Life on a Young Planet (40 page)

One school of thought, championed with particular eloquence by astrophysicist Paul Davies, holds that Mars actually beat Earth to the mark. Davies believes that life may have begun on Mars and colonized the Earth by traveling in meteoritic “spacecraft.” ALH-84001 and its kin show us that the required trade route exists, and it is indeed possible that simple organisms in meteorite interiors could have survived launch, prolonged exposure to radiation in space, and arrival on Earth. The biggest hurdle might well have been ecological—what is the likelihood that a meteorite would land on a substrate its microscopic occupants could metabolize? The “we are all Martians” hypothesis is imaginative, as well as a bit subversive. It subtly undermines the notion that life might arise wherever Earth-like planets exist, because if it is correct, life didn’t originate on Earth. Before worrying about this overmuch, however, we should probably learn whether Mars ever supported biology.

Bruce Jakosky, a planetary scientist at the University of Colorado, and I once spent an idle afternoon concocting a title for a brief essay about Mars astrobiology. After (too?) much discussion, we arrived at “The Search for Life on Mars: Fish in a Barrel, Needle in a Haystack, or Pig in a Poke?” We chuckled over our cleverness for weeks afterward, but, perhaps wisely, never published the essay. After that title, there wasn’t much else to say.

We can readily eliminate one possibility raised in that tongue-in-cheek title—looking for life on Mars won’t be like shooting fish in a barrel. By the most optimistic estimates, we will set virtual foot on Mars a half dozen times in the next twenty years. Given NASA’s current ban on launching nuclear power sources, landing missions will be relatively short and the exploration radii consequently small. (On an early field test of the 2003 Mars rover conducted in a forlorn valley not far from Las Vegas, I wandered over to a nearby outcrop, looked it up and down, chipped off a bit of rock with my hammer, and examined it briefly by hand lens. Satisfied with my observations, I discarded the rock chip and walked back to the group. “That took three minutes,” noted Cornell’s Steve Squyres, principal investigator for the project. “On Mars, the rover will need a full day to do the same thing.”)

Figure 13.3.
Channel network carved by water into the surface of Mars. These channels formed early in the red planet’s history, principally by groundwater sapping and catastrophic melting. How much liquid water existed at any one time—whether early Mars had persistent oceans or long-lived rivers and lakes—remains a subject of debate. (Viking Image courtesy of NASA/JPL/Caltech)

Geologists interested in Mars’s planetary history favor landing sites that resemble the Grand Canyon; engineers charged with landing safely prefer Kansas. Given that we can explore only a small fraction of the Martian landscape by lander, there is a premium on learning all we can by orbital observation in order to choose sites that optimize scientific opportunity and technical feasibility. Our experience in Precambrian paleontology tells us where to look and how—promising targets include precipitated spring deposits and fine mudstones formed beneath
ancient water bodies. But, as noted above, terrestrial experience may not prepare us for what we find—our biological and paleobiological knowledge of terrestrial life furnishes blinders as well as a guide. Opportunities for false positives and false negatives abound, and it will be a challenge to get it right in the few precious opportunities we are granted.

We have no guarantee that life ever existed on Mars, and the aggressively oxidizing nature of its present surface will undoubtedly have erased some evidence that may once have lain within the grasp of rovers. Even if we succeed in coring sedimentary rocks to obtain unaltered samples of ancient shales, astropaleontology may prove frustrating. On Earth, the fingerprints of biology are everywhere because of photosynthesis, which enabled life to expand across our planet’s surface. In the absence of photosynthetic organisms, traces of Martian life (if it existed) might be common only in the immediate vicinity of hydrothermal springs.

There is, of course, another possibility, and that is to search for living and not fossil organisms. For years a small band of optimists has reasoned that chemosynthetic microorganisms might persist today in hydrothermal oases far below the Martian surface. It’s a long shot, to be sure, but in a world without data, all things are possible and most are worth investigating. It may be possible to detect subsurface water by microwave imaging from orbiting satellites, but at present deep drilling is well beyond our capabilities. For this reason, astrobiologists were energized by the recent announcement that liquid water has been present on the Martian surface within the past few million years and conceivably could be there today. The evidence, recognized by planetary scientists Michael Malin and Kenneth Edgett in images from the
Mars Global Surveyor
, consists of erosional gullies that originate in crater and valley walls and cut into the aprons of talus at their bases (
figure 13.4
). The gullied talus slopes contain few craters, a sign of relative youth among Martian landforms. Some gullies cut across dunes, but no dunes obscure gullies, reinforcing the view that these gullies are no more than a few million years old. On Earth, water sculpts similar features, and so Malin and Edgett favor groundwater seepage and surface runoff as an explanation for the Martian gullies. Quite a few planetary scientists have welcomed this interpretation, although Michael Carr, the doyen of Martian water, cautions that other explanations may be possible. As Carr reminds us, the gullies occur at mid- to high latitudes where the Martian surface is so cold that the presence of liquid water is hard to understand. One possibility is that Mars’s axis of rotation swings widely through time. This means that features currently in a deep freeze might thaw every few million years. That idea is intriguing but not necessarily cheering to astrobiologists—environments where liquid water is present for 1 million years and then absent for the next don’t easily sustain biology.

