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.

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.

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.

Tilting toward Life

But Mars goes through cycles of obliquity, or changes in its orbital tilt. Currently, Mars is wobbling back and forth between 15 and 35 degrees' obliquity, on a timescale of about 100,000 years.

But every million years or so, it leans over as much as 60 degrees. Along with these changes in obliquity come changes in climate and atmosphere.

Some scientists speculate that during the extremes of these obliquity cycles, Mars may develop an atmosphere as thick as Earth's, and could warm up considerably. Enough for dormant life to reawaken.

"Because the climate can change on long terms," says Clark, ice in some regions on Mars periodically could "become liquid enough that you would be able to actually come to life and do some things - grow, multiply, and so forth - and then go back to sleep again" when the thaw cycle ended."

"There are organisms on Earth that, when conditions become unfavorable, can form "spores which are so resistant that they can last for a very long time. Some people think millions of years, but that's a little controversial."

Desulfovibrio is not such an organism. It doesn't form spores. But its bacterial cousin, Desulfotomaculum, does.

"Usually the spores form because there's something missing, like, for example, if hydrogen's not available, or if there's too much [oxygen], or if there's not sulfate", says Clark.

"The bacteria senses that the food source is going away, and it says, 'I've got to hibernate,' and will form the spores. The spores will stay dormant for extremely long periods of time."

"But they still have enough machinery operative that they can actually sense that nutrients are available. And then they'll reconvert again in just a matter of hours, if necessary, to a living, breathing bacterium, so to speak. It's pretty amazing," adds Clark.

That is not to say that future Mars landers should arrive with life-detection equipment tuned to zero in on species of Desulfovibrio or Desulfotomaculum.

There is no reason to believe that life on Mars, if it ever emerged, evolved along the same lines as life on Earth, let alone that identical species appeared on the two planets.

Still, the capabilities of various organisms on Earth indicate that life on Mars - including dormant organisms that could spring to life again in another few hundred thousand years - is certainly possible.

Clark says that he doesn't "know that there's any organism on Earth that could really operate on Mars, but over a long period of time, as the martian environment kept changing, what you would expect is that whatever life had started out there would keep adapting to the environment as it changed."

Detecting such organisms is another matter. Don't look for it to happen any time soon. Spirit and Opportunity were not designed to search for signs of life, but rather to search for signs of habitability.

They could be rolling over fields littered with microscopic organisms in deep sleep and they'd never know it. Even future rovers will have a tough time identifying the martian equivalent of dormant bacterial spores.

"The spores themselves are so inert," Clark says, "it's a question, if you find a spore, and you're trying to detect life, how do you know it's a spore, [and not] just a little particle of sand?"

"And the answer is: You don't. Unless you can find a way to make the spore do what's called germinating, going back to the normal bacterial form."

That, however, is a challenge for another day.

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