The ultimate scientific study of Mars will be realized only with the coming of man—man who can conduct seismic and electromagnetic sounding surveys; who can launch balloons, drive rovers, establish geologic field relations, select rock samples and dissect them under the microscope; who can track clouds and witness other meteorological transients; who can drill for permafrost, examine core tubes, and insert heat-flow probes; and who, with his inimitable capacity for application of scientific insight and methodology, can pursue the quest for indigenous life-forms and perhaps discover the fossilized remains of an earlier biosphere.
—Benton Clark,
“The Viking Results—The Case for Man on Mars”
New visualization technologies and higher data rates will change the way we see the surface of Mars. But many scientists are eager for something less superficial. Just as European explorers moved from the sea coasts to the continental interiors in their second great age of exploration, so Martian explorers, too, are looking beyond what can be seen from their ships. They are interested in the planet’s depths. Much of what matters most about Mars is clearly beneath its surface and invisible to our eyes: lost terrains now covered in sediment, buried lenses of ice, aquifers (if any), magma chambers. And, most important, if also most speculative, life.
The case for life deep below the Martian surface was first made in a paper published in Icarus, the leading planetary science journal, in 1992. The authors were Penny Boston, Mikhail Ivanov, and Chris McKay. Ivanov is an expert on microbial life below the surface of the Earth; Boston and McKay are experts on Mars. More than experts: believers. They met in the 1970s, when they were both in a gifted-students program at Florida Atlantic University. From there they more or less coincidentally both moved on to Boulder, Colorado, which is where they were in 1976, the summer of the Viking landings. Neither was a planetary scientist—Boston was a biologist, McKay was moving toward astrophysics—but they and a handful of others in their circle became completely fascinated by the new planet that Viking was revealing to them. They devoured the technical papers written by the Viking team and started to ask themselves what they could do to take things further. Mars became a consuming passion. As scientists they yearned to understand it; as dreamers and adventurers they yearned to go there.
“I’ve never been involved in another group quite like it,” says Boston. “The magical thing was that everyone’s personalities dovetailed. It wasn’t the relationship of one person to the group that was important—it was the different relationships we had with each other.” Boston and Carol Stoker, who shared an office with McKay, gave the group huge energy. Tom Meyer, Stoker’s boyfriend, who had moved to Boulder after a few years working on the wonderfully science-fictional 1970s idea of mining sea floor manganese nodules, provided real experience of opening up a new world through technology. (As a child, he had developed a system for relaying recordings through his hometown’s fire alarm wiring so that, with the aid of a bizarre coxcomb antenna attached to military-surplus aviator headphones, he could listen to classical music as he cycled around delivering papers. To Tom Meyer, this was a fairly obvious thing to do.) Boston’s partner Steve Welch was a great hands-on technician, the sort of person whom anyone doing experiments wants around. Carter Emmart, the youngest of the bunch, used his artistic skills to help everyone visualize what they were talking about. And McKay provided, among other things, an essential optimism. “Chris’s main function for me was not getting discouraged by discouraging things,” Boston told me a few years ago. “I can get fairly blue—‘No one cares, no one wants to do this’—and he is fairly impervious to that. He’s a very complicated man—there’s a lot more to him than you see on the surface—but not a moody man.” McKay was not exactly the leader, but he was to some extent the focus. He was the one undergraduates would come up to and say, “You’re the Mars Guy, right?” Being exceedingly tall and thin helped; so did having a mind that was sharp and quick even by the standards of an extremely intense, intelligent group; so did genuine charisma.
They ran seminars; they imagined new ways of making use of Martian resources; they grew radishes in jars containing carbon dioxide at near Martian pressures. They dreamed with a passion of exploring Mars themselves and despaired—inasmuch as McKay’s presence would permit—as they watched NASA’s shuttle-driven retrenchment. They went on road trips to meetings where Mars would be discussed and sought out like-minded dreamers, people who saw the self-evident truth that Mars was the most important thing, the most exciting thing—the next thing. And they still found time for naked whitewater rafting. A like-minded journalist, Leonard David, called them the Mars Underground; the name stuck.
