5. Life on Mars?

Ups and Downs in the Search for Life. The question of life on Mars has fascinated the public imagination since the 1890s, when Percival Lowell began popularizing his theory that an alien civilization built the Martian "canals". Lowell's ideas convinced few of his colleagues. But up until the 1960s, it was widely believed that seasonal changes observed on the Red Planet might be due to vegetation. Then in 1965, the first successful flyby of Mars dashed hopes for any life on the planet. The Mariner 4 spacecraft sent back pictures of the Martian southern hemisphere, which unexpectedly showed a bleak cratered landscape like that of the Moon.

The picture changed again in 1971, when Mariner 9 made the first space-based global map of Mars. Mariner 9 revealed that the northern hemisphere of Mars has an almost Earthlike surface, with spectacular volcanoes and features that look like dry flood channels and river valleys. Suddenly, the prospects for life on Mars looked better. (But not intelligent life -- Mariner 9 proved once and for all that Lowell's "canals" never existed.)

In 1976 two Viking spacecraft arrived at Mars. Each included an orbiter and a lander. The orbiters mapped the Martian surface in amazing detail. They revealed that the flood channels and valley networks are overprinted by impact craters that theoretically date from the first two billion years of Martian history. Thus there was evidently running water on Mars but only in the distant past. The Martian climate must then have been dramatically different than it is now. Somehow, Mars has evolved from the ancient wet and warm conditions to the frozen desert we see today.

Martian Meteorite

The Viking 1 and 2 landers were sent to the surface of Mars specifically to search for chemical evidence of life. But the results were negative. One of experiments was designed to look for organic molecules, but it found not even the slightest trace of such material. Most of the scientists concluded that this ruled out any life on Mars, past or present. They assumed that even if there was life on Mars long ago, it should have left at least some organic residues in the soil.

Then, in 1996 came the startling report of possible evidence for microscopic fossil life on Mars, based on the analysis of a Martian meteorite. Clearly, the story of life on Mars has had its ups and downs, and it is far from finished.

Signs of Life in the Mars Rock? About a dozen meteorites from Mars have been identified on the Earth. All of them contain trapped gases in the same proportions as found by the Viking landers in the atmosphere of Mars. The evidence for their Martian origin is thus quite compelling.

The Martian meteorites escaped from the planet in explosions caused by collisions of asteroids or comets with the planet. Most of those meteorites are less than 1.5 billion years -- that is, they formed after the time when the surface of Mars apparently had liquid water. The sole exception, the Martian meteorite ALH 84001[credit Lunar and Planetary Institute], is about 4.5 billion years old, or as old as Mars itself. So it must have already been there when the surface of Mars had liquid water, and perhaps life. For that reason, David McKay and his team studied it carefully for signs of life.

Martian microfossils?

The evidence for fossil life in ALH 84001, according to the investigators, was found in carbonate globules deposited in cracks in the rock. These tiny blobs, resembling limestone, formed when the rock was exposed to liquid water. Inside the carbonate globules, the scientists found three major indicators of possible life: (1) complex organic molecules (the so-called polycyclic aromatic hydrocarbons), (2) microscopic grains of magnetic iron (magnetite) similar to the kind made on Earth by certain bacteria, and (3) tiny rod-shaped objects that resemble fossil bacteria.

Each of these unusual features might have a non-biological explanation. But it was the combination of all of them in the same part in the rock that persuaded McKay and his team to consider that ancient life on Mars might be the simplest explanation. There is now a healthy and vigorous scientific debate about every aspect of the Mars rock evidence, and as yet no consensus has emerged. So stay tuned. (For more details, see "Fossil Life in ALH84001?", compiled by the Allen Trieman)

The Hostile Surface of Mars. In retrospect, it's not surprising that the Viking landers found no signs of life on Mars. The surface is a very hostile environment to life as we know it, in large part because the atmosphere is so thin. The atmospheric pressure is much too low to allow any liquid water to exist on the surface. The atmosphere also has no oxygen, so it lacks any ozone (a form of oxygen) to shield the surface against deadly ultraviolet light from the Sun. Furthermore, the low atmospheric pressure and the weak magnetic field of Mars offer little protection against cosmic radiation from the Sun and from deep space.

The Viking lander experiments found not a trace of organic matter in the dust of Mars. This was taken as the strongest evidence against any life on Mars. But the investigators overlooked a curious feature of the Martian environment: With the dry surface exposed to very low atmospheric pressure and high winds, the Martian dust storms should generate an electrostatic phenomenon called "glow discharge" (the swirling dust will actually glow faintly in the dark). Glow discharge is very efficient at destroying any exposed organic molecules. Thus, even if the subsurface of Mars were loaded with organic matter, perhaps even living microbes, we should not expect the dust on the surface to show any traces of it.

In fact, the Viking experiments told us nothing about the possibility of life below the surface of Mars, where organic matter is shielded from the hostile surface environment. The Martian meteorites almost certainly originated from well below the surface. That's because only rocks ejected from the impact explosions that make big craters can escape from Mars, and most of the debris volume of a big crater comes from well below the surface. We now know that several of the Martian meteorites contain complex organic matter. This shows that the subsurface of Mars is chemically very different from the surface.

Could Life Exist on Mars Today? The discovery that water once ran on the surface of Mars raised the real possibility of ancient life on the Red Planet. Perhaps future missions will find Martian fossils. But what about life on Mars today? At depths of hundreds of meters below the surface, the pressures and temperatures are high enough to allow liquid water to exist. The subsurface is also shielded from glow discharge, solar ultraviolet light, and cosmic rays. Could life exist down there today?

In the last few years scientists have discovered that microbial life is thriving deep below the surface of the Earth, in pore fluids and fissures of the crustal rock. This subterranean life gets its food and energy from the chemistry of the Earth itself, and is entirely independent of sunlight. Such a "deep biosphere" might also exist below the surface of Mars (and perhaps in the larger satellites of some of the outer planets). We know of nothing to prevent it.

If microbes once lived below the surface of Mars, as suggested by the famous Martian meteorite, then their descendants may still live there today. The subsurface environment of Mars has probably not changed very much in the last few billion years, compared to the surface

Microbes on Earth are a very hardy and diverse lot. They evolve rapidly to take advantages of all possible opportunities offered by the environment. This might also be the case for any microbial life forms on Mars. Thus, if microbes actually lived on Mars billions of years ago, their descendants may still be there today. But only well below the surface, in a relatively stable and protected environment, with liquid water and chemical energy.