Figure 13.4.
Gullies cut in relatively recent times into talus slopes beneath the walls of Martian craters. Such features suggest that liquid water can still form at the Martian surface—but how continuously it forms and how long it lasts remain uncertain. (Mars Global Surveyor Image courtesy of NASA/JPL/Malin Space Systems)

The newly discovered Martian gullies present an exciting research challenge that promises to tell us much about Mars as a planet, regardless of what it reveals about habitability. Landing on them, however, won’t be easy—in this instance the “Grand Canyon problem” looms large.

My thoughts on Mars astrobiology emphasize problems over opportunities—a needle-in-a-haystack view, if you will. Am I too skeptical? Maybe, but naive enthusiasm has pervaded so much Mars rhetoric that
a mild corrective seems in order. We must not assume that the astrobiological exploration of Mars will be easy or that unambiguous answers will soon be in hand. The search for life on Mars will be difficult and may turn up nothing, but as part of a balanced effort to understand our planetary neighbor, it is worth doing and worth doing right. Bear in mind that a negative conclusion to our search will be just as important as a positive answer. If a thick atmosphere, volcanism, and liquid water do
not
provide a foolproof recipe for life, we’ll have to rethink our models of biogenesis—while feeling a bit lonelier than before.

The new age of Mars exploration is just beginning, and it is likely to continue well beyond my lifetime. No knows what we will find. Perhaps, then, we should leave the final word to a poet, at least for now. In his great novel
The Glass Bead Game
, Hermann Hesse wrote two beautiful lines that provide a fitting credo for Mars astrobiology:

Nothing is harder, yet nothing is more necessary, than to speak of certain things whose existence is neither probable nor demonstrable. The very fact that serious and conscientious men treat them as existing things brings them a step closer to existence and to the possibility of being born.

Mars may be a particularly attractive target for astrobiological exploration, but it is hardly the only one. Within our solar system, there is Europa, a Jovian moon whose icy surface may cover an ocean of water, and Titan, a moon of Saturn with an atmosphere of methane and hydrocarbon smog. Farther afield, the targets are potentially endless, but as we leave our solar system the rules of the game change. Some day, our descendants may find a loophole in the laws of physics, but until they do, astrobiological exploration of the greater universe will be done remotely.

One of the most exciting recent developments in astronomy has been the discovery of planets in orbit around nearby stars. So far, we’ve observed only giant planets (about the size of Saturn, or larger) in tight orbits around their stars, but that isn’t necessarily because such planets dominate the universe. At present, these are the only planets we can detect. By the end of the next decade, however, an ambitious project called the Terrestrial Planet Finder may enable us to detect Earthlike planets in nearby solar systems. Not only that, spectroscopic images may reveal the compositions of their atmospheres.

In the early 1970s, James Lovelock, father of the Gaia hypothesis, proposed that planetary atmospheres provide sensitive indicators of biological activity. To appreciate why, we need only look at the Earth. In addition to nitrogen gas and carbon dioxide, our atmosphere contains water vapor, oxygen, and a small but measurable amount of methane. It isn’t easy to see how oxygen and methane could coexist in an atmosphere governed by equilibrium chemical processes, but when cyanobacteria (or their chloroplast descendants) and methanogens are afoot, this mixture can be sustained indefinitely. (Of course, by this methodology, we couldn’t have pegged the Earth as biological for the first half of its history.)

We will not soon learn whether the inhabitants of extrasolar planets synthesize DNA or proteins, whether they are unicellular or include many-celled forms, or whether they live on land as well as in water. But on a distant planet where organisms are abundant and include the right metabolisms, we might recognize life by its environmental impact.

At distances greater than a few light years, however, even planet finders will fail us—planets in more distant galaxies are simply too dim and too far away to be detected in this way. For most of the vastness of the universe, then, we can search for life in only one way—by listening for technologically gifted beings who can answer our signals or initiate dialogue. On Earth, such capabilities have become available only within the past century.

In their book
Rare Earth
, Peter Ward and Donald Brownlee argue that intelligent life may be exceedingly uncommon in the universe, citing the myriad of astronomical and tectonic circumstances that contributed to the evolution of neurological complexity on Earth. Hap McSween earlier presented similar arguments in his fine volume
Fanfare for Earth
. Sometimes lampooned as “Goldilocks” hypotheses because they require everything to be “just right” for the evolution of intelligence, these arguments assume that because the conditions that facilitated our own evolution are particular, they must be rare. But we have no way of knowing whether this is true. We can agree that not all solar systems contain planets that are Earthlike in every way, but what if 10 percent do, or 1 percent, or even one in a million. Given the dimensions of the universe, this would provide untold millions of potential incubators for intelligent life. Put another way, planets with intelligent life could be
proportionally rare but absolutely abundant. We really don’t know how to assess the odds. And there is another kicker: we have no idea whether the route by which we achieved intelligence is the only one available. I doubt that it is, but I can’t come up with a convincing way to couch my prejudices in theory. The issue must be solved empirically.

It is ironic that the type of life most likely to be rare is the only one we can search for through most of the universe. It is also unfortunate, because the vastness of space makes it difficult and perhaps impossible to address one of astrobiology’s most fundamental questions: which attributes of terrestrial biology are general features of life and which are local consequences of our own peculiar history? If we don’t find life on Mars, and if we fail to detect organisms in Europan ice or oceans, we may never learn the answer—unless some alien with a crystal set tells us.