In 1981 the Underground gathered their network of sympathizers for a conference to marshal the arguments for sending people to Mars. Benton Clark, who had worked on Viking at Martin Marietta, the company that built the lander, had written a paper in 1978 called “After Viking—The Case for Man on Mars” that had inspired them; shortening his title and correcting its sexual politics, they called their conference the Case for Mars. That meeting was the first in what turned out to be a triennial series. The meetings often had a strange, fringe-y feeling (McKay, in particular, insisted that no one be excluded as a flake) but that was part of their charm. Where else could you see Hal Masursky slowly and carefully study computer enhancements of the “Face on Mars” before delivering his damning verdict: a drawn-out “Naah” spoken with the authority of a man who had looked at Mars as much, if not as meticulously, as anyone alive.
The case they assembled was this. Mars was worth studying because it was much more similar to the Earth than any other planet and had in the past been more similar still. In particular, its rocks might well contain evidence of past life. Robotic explorers could undoubtedly tell us much about the planet. But those who said that everything worth doing could be done by robots—a fairly common view among space scientists, who resent the far greater budgets with which the manned program produces far fewer results—were wrong. The evidence for life might be subtle and hard to parse. It would require real fieldwork and real laboratory skills on site to put it together. And to do it right you would need humans.
The same things that make Mars interesting also make its human exploration feasible—it has an environment that can supply things explorers would need. They could grow plants there in greenhouses irrigated with melted ground ice. They could make rocket fuel from the atmosphere. Life was thus the beginning of the case, because life on Mars in the past was what most needed studying, and the end of the case—because those studies required that we learn how to live on Mars ourselves. Across billions of years and hundreds of millions of miles, life was calling out to life.
Thomas Paine, who had run NASA from 1968 to 1970, was convinced to give a keynote address at the second of the Case for Mars meetings: The man who had celebrated the “squares” of Apollo was delighted by the longhairs of Boulder and their dreams of living off the Martian land. The Underground was invited to Washington to present its case. Meyer remembers overhearing the head of manned space flight lean over and whisper to a colleague, “I had no idea this was going on. We’ve got to look into this.”
NASA did look into it. Paine wrote a much-discussed report, “Pioneering the Space Frontier,” that called for the exploration and colonization of Mars, among other things. It attracted the interest of the White House. In 1989, on the twentieth anniversary of the first Apollo landing, George Bush the Elder called for a return to the moon and an outpost on Mars. But the president’s enthusiasm got little further than Spiro Agnew’s had twenty years before. NASA came up with a spectacularly complicated and ornate plan that included more or less everything its engineers had ever wanted to do and was perhaps ten times costlier than it could have been, which left the idea—for which the president had done little to prepare Congress—dead in the water.
The Underground was not just about proselytizing, though; it was also about science and technology. Stoker became an expert on what rovers can and cannot do on planetary surfaces: She was one of the leaders of the team that produced the virtual reality Marsmap system for Sojourner. Boston threw herself into studying the complex interactions within biospheres in order to make sense of what biology could do on a planetary scale; she was one of the people who got the American Geophysical Union to take its first serious look at Jim Lovelock’s Gaia theory, which claims that the planet Earth and its biosphere together form a self-regulating system. And McKay started to study life in the seemingly sterile dry valleys of Antarctica, where the temperature is below freezing for almost all of the year and the precipitation is dauntingly close to zero.
It was Imré Friedmann, a microbiologist from Florida State University, who first suggested how life might persist in such a desolate place. In the 1960s Friedmann’s search for life in the Negev desert had revealed lichen growing inside sedimentary rocks, staying close enough to the surface for a little light to get through to them, but keeping enough rock between them and the outside world to stop any moisture from getting out. He thought similar “cryptoendoliths” might be found in Antarctica, but he couldn’t get a grant to go and find out. However, a friend of his, Wolf Vishniac, had a NASA grant to go to the dry valleys as part of his work on the Viking program; they were already recognized as arguably the most Mars-like places on the Earth in their frigid aridity. Friedmann asked Vishniac to pick him up some random sandstones so that he could look for lichen within them. Vishniac died on the trip; hiking into the Asgard Mountains to see whether any microbes had grown on nutrient samples he had left there, he stumbled and fell. But when his effects were shipped home, his wife found some stones in them labeled “for Imré.” She sent them on; beneath their crust there was a layer of life. Friedmann published and the world took note. Grants were no longer an insurmountable problem.
