Each year, scientists estimate, a ton of fist-size and larger rocks from Mars land on Earth, and of those, a few make the trip quite quickly, in as little as one year.
May some of those at some time have carried some Martian life to Earth?
Magnetic studies of a Martian rock that crashed in Antarctica 13,000 years ago show that Martian bacteria, if they ever existed, could at least have made a safe departure from the planet. The interior of the meteorite never exceeded the temperature of a hot summer day, even as it was blasted off the surface of Mars by a large meteor impact, scientists reported last week.
This particular meteorite, known as ALH84001, almost certainly would not have ferried any microbial passengers. Although this meteorite has been the subject of a controversial claim that it contains fossils and other chemical traces of ancient, tiny Martian bacteria, by the time it was blasted into space, the surface of Mars had long since turned cold, dry and lifeless.
And even if any microbes were alive within ALH84001 when it was ejected from Mars, 15 million years of exposure to cosmic ray radiation during the meteorite's circuitous path to Earth would probably have killed all of them.
But not all Martian meteorites may have been lifeless.
Early in the history of the solar system, Mars is believed to have been warm and wet. ''It conceivably is a better place to get life going,'' said Dr. Joseph Kirschvink, a professor of geobiology at the California Institute of Technology in Pasadena.Continue reading the main story
Had life arisen on Mars, some of it would almost certainly have been carried to Earth through the continuing shower of meteorites.
That leads to the intriguing possibility that all living organisms on Earth are the descendants of microbes from Mars.
''So, conceivably,'' Dr. Kirschvink said, ''we're all Martians.''
The new ALH84001 research, reported in the current issue of the journal Science by Dr. Kirschvink and his collaborators at Caltech, Vanderbilt University in Nashville and McGill University in Montreal, focuses on a much narrower question: the temperature within the meteorite during the last four billion years.
Until a couple of decades ago, scientists believed that a high-pressure shock wave radiating outward from the impact would inevitably vaporize or melt the rocks that were knocked into space. But the Martian meteorites found on Earth show few signs of such damage.
In the early 1980's, Dr. H. Jay Melosh, a professor of planetary sciences at the University of Arizona's Lunar and Planetary Laboratory, suggested that with nothing but air pushing them, rocks at the surface would escape the annihilating pressure waves and be lifted off the planet intact.
''Kind of like squeezing a watermelon seed between your fingers,'' Dr. Melosh said. ''It can be gentle for certain rocks. This one has suffered no great insult and has been very gently blasted to planetary escape velocity.''
Earlier work had indicated that temperatures in the interior of ALH84001 had remained below 230 degrees, but 230 degrees is still hot enough to kill most bacteria.
In the latest work, the researchers examined grