon dioxide icecap advances and retreats with the seasons (as of Nov. 5, the edge of dry ice had just shrunk back from the landing site). It is also what scientists call a cold trap: a region where volatile compounds in the Martian atmosphere, after blowing around the planet, finally fall to ground. "There are a lot of cool places to explore on Mars, but if you're looking for water and ice, the compound associated with life, then the place you want to be is the place where it got stuck," says David Paige of UCLA, the principal investigator for the mission's main experiments.

The question of life on Mars has a long pedigree. Ever since the late 1870s, when Italian astronomer Giovanni Schiaparelli reported seeing canali, or channels, on Mars, the planet has offered hints that humankind might not be alone. A few years later, American businessman Percival Lowell made telescope observations that convinced him the "canals" (a mistranslation) were built by an alien civilization. Visionaries dreamed of transmitting messages to Mars with huge mirrors. In 1922 and 1924, the U.S. government asked radio stations to go quiet for a while so radio operators could listen for signals from Mars, which was making two of its closest passages to Earth. Needless to say, no one picked up any cosmic hellos. And images returned from spacecraft orbiting or flying by Mars in the 1960s showed a desolate world, pockmarked with craters, with nary a canal-building crew in sight. Experiments by the Viking landers in 1976, which searched for chemical signs of life, were widely interpreted as a down-arrow for little green men, and even tiny brown microbes. But then came the summer of 1996. After alerting the White House, a team from NASA's Johnson Space Center and Stanford University announced that a hunk of rock from Mars, which had been blasted off the planet by asteroid or meteor impacts 17 million years ago and landed in Antarctica, contained microscopic fossils and other signs of life. That claim has been hotly disputed (sidebar). But it pushed NASA to declare the "search for evidence of past or present life" the first of the "highest priority scientific objectives" for its Mars missions--and to throw $50 million more a year into the effort.

What makes Mars hospitable to life is the presence of liquid water billions of years ago. Earlier missions discovered canyons and deep outflow channels that wind hundreds or thousands of miles downhill to the northern plains. They seem to have been formed by running water--lots of running water: some features look like they were carved by torrents raging with a force of 10,000 Mississippi Rivers. But more recent observations go further. Images taken by the Mars Orbiter Camera spied a 500-mile-long channel. Named Nanedi Vallis, its "sinuous shape... suggests that the river that cut the valley was fed largely by groundwater," says geologist Michael Carr of the U.S. Geological Survey. If so, then liquid water may still exist deep below the surface of Mars, even though the planet today is too cold and the atmosphere too thin to keep water liquid at the surface. (In such thin air, water boils instantly.) Liquid water is necessary, though not sufficient, for life.

Now the signs of water on Mars have grown even more tantalizing. "The outflow channels do not continue far into the northern highlands," points out geologist James Head of Brown University. "They disappear really quickly." Where did the water go? Ten years ago scientists led by Timothy Parker of JPL scrutinized some of the old photos of Mars and thought they spied long, curving lines in the northern lowlands. Now Head and colleagues are presenting findings from the altimeter on the Mars Global Surveyor (which is orbiting the planet) that suggest at least one of the marks may be an ancient shoreline. For one thing, the line stays at a near-constant elevation, which is what you'd expect of a shoreline (sea level is sea level on Earth, after all). Also, the lowlands that the line curves around are extraordinarily flat and smooth. One way such terrain forms is that sediments that have washed into a sea fall out, says Head, just as they did to form the floors of the deepest seas on Earth. Standing water would provide a source of water vapor, through evaporation, notes Head. Water vapor absorbs heat, and so would have warmed the atmosphere on ancient Mars from its current frigidity into a toastier range.

For years the party line on Martians has been that the environment is too hostile for them: it's cold and it's dry, and the thin atmosphere is no more effective against damaging solar radiation than a paper umbrella against hail. But standing water blocks solar radiation. And those shorelines are also interesting. For biology to emerge from mere chemistry, you need the basic ingredients of life (compounds like nucleic acids and proteins), water to mix them all together and an energy source to zap them with that vital spark. The heat of hydrothermal vents might do the trick. So might lightning. But so might something as gentle, and as simple, as waves lapping on an ancient shore. The ebb and flow of tides could also have provided the alternating wet-and-dry conditions that some theories of life's origins say transform not-quite-biological molecules into fully biological ones. "If Mars did sustain a great northern ocean," says Head, "then there would have been tens of millions of years of an environment compatible with what we know about the origins of life." Or, as planetary scientist William Boynton of the University of Arizona puts it, "If life didn't get started [on Mars], we'd really have to wonder why not."