By 1980, Friedmann and his wife had made a number of trips to Antarctica to further their research, and all sorts of people were eager to come along. Rather to his surprise, one of the few Friedmann ended up taking was a young physicist from Boulder with no real biological training and no field experience. “I just thought, ‘This young man is serious,’ ” says Friedmann, now semiretired, in a voice that suggests there is no greater attribute under the sun than seriousness. With a chuckle, he adds, “I have a good nose. Chris McKay’s the calmest and most balanced man I have ever met. He is also very, very intelligent. I would, without exaggeration, call him a genius.” McKay quickly became a key member of Friedmann’s team, cooking up new ways to measure the “nanoclimates” that allowed the microbes to keep tiny amounts of water liquid within rocks despite what was going on outside. He also became part of the human glue that holds such ventures together, remaining pleasant and sharp in conditions that can make people alarmingly unstable.
Establishing a reputation in Antarctica led to invitations to study other extreme environments. In Siberia, Chris McKay found microbes that had been frozen for three million years, but that could still reproduce when thawed out. In the Gobi Desert he found microbial life all too present when dysentery-bearing dumplings struck down his whole expedition. In the far north of Canada he found springs that welled up through permafrost a half-mile deep. Everywhere McKay found liquid water and a supply of nutrients, he found life. Only in Chile’s Atacama Desert was there no life to be measured. Bacteria fell to the surface from the sky (as they do all over the world) but failed to flourish. And the Atacama was the driest place McKay had been—no precipitation in four years. “It gives us our first data point on the other side of the line: too dry for life,” says McKay.
The Martian surface looks like one great big Atacama, except worse. It is not only arid and frozen but bathed in harsh ultraviolet radiation; the Martian atmosphere, lacking oxygen, necessarily lacks an ozone layer. The Viking landers had discovered that the regolith appeared to be laced with highly reactive chemicals, which no one had expected and which led to initially confusing results. Experiments trying to detect life by seeing if nutrients were broken down by something in the soil scored a big yes, which led to great excitement. But these reactions quickly tapered off, whereas metabolic reactions would be expected to persist; and they produced gases life would not be expected to produce (oxygen was given off by the samples being incubated, which in the absence of sunlight didn’t seem a likely sign of life). What’s more, studies of the soil itself showed that it contained no organic compounds. So the soil reactions were agreed by most to be chemical, rather than biochemical. There were dissenters—most notably Gil Levin, who still claims his experiment on the landers found life—but the consensus opinion was and remains that the soil was laced with peroxides produced by the high levels of ultraviolet light* and utterly without life.
A lifeless surface was assumed to be the same as a lifeless planet. In the late 1980s, though, that assumption started to come under renewed scrutiny. The floors of the Earth’s oceans had long been thought more or less lifeless; there was obviously water there, but there seemed to be no obvious source of nutrients. With no sunlight there couldn’t be any photosynthesis and photosynthesis sits at the bottom of almost all the food webs that biologists study. But just a few months after the Viking landings the submersible Alvin discovered a profusion of life around volcanic vents in the Galápagos rift, a mile and a half below the surface. Since then many such systems have been found, containing bacteria living under immense pressure and appreciably above the normal boiling point of water.
In the 1980s the SNC meteorites—and in particular the youngest of them, the Shergotty meteorite—suggested that there might still be volcanic activity going on within the Martian crust. If it was close to the surface and near the poles, that volcanism would surely melt ground ice and cause liquid water to circulate. So Mars might well have had hydrothermal systems of its own in the past and might still have them today. If there was life clustered around the lightless hydrothermal vents in the depths of the Earth’s ocean, why should there not be life in the hydrothermal systems deep inside the Martian crust?
