Like stickers on a traveler’s suitcase, name badges cover a wall of Jack Farmer’s office. They’re a record of his two-year journey through the world of scientific conferences, into covens of biologists and geologists, paleontologists and space scientists. At each meeting, he has tried to enlist the help of his audience--to convince them that his quest, while it may sound quixotic, is not crackpot. Farmer wants to find fossils on Mars.
Based at the NASA Ames Research Center in Moffett Field, California, Farmer is the world’s first and only full-time exopaleontologist, a job title he had to invent himself. Until now the search for life on other planets, known as exobiology, has been a search for live, squirming organisms. Mars was once the great hope; finding evidence of life there was a central objective of the Viking missions of the 1970s--which in that respect, at least, were a total bust. Most researchers doubt that anything could still be alive on Mars today.
On the other hand, many of the people Farmer meets in his travels do accept the possibility that life once existed on the Red Planet. Before 3 billion years ago, it is widely believed, Mars was a much warmer and wetter place. The valleys and channels visible on the planet today suggest that liquid water--a prerequisite for life as we know it--once flooded its surface. Unlike Earth, Mars lost most of its heat-trapping atmosphere early on; today its water exists only as ice. But during the epoch when life was beginning on Earth, Mars too may have had the necessary conditions. Fossils of 3.5-billion-year-old microbes have been found on
Earth, and Farmer thinks we might have a good chance of finding similar fossils on Mars--certainly a much better chance than we have of finding living organisms.
If we’re going to go back to Mars, and exobiology’s going to have a role, you can’t say, ‘We’re going to look for extant life,’ Farmer says, because no one’s going to accept that. But looking for a fossil record is a whole different ball game. Life does leave an imprint. Even long after it’s gone we can find it.
From the mundane to the sublime--Farmer’s career in fossil hunting has spanned the spectrum. In 1981, a few years out of grad school, he became a corporate petroleum geologist, analyzing drill cores for fossil clues that might point to hidden oil reserves off the California coast. But after a few years of that he wanted a change. He was tired of reporting leaks on the oil rigs, tired of being treated like the office oddball for keeping a recycling bin, and tired of doing work that was so narrowly practical. I wanted to do real science, he says.
He got his chance in 1988, when he went to UCLA and joined a research group led by J. William Schopf--the man who discovered the 3.5- billion-year-old microbe fossils in Western Australia. Those fossils, like most fossils more than a billion years old, were found in association with unusually layered rocks known as stromatolites. The stromatolites were formed by generations of microbes that trapped sediment and minerals from the water around them. To understand exactly how, you have to look at similar communities of microbes alive today. At UCLA, Farmer was given the job of studying microbial mats that grow in Mexican salt ponds.
At the time he knew little about Mars, beyond the fact that in 1976 the two Viking landers had failed to find life there. (Indeed, they had found a soil so full of destructive chemicals, such as peroxides, that it seemed impossible even for basic organic compounds to survive.) In Mexico, though, Farmer started collaborating with biogeochemist David Des Marais of NASA Ames. Des Marais told him there were people at NASA who still quietly believed there might be microbial life on Mars, perhaps hiding out in some underground oasis. With plans in the works for a return to Mars, NASA researchers were also thinking seriously about looking for fossils.
In Farmer, Des Marais saw a scientist with the right training in paleontology and geology to lead the fossil hunt. He invited Farmer to come to Ames to find fossils on Mars. I was skeptical at first about whether this would be a realistic goal, Farmer recalls. But he also thought he could do better than Viking had done. So in 1991 he moved to Ames and started studying rocks again--stromatolites and other fossil-bearing rocks that might turn out to have Martian analogues.
Eventually he mapped out a three-step strategy for doing paleontology on Mars: first, identify likely sites from space with an orbiting spacecraft; second, drop a lander down at one of those sites to scour it for possible fossil-bearing rocks; and third, send another probe to retrieve rock samples and bring them to Earth, where researchers could put slices of rock under a microscope to look for the fossils themselves. The strategy will take more than a decade to execute if it is ever executed at all. But Farmer is actively planning steps one and two.