The standing water is long gone, of course: the Noachian period on Mars ended at least a billion years ago, maybe two. With no liquid water on the surface, the planet would be hard pressed to sustain life. Or so it seemed. "We have broadened our thinking about when and where life might occur because we have found [microbes] living in complete darkness in thermal vents, and inside rocks in the dry valleys of Antarctica eating hydrogen," says Hubbard. Other newly discovered "extremophiles" on Earth sustain life by dissolving minerals--they eat rocks. Others live in sulfuric acid, at 212 degrees Fahrenheit, or in environments as acidic as vinegar or as harsh as ammonia. Apparently, life is pretty loose about where it lives. "If life ever got started on Mars," says Jim Head, "then I'd say you'd have a helluva time eradicating it. Once the surface became inhospitable, life would go underground."

That possibility has re-energized the quest for life on Mars. Although life there might not be so abundant as to require reclassifying "My Favorite Martian" as a documentary, it may be holding on in niches deep below the cold, arid eolian surface. Experiments on Mars Polar Lander during the planned 90-day mission will seek out signs of conditions conducive to life. As the probes, named after (Earth's) South Pole explorers Amundsen and Scott, search for water and water ice about 60 miles to the north, the lander will cook up its own experiments. A 6-foot-6 robotic arm, resembling a shiny steel backhoe, will scoop up minuscule soil samples, putting thousandth-of-an-ounce dollops into the $4.4 million Thermal and Evolved Gas Analyzer. TEGA's eight tiny ceramic ovens, each as big as a piece of elbow macaroni, will heat the samples. As gases are released, they will waft into an adjoining chamber where lasers will determine the concentrations of water, carbon dioxide and water-bearing minerals in the Martian soil. Depending on how much heat was required to drive off the water, the scientists will be able to infer whether the water was present as plain old ice, in carbonates (like limestone), or in hydrates. These minerals form almost nowhere but in water, and so would suggest ancient seas. Although Polar Lander isn't designed to look for life itself, scientists wax eloquent about what it would mean if a future mission detected signs of ancient or extant life. About how the discovery might shed light on how life emerged from nonlife on Earth. About how it might show whether there is more than one way to build life--do Martians have DNA? About how it might show the way an abrupt environmental change makes life fizzle out--or at least stop evolving beyond primordial ooze. But the deeper implications might come from a discovery that Mars never supported life. For that would challenge scientists to identify what vital spark was missing in a place that seemed to have all the right ingredients, and challenge the rest of us to see the single known example of life in the universe as that much more wondrous.

But first, the Polar Lander must elude the Martian jinx. The Russians lost several Mars missions in the 1960s and their Mars 3, the first spacecraft to land on the Red Planet, went silent seconds after touchdown. JPL engineers joked that a "great galactic ghoul" was gobbling up Mars probes, but it was no laughing matter when their $1 billion Mars Observer went AWOL in 1993. And this past September, Polar Lander's companion, the Climate Orbiter, performed a flight maneuver that had been calculated in pound-seconds (a British unit), rather than newton-seconds (metric). As a result, the $125 million spacecraft tried to enter Martian orbit at an altitude of 34 miles rather than the planned 135. It either crashed or skipped off into space. During the investigation, an engineer discovered that Polar Lander was wending its way through 137 million miles of space with its own potentially fatal flaw: fuel lines on the descent engine are at risk of freezing. If they do, the craft will crash, but JPL director Edward Stone assures that controllers "will turn on the [engine's] heaters earlier."

If Polar Lander survives and sends back data that strengthen the case for ancient life on Mars, enthusiasts hope it will give a boost to dreams of sending humans there. Says Robert Zubrin, founder of Pioneer Astronautics, "The human presence--whether to drill for signs of life in liquid water deep underground or to look for fossils--becomes increasingly important." Zubrin envisions a series of paired launches, in which one spacecraft of each pair would deposit an Earth-return vehicle on Mars and the other, up to three months later, would send a crew of four, a "habitat" module and a duplicate return vehicle. After a six-month flight and an 18-month sojourn on Mars, the crew would return home in the first vehicle, using fuel that it had made from Mars's carbon-dioxide atmosphere (carbon dioxide reacts with hydrogen, which the ship would bring from Earth, to produce methane fuel). NASA administrator Daniel Goldin has said that, within a few years, he would like to present to the (next) president a blueprint for sending astronauts to Mars. Zubrin hopes for an announcement even sooner. "A new millennium and a new administration is the time for great beginnings," he says. "No group of nations has ever been better equipped to launch a new age of exploration."

Still, a program that loses robotic Mars missions will not be entrusted with human lives until it can show that it can get them there and back safely. And that requires assurances that if life once did, or still does, exist on Mars it poses no threat to astronauts. That may become clearer in 2008, when a mission brings samples of Martian dirt and rock back to Earth for analysis. There may have been similar exports in the past: some scientists suspect that if life arose on Mars, it might have seeded Earth with primitive microbes. Mars, having a weaker gravitational field than Earth's, tends to lose whole chunks of itself when bombarded by comets or asteroids. If its primordial life rode a meteorite to Earth, then we have already discovered Martian life: the descendants of that ancient interplanetary vagabond would be... us.