Now that there's conclusive evidence that at least part of Mars was once a water-soaked place where living things could have wriggled, swum or slithered, it takes only a few more leaps of speculation to wonder how they might have died.
Did their eyes bug out like Arnold Schwarzenegger's in "Total Recall"? Not likely — hypothetical Martian creatures probably wouldn't have had enough time to evolve eyes before the planet became the cold and arid place it is today.
In the optimistic picture of life on Mars, a thick blanket of carbon dioxide created a greenhouse effect that warmed the planet for its first billion years or so, and lakes and oceans dotted the surface. (The pessimistic view is that it was always cold and lifeless.)
But for at least 500 million years, Mars, like Earth, endured a period known as "heavy bombardment," when it was repeatedly whacked by meteors large enough to vaporize the oceans.
Life probably wouldn't have had enough time to gain a foothold between impacts, and even if it had, it would have been boiled and steamed to oblivion by the next meteor. Thus, life on Mars would likely have had, at best, a run of a few hundred million years.
Even so, some scientists speculate that life could have been more precocious on Mars than on Earth. Life on Earth started between 3.9 billion and 3.5 billion years ago. Mars, only half as wide as Earth and with only one-tenth its mass, would have been a smaller target for meteors and might have become habitable sooner.
In addition, Mars doesn't have plate tectonics, and some scientists have argued that the lack of movement in the planet's crust could have led to a different chemistry in the air and rocks, leading to an early build-up of oxygen produced by the photosynthesis of plants.
While life remained stuck at the single-celled level for a billion years on an oxygen-poor Earth, evolution on Mars might have led much more quickly to microscopic animals. But then, about 3.5 billion years ago, the planet turned chilly, and the Martians — if they ever existed — would have been snuffed out in any of several ways:
They froze. Most of the planet's carbon dioxide either leaked into space or was permanently transformed, via chemical reactions, into rock. On Earth, through plate tectonics, whatever rock goes down comes back up through volcanoes, and the carbon dioxide is returned to the air. On Mars, it's a one-way street, and its carbon dioxide, locked into place, disappeared from the air.
As the carbon dioxide vanished, so did the greenhouse effect. As a result, Mars today is like an Antarctica without an atmosphere.
They dried up. Even at Meridiani Planum, the place where scientists said last week the Mars rover Opportunity had found evidence of "soaked" rocks, the discovery of vast quantities of sulfur salts suggests the former presence of a wet area like a lake that dried up.
They suffocated. If Martian life reached the animal level, with creatures able to live off oxygen produced by plants, they died off as the plant life dried up or froze. Kill off Earth's plant life, and animals here would run out of stuff to breathe, too.
They starved. As the planet's flowing water froze or dried up, Martian microbes chomped up all the available food. "You need phosphate," said Dr. Andrew Knoll of Harvard University. "You need nitrogen. It's not clear to me that biologically useful forms of those nutrients would be abundant."
They were fried. Mars does not have a magnetic field to deflect the solar wind, and its thin atmosphere does not block out ultraviolet light. Thus, Martian organisms were done in by sunburn and cancer.
They went underground. The truly optimistic say life could persist underground on Mars, just as microbes fill the top few miles of Earth's crust. The overlying soil would protect them from the cosmic and ultraviolet radiation, and liquid water would be available from underground ice melted by heat from the interior. "It could easily have migrated to the subsurface and exist today," said Dr. Bruce Jakosky, a professor of geological sciences at the University of Colorado.
They are us. Billions of pieces of Mars, blasted into space by meteors, have landed on Earth. Scientists have shown that the interior of a Martian meteorite found in Antarctica never reached temperatures hot enough to kill bacteria.
If life actually did arise on Mars, then at least some Martians possibly made it here.
At least one species of earthly bacteria, Bacillus subtilis, is capable of surviving the rigors of space travel. In one experiment, 10 percent of this species sent into orbit on a satellite survived six years in the vacuum of space. Other experiments show that Bacillus subtilis, as well as another common bacterium, Deinococcus radiodurans, can also survive the tremendous jolt of being blasted into space by a meteor — up to 100,000 times the normal pull of gravity on Earth — as well as the bombardment of cosmic radiation during the trip.
In a Martian rock that originally carried a few million microbes, a 10 percent survival rate would still leave a few hundred thousand Martian microbes to populate Earth.
Or maybe there never were native Martians or Earthlings at all, and life originated from a third planet.
"We can always blame it on Venus," Jakosky said.