One answer is that the Earth’s hydrothermal ecologies are not independent; they rely on imports from other parts of the Earth’s biosphere. Life requires a supply of chemicals willing to give up electrons and a supply of chemicals willing to take those electrons up. At the surface of the Earth the giving of electrons is mostly done by organic carbon compounds—food, if you want to get technical about it—and the taking up is done mostly by oxygen produced by plants. That is why the loss of electrons is called oxidation. The reverse process is called reduction (because it reduces oxide ores to pure metals) and the two processes have to happen in concert: If an oxidizer is to gain electrons, a reducer must give them up. Although there’s only a tiny amount of photosynthesis at deep ocean vents,* there is a steady supply of oxygen-bearing compounds drifting down from the sunlit shallows above, and they are what the vast majority of the creatures around the vents use to oxidize their food.
On Mars there’s no supply of oxygen at the surface to seep into the depths. However, though oxygen is a particularly powerful oxidizer, it’s not the only one life makes use of. Some earthly bacteria oxidize hydrogen with carbon dioxide to make methane (or acetate) and water; others reduce water with carbon monoxide to make methane and carbon dioxide. There are bacteria that use hydrogen to reduce sulfates, or sulfur itself, or electron-hungry forms of iron. These reactions don’t liberate anything like as much energy as reactions between organic matter and oxygen. But they can still be made to work. If there is a constant supply of some fairly simple chemical feedstuffs to work with, life can make do without any oxygen or organic matter to feed on. And hydrothermal systems—mixtures of groundwater and volcanic gases—are often rich in the highly reduced chemicals needed.
In their 1992 Icarus paper, Boston, Ivanov, and McKay laid out the argument that life of this sort might be possible in aquifers deep below the Martian surface. In the same year the famously controversial astrophysicist Thomas Gold published a paper that, while based on a very different chain of reasoning, brought similar ideas about the Earth to a wider audience. Gold has long believed that the Earth and the other rocky planets contain vast primordial hydrocarbon reserves, and that it is the upwelling of these hydrocarbons, rather than the burial of organic matter from the surface, which provides our reserves of oil, gas, and coal, an idea summarily rejected by most geologists. A corollary to this idea is that there is a vast number of microbes feeding off these hydrocarbons in the depths of the crust: Gold’s paper called them “the deep hot biosphere,” and the term started to slip into scientific usage, even though Gold’s explanation of the processes that might feed such a biosphere remains very much a minority view.
Soon afterward, an outpost of the deep, hot biosphere was apparently discovered inside the Columbia River basalts in central and eastern Washington State. Deep below the surface, warm water was reacting with minerals in the basalt to release hydrogen, and the hydrogen was apparently being used by bacteria as a source of energy; they caused it to reduce carbon dioxide, thus producing methane. Microorganisms had been found at such depths before—Ivanov was an expert on the subject—but normally in sedimentary rocks and oil fields. Here they seemed to be prospering in an aquifer within igneous rock just like that which makes up the surface layers of Mars.* McKay came across the data in a poster session at a conference and says he’s never been more surprised by anything in his life. The idea of some sort of deep, hot biosphere on Mars started to feel a lot more real.
In 1994, Boston and McKay decided to go and have a closer look at the life below. Probably the first spelunkers ever funded by NASA, they and their friend Larry Lemke descended into the vast Lechuguilla cave in New Mexico to look for new forms of microbial life. Unlike most large caves, which are created by carbonate rocks dissolving in water, Lechuguilla was carved by diluted sulfuric acid and might provide intriguing niches for sulfur-metabolizing microbes. The first trip was, says Boston, an awful experience; unprepared for the rigors of caving, by the end her objective “was just getting out alive.” McKay thinks it was the toughest field trip he’s ever done—which is saying quite a lot. He also found that caves hold little interest for him from a professional point of view—his role in fieldwork is to monitor nanoclimates that persist in spite of changes in the environment outside, and, in general, cave environments are pretty stable—and so after that first expedition he saw little reason to repeat the experience.