He has already decided, for instance, that one good site to look for on Mars is an extinct hot spring. Many researchers now believe that hot springs are a likely site for the origin of life. Moreover, such springs are certainly a good environment for preserving microbes that have died. If they don’t get encased in something, they’re gone, Farmer explains. Just as tree resin can entomb bugs in amber, certain minerals dissolved in water can coat a microorganism. The best minerals for the job are silica and phosphate, but carbonate can work, too. These are the minerals that record the earliest signs of life on Earth, and they are rife in hot springs.
Since early Mars had active volcanoes as well as water, it probably had hot springs. Finding the relics of those springs from space, though, won’t necessarily be easy. Farmer is testing the infrared cameras slated for upcoming Mars missions to see if they’ll be of use to him. Next year, from high-altitude aircraft above Yellowstone National Park and other hot-spring sites, he and NASA Ames remote-sensing scientist David Peterson will see how well the equipment can pick out silica, iron, and carbonate deposits formed by the springs.
At the same time, Farmer has been trying to learn everything he can about how life in such springs might leave its mark. One particularly striking example of microbial sculpture he has found is made by the Phormidium genus. These microbes form mats of spaghetti-like filaments that line up in streaming water. As time passes, silica encrusts the Phormidium mat and preserves the pattern in rock. While studying extinct hot springs in Australia with Des Marais and Australian paleontologist Malcolm Walter, Farmer discovered rocks in which Phormidium’s pattern was still clearly preserved--after 350 million years.
If a Martian lander were to find such a distinctive pattern, Farmer would know that he had found an excellent place to look for fossils. Another good sign would be a tufa tower. Tufa towers are eerie spires and pagodas made of calcium carbonate; they form not at hot springs but at cool springs on lake bottoms. But bacterial mats are common at cool springs too, and as the calcium carbonate precipitates out of the water, it often traps microbes. Farmer has been diving into Mono Lake, east of Yosemite National Park, to study the process by which microbes get trapped in the tufa towers there.
Meanwhile, some of his NASA Ames colleagues are investigating the possibility that a Martian probe might find hints of life in the chemical composition of the rock. Ultimately, life is chemistry, says Des Marais, and if you try to look for it at that basic chemical level, you’re going to cast a broader net. A Martian rock’s ratio of carbon isotopes, for instance, might reveal whether the carbon had been tampered with by photosynthetic organisms, which prefer the lighter isotopes. Even stronger evidence for life would be the presence of certain fatty-acid molecules-- which help form microbial cell walls, and which have been found to survive intact in rocks on Earth for at least 1.8 billion years.
Exopaleontology’s first test will be the $170 million Mars Pathfinder mission, scheduled to launch in December 1996 and parachute to the planet’s surface on the following Fourth of July. After Pathfinder lands on giant air bags, it will release a two-foot-long rover that will explore the surface in a 30-foot radius from the lander. Exciting as the Pathfinder mission is, it is not the introduction to Mars Farmer had expected--it puts step two of his strategy before step one. He had originally planned to pick his fossil-hunting sites with Mars Observer, which was supposed to map the surface of Mars. But that probe went mute in August 1993 just as it was going to enter the planet’s orbit.
Farmer had to scramble to pick landing sites for Pathfinder. They had to be on a flat area, and they had to be no more than 30 degrees from the equator to guarantee enough sunlight for the craft’s solar cells. He decided that his first choice would be an expansive plain called Ares Vallis. The idea to go here became obvious to me when I started focusing on this area, but before being forced to do that by the Pathfinder constraints, I hadn’t really identified this spot, he says. What made it particularly attractive was the geologic grab bag of rock types that had been funneled over the plain by ancient floods. He would have a better chance of finding fossil-bearing rock, and other researchers with other agendas would have something to look at as well. In July 1994, NASA selected Ares Vallis for Pathfinder’s elliptical 60-by-120-mile drop zone.
Pathfinder will land at the mouth of a channel originating in a rugged spread of what looks like thermokarst. On Earth that type of terrain is created when underground heat melts ice in the ground and causes surrounding sediments to collapse; so the presence of similar-looking terrain in Ares Vallis is an encouraging sign that a hydrothermal system might once have existed there. But the Pathfinder mission remains a difficult first chapter for exopaleontology. Pathfinder is really not designed to look for fossils, says Des Marais. Nevertheless, it does carry a camera that can distinguish some rock types, and Farmer is pushing to get it equipped with special filters to spot aqueous mineral deposits.
