From Astrobiology Magazine 30 August 2004
Was Mars once a living world? Does life continue, even today, in a holding pattern, waiting until the next global warming event comes along? Many people would like to believe so. Scientists are no exception. But so far no evidence has been found that convinces even a sizable minority of the scientific community that the red planet was ever home to life. What the evidence does indicate, though, is that Mars was once a habitable world. Life, as we know it, could have taken hold there.
The discoveries made by NASA's Opportunity rover at Eagle Crater earlier this year (and being extended now at Endurance Crater) leave no doubt that the area was once "drenched" in water. It might have been shallow water. It might not have stuck around for long. And billions of years might have passed since it dried up. But liquid water was there, at the martian surface, and that means that living organisms might have been there, too.
Left: rover computer rendering on the edge of a depression, much like Opportunity's perch on the edge of Endurance Crater. Image credit: Don Maas/NASA/JPL. Right: liquid water may have flowed episodically over the surface of Mars in the planet's distant past. Artist conception of a delta filling a crater. Image credit: NASA. So suppose that Eagle Crater—or rather, whatever land formation existed in its location when water was still around—was once alive. What type of organism might have been happy living there? Probably something like bacteria. Even if life did gain a foothold on Mars, it's unlikely that it ever evolved beyond the martian equivalent of terrestrial single-celled bacteria. No dinosaurs; no redwoods; no mosquitoes—not even sponges, or tiny worms. But that's not much of a limitation, really. It took life on Earth billions of years to evolve beyond single-celled organisms. And bacteria are a hardy lot. They are amazingly diverse, various species occupying extreme niches of temperature from sub-freezing to above-boiling; floating about in sulfuric acid; getting along fine with or without oxygen. In fact, there are few habitats on Earth where one or another species of bacterium can't survive.
What kind of microbe, then, would have been well adapted to the conditions that existed when Eagle Crater was soggy? Benton Clark III, a Mars Exploration Rover (MER) science team member, says his "general favorite" candidates are the sulfate-reducing bacteria of the genus Desulfovibrio. Microbiologists have identified more than 40 distinct species of this bacterium.
Eating rocks We tend to think of photosynthesis as the engine of life on Earth. After all, we see green plants nearly everywhere we look and virtually the entire animal kingdom is dependent on photosynthetic organisms as a source of food. Not only plants, but many microbes as well, are capable of carrying out photosynthesis. They're photoautotrophs; they make their own food by capturing energy directly from sunlight. But Desulfovibrio is not a photoautotroph; it's a chemoautotroph. Chemoautotrophs also make their own food, but they don't use photosynthesis to do it. In fact, photosynthesis came relatively late in the game of life on Earth. Early life had to get its energy from chemical interactions between rocks and dirt, water, and gases in the atmosphere. If life ever emerged on Mars, it might never have evolved beyond this primitive stage.
Desulfovibrio makes its home in a variety of habitats. Many species live in soggy soils, such as marshes and swamps. One species was discovered all snug and cozy in the intestines of a termite. All of these habitats have two things in common: there's no oxygen present; and there's plenty of sulfate available.
Round spore of sulfur-reducing bacterium. The rod-shaped Desulfotomaculum exists both in an unsporulated, "free-tumbling" form in sulfate-rich water and this round sporulated form. Image Credit: Mazák Károly. Sulfate reducers, like all chemoautotrophs, get their energy by inducing chemical reactions that transfer electrons between one molecule and another. In the case of Desulfovibrio, hydrogen donates electrons, which are accepted by sulfate compounds. Desulfovibrio, says Clark, uses "the energy that it gets by combining the hydrogen with the sulfate to make the organic compounds" it needs to grow and to reproduce.
The bedrock outcrop in Eagle Crater is chock full of sulfate salts. But finding a suitable electron donor for all that sulfate is a bit more troublesome. "My calculations indicate [that the amount of hydrogen available is] probably too low to utilize it under present conditions," says Clark. "But if you had a little bit wetter Mars, then there [would] be more water in the atmosphere, and the hydrogen gas comes from the water" being broken down by sunlight.
So water was present; sulfate and hydrogen could have as an energy source. But to survive, life as we know it needs one more ingredient carbon. Many living things obtain their carbon by breaking down the decayed remains of other dead organisms. But some, including several species of Desulfovibrio, are capable of creating organic material from scratch, as it were, drawing this critical ingredient of life directly from carbon dioxide (CO2) gas. There's plenty of that available on Mars.
All this gives reason to hope that life that found a way to exist on Mars back in the day when water was present. No one knows how long ago that was, or whether such a time will come again. It may be that Mars dried up billions of years ago and has remained dry ever since. If that is the case, life is unlikely to have found a way to survive until the present.
Drill hole from rock abrasion tool showing spiral or fluted rock form below the surface of El Capitan. Image credit: NASA/JPL/Cornell.