Boston, on the other hand, came away fascinated, and now focuses a great deal of her research on the weird and intimate relationships between biology and mineralogy to be found in caves, studying rocks that have life running through their veins and living slimes on the brink of turning to stone. The science is fascinating, but there’s something more personal to it than that. McKay talks of finding a certain oneness with the world in the seemingly lifeless wastes and solitary places of Antarctica. That’s not why he goes there—and he says he can find it in his office at Ames on a quiet Saturday afternoon, if he’s lucky—but it definitely counts as one of the perks of the job. Boston gets closest to that feeling in the closeness of her caves, and that’s one of the reasons she returns to their sometimes unbreathable atmospheres, their dripping bacterial “snottites,” and their battery acid streams again and again. There is something about life at the edge of where life is possible that makes everything, living and nonliving, seem valuable.
In the early summer of 1996, one of the discoverers of the Columbia River basalt bacteria gave a talk at NASA’s Johnson Space Center and provided some samples of the bacteria-bearing rock for the scientists there to study. A summer intern was put in charge of making electron microscope images of them. The images showed what appeared to be small bacteria (small, that is, even by bacterial standards) nestled in tiny cracks in the rock. When the intern showed Polaroids of the images to her boss, David McKay—an expert on meteorites and the moon’s regolith, and no relation to Chris—he smiled and passed her a set of pictures that seemed remarkably similar. They were electron micrographs of formations inside a Martian meteorite called ALH 84001, pictures that six weeks later would be on the front page of half the newspapers in the world. McKay and his colleague Everett Gibson had found what seemed to them to be signs of fossilized life in the meteorite a year before, but they had been playing their cards very close to their chests, letting only a few colleagues into the secret as they tried to muster as much support as they could through further studies.
ALH 84001 is the oldest of the Martian meteorites that have been thrown to Earth and recognized for what they are; radioisotope analysis dates it to the very beginning of Martian history, four-and-a-half billion years ago. The carbonates within it are younger, perhaps as much as a billion years younger, but they are still old enough to date from Mars’s putative warm, wet phase, which was helpful to the argument that they might contain—or be—signs of life. The rock was kicked off Mars by an impact about sixteen million years ago and landed in Antarctica about thirteen thousand years ago. It was found in 1984 as part of a program to collect Antarctic meteorites and identified as Martian in 1993.
By 1996 David McKay and his colleagues in the Johnson team had found various possible signs of life in their rock. There were small hydrocarbon molecules called PAHs, which might be produced by the breakdown of organic matter. There were carbonates in which the ratio between the two stable isotopes of carbon—carbon-12 and carbon-13—was oddly skewed; such carbon-isotope ratios are often associated with living beings. Some of the carbonates were in shapes that looked like tiny fossilized bacteria and one of them, “the worm,” appeared to be segmented. And there were very regular little grains of the iron-bearing mineral magnetite. Some earthly bacteria produce magnetite grains of a very particular size and shape, and the Martian magnetite grains looked just like them. On Earth, according to experts on biological magnetite, no inorganic processes produce magnetite grains with this same crystal structure.
The Johnson team’s paper was accepted by the journal Science in July. Knowing that its publication in August would set off a storm of interest, David McKay and his team headed off to Washington to brief NASA administrator Dan Goldin. Goldin gave them a thorough grilling and then went to brief the White House. The micrographs were not exactly what Chief of Staff Leon Panetta had been expecting as evidence of life on Mars—after all, this was the summer when according to the movie Independence Day alien life was the sort of thing that arrived in spaceships the size of cities and bent on blowing up Panetta’s office—but Goldin convinced him that the little worm was a big thing. President Clinton was briefed, as was Vice President Gore. A few days later, Clinton announced the discovery to the world.*
ALH 84001—Penny Boston calls it “Big Alh”—reshaped the Mars program and it refocused a great deal of NASA’s science. It is, in part, due to ALH 84001 that NASA has turned itself heavily toward what is now known as “astrobiology.” For all its effects, though, Chris McKay and many others were largely unconvinced by ALH 84001. Carbonate blobs are carbonate blobs, and some experts were saying that these particular blobs could only have been created at 1,300°F, far too high a temperature for any sort of life. Microscope images are notoriously open to interpretation and who knows what processes might have influenced carbon isotope ratios billions of years ago on an alien planet. PAHs are found in all sorts of rocks, including meteorites that have never been near Mars. But the magnetites were interesting. Chris McKay and others at Ames quickly organized a two-day seminar on the subject. One of the speakers was the world’s greatest expert on biological magnetism, Joe Kirschvink of Caltech.