Farmer hopes that future Mars missions will be more suitable for finding fossils. As part of the Mars Surveyor program, NASA plans to send a low-cost orbital probe and lander to Mars every two years until 2006. A joint U.S.-Russian mission is under discussion as well, with a launch tentatively planned for 1998. These probes may carry equipment capable of detecting the mineral composition of rocks and low levels of organic compounds. With the help of Ron Greeley of Arizona State University, Farmer has already picked the top 50 sites on Mars worth investigating for fossils. He ranked them based on several factors: whether evidence suggested there had been an aquatic habitat there, such as thermal springs or a lake; whether the water (and with it perhaps life) had been there for a long time; and whether fossilization was likely to have taken place. Among his favorite spots are Dao Vallis, a channel possibly formed by water from underground hot springs, and Gusev Crater, which was once a lake and delta. Both Viking lander sites, incidentally, scored low on all counts.
As Farmer has built up his name badge collection, he has inspired enthusiasm from his audiences but also skepticism. Some people aren’t convinced Mars was ever particularly warm and wet. The planet, they say, may never have had a thick, heat-trapping atmosphere; its channels and valleys may all be like Dao Vallis, carved not by runoff from rainfall on a generally warm planet but by water flowing from isolated hot springs. In the subfreezing chill that prevailed on most of Mars, the chemical reactions required for the origin of life would have happened much more slowly. Life might still have developed at a Martian hot spring, but it would have been a much chancier matter.
Even if life did evolve on Mars, some researchers think our prospects of finding actual fossils are slim. One such skeptic is Farmer’s old boss at UCLA, J. William Schopf. Having spent years searching for his 3.5-billion-year-old fossils, Schopf knows how rare they are on Earth. He thinks Farmer should be content to look for a more general target, like fatty-acid molecules or other signs of organic matter. If you want to look for fossils, you’re really looking for a needle in a haystack, he says. If you want to find organic matter, then you’re looking for the haystack.
Farmer has one reason to hope, though, that Mars has richer haystacks than the ones Schopf sifts through on Earth. The rocks in the frozen Martian crust have not been subjected to the heat and crunching produced by plate tectonics, so if fossils ever formed on Mars, they would have had a better chance of remaining intact. Younger rocks on Earth, which have been less battered by plate tectonics than Schopf’s 3.5-billion-year- old ones, are more crammed with microbial fossils. On Spitsbergen, an island north of Norway, Harvard paleontologist Andrew Knoll has studied sedimentary rocks formed in a shallow sea 850 million to 600 million years ago. Those rocks are just oozing biology, Knoll says. A quarter of his rock samples contain the fossilized bodies of bacteria and microscopic algae, half have organic matter, and nearly all contain the characteristic ratio of carbon isotopes created by photosynthetic life. Knoll thinks Farmer might have a fighting chance of success if he looks for all three of these signatures of life on Mars. The trick, of course, he adds, is how to do this by remote control.
Indeed, the third part of Farmer’s plan--actually getting hold of the fossils themselves--is also the sketchiest part. NASA hopes to send another mission to Mars between 2005 and 2010. If previous landers have by then found a site with fossil potential, this probe would dig up some of the rock and place it in a capsule that would be fired back to Earth. Researchers would then be able to slice the rock into thin sections and look for dead Martians. Whether such a mission will ever come to pass is uncertain.
Farmer himself tries to remain agnostic about his prospects of one day finding Martian fossils. You don’t know till you go there, he says. That will be the fair test. Even if he fails to find fossils, he says, his research program will probably produce good science. It may find on Mars the sorts of primitive organic molecules that let life evolve on Earth. It may uncover clues to the ancient Martian climate. It may do those things and more, and yet that is not why Farmer or anyone else is excited about the program. What’s exciting is the prospect that Farmer and his colleagues might succeed in making an epochal discovery: that Earthlings aren’t, or at least weren’t, alone.