In the early 1970s Kirschvink was a student at Caltech at the time that Gene Shoemaker, then head of the geology department, was setting up a lab to look at the magnetic fields trapped in the Moenkopi sandstones of the Colorado Plateau. Shoemaker had an exquisitely sensitive magnetometer built for his work, and over the following twenty years Kirschvink used it for a wide range of studies, looking for biological magnetite in fossils, in racing pigeons, and in human brains. Kirschvink has a broad mind, an argumentative Caltech stance and a tendency to champion provocative, dramatic theories. He’s one of the leading advocates of the snowball Earth theory; he’s recently reapplied the idea of true polar wander—the notion that Peter Schultz used to try to explain layered deposits all over Mars as the fossils of bygone polar caps, and oddly enough another idea originally thought up by Thomas Gold—to the history of the Earth, arguing that there’s evidence for a massive concerted flopping-over of all the lithospheric plates at the beginning of the Cambrian period 540 million years ago. A man with a taste for the provocative and a profound belief in the significance of biological magnetite, Kirschvink found the evidence of biology in ALH 84001 incredibly exciting. (He has since gone on to work closely with the Johnson team.) Chris McKay left the seminar still not convinced, but even more intrigued by the magnetite, partly because it might fit into a favorite speculation of his own.
Some bacteria—called aerobes—need oxygen, some—the anaerobes—never touch the stuff. Most of the bacteria that produce grains of magnetite are picky in-betweeners. They tend to live in sediments where oxygen levels are low but not too low; the chains of magnetite that run along their bodies keep them oriented along the lines of the Earth’s magnetic field and thus help them move up and down in the sediment to the oxygen level that suits them best.* This approach, though, presupposes that there is some oxygen around in the first place. On the Earth, this has not always been true. It is not yet clear when the photosynthetic production of oxygen evolved, but only about 2.3 billion years ago did significant amounts of oxygen start to make it into the atmosphere.
When scientists talk about the possibility of life on Mars early in its history, they normally assume that it would have been purely anaerobic, with little if any photosynthesis. The history of life on Mars would have been a billion years of bugs that went extinct when the climate got worse. A wrinkle on this argument is that the advent of photosynthesis may actually have killed them. James Kasting of Pennsylvania State University, an expert on models of the early climate on both the Earth and Mars, thinks that the only greenhouse gas powerful enough to have given Mars a warm and wet early period would have been methane, and that that methane would probably have had to be biologically produced. Organisms living in a methane greenhouse would be very ill advised to start pumping out oxygen, since it would react with the methane and deprive them of their warmth. On the Earth, according to Joe Kirschvink, the appearance of oxygen 2.3 billion years ago seems to have triggered one of the snowball-Earth events that covered the whole globe with ice, possibly by removing methane from the atmosphere. On Mars the effect would have been even more dramatic and probably terminal.
The argument that Penny Boston and Chris McKay had put forward with Ivanov was that whatever had happened on the surface, survivors might persist in deep hydrothermal systems to this day. But McKay has since become intrigued by another possibility: that life on Mars might have moved much more quickly than life on Earth. Maybe life on Mars had started to pump out oxygen very early on. Since Mars was less volcanic than the Earth, the supply of reduced chemicals available to react with that oxygen would have been much less copious, and free oxygen might have built up at the surface where it could be used to power far more energetic metabolisms than those available to anaerobes living off rocks. Mars might have had oxygen in its atmosphere long before the Earth did and might have evolved more complex organisms than simple bacteria. Rather than being a sickly weakling, failing to thrive and never getting beyond infancy, it might have been a James Dean of a planet, living fast before dying young. Its rusted face might be the legacy of that life. Maybe it wasn’t oxidized by ultraviolet radiation. Maybe it was oxidized by oxygen produced by photosynthesis. This is obviously hard to reconcile with Kasting’s notion of a methane green-house, but as both of them would quickly point out, that’s why we need more data.
Biological magnetite fits into McKay’s speculation in an intriguing way. Kirschvink suspects that on the Earth the original evolutionary pressure that drove bacteria to produce magnetite was linked to the chemical problems faced when oxygen becomes abundant. The way magnetite is used for navigation only makes sense in a world with oxygen gradients in its sediments. So if Martian bacteria were producing magnetite in large quantities more than three and a half billion years ago for either of the reasons that earthly bacteria produce it, it would seem likely that there was oxygen around.
Chris McKay is still not convinced that the magnetite in ALH 84001 is evidence of life, but he’s leaning more toward the idea than he did originally. One thing that has swayed him is that his old friend Imré Friedmann thinks he has found magnetite in the meteorite that is actually arranged in chains, as the magnetite crystals in earthly bacteria are. That said, there are others who find the evidence for chains—and indeed the whole argument that the presence of this sort of magnetite necessarily implies that there were bacteria around to make it—poor. And there is no clear reason to believe that the two sides will ever really come to an agreement until there is more evidence to go on. Probably the most quoted thing that Carl Sagan ever said (bearing in mind that he used to insist that he never actually said “billions and billions”) was that “Extraordinary claims require extraordinary evidence.” On its own, ALH 84001 may simply not be quite extraordinary enough.* But it has certainly continued the recasting of expectations about where Martian life, or its relics, might be found. ALH 84001 was not a surface rock. It came from underground.
Long before ALH 84001, it was clear that the possibility of life, even fossilized life, on Mars was the most exciting of the planet’s mysteries. The fact that seeking out and interpreting fossils is a very hard thing to automate has always been one of the key arguments in any case for humans on Mars. If the exploration has to be done underground, the case becomes stronger still. There may be robotic ways to choose a site from which to explore the underground. Infrared scanners might be able to pick up the faint surface warmth that betokens an active hydrothermal system below. Radar used from orbit or from an aircraft should be able to reveal liquid water aquifers to drill into, if there are any, or subterranean ice deposits worth sampling. Aircraft with chemical sensors might be able to sniff out minute traces of gases produced by subterranean life; any methane or formaldehyde produced by underground anaerobes would last only hours in the Martian atmosphere, and so would tend to be found around vents that connected the surface to the life below.
Once you have found a place from which to start your underground explorations, though, whether it be by means of a drill rig or—a scenario to delight the imagination of Penny Boston—by actually exploring a cave system, you will have reached the limits of today’s remote sensing technology. Serious drilling in unknown territory requires a drilling crew; in this, if in no other particulars at all, the movie Armageddon was right. And caving requires speleologists. If you’re going to usher in a new age of Martian exploration with a serious attempt to understand what lies beneath the surface of Mars, you’re almost certainly going to need to send people to do it.
*In Geoffrey Landis’s Mars Crossing, a novel that tries to deal with a near-future mission to Mars as realistically as possible, the explorers go blond and get itchy as traces of peroxide-laced dust build up in their living quarters.
*Some deep-sea bacteria apparently photosynthesize using the light given off by their phosphorescent neighbors, which is remarkable but not very significant.
*The Columbia River basalts are also the basement rock beneath much of the channeled scablands, which makes them doubly Martian.
*Well-connected Mars Underground fellow traveler Leonard David got the story a day early.
*For a little thing like a bacterium, the twisting force exerted by the Earth’s magnetic field is much stronger than the force of gravity.
*There’s a wry little story by Stan Robinson, written in the form of a set of abstracts to articles in scientific journals, which suggests that even after people have been able to study the site on Mars from which the meteorite came they still won’t be able to agree. “Selected abstracts from The Journal of Areological Studies,” in The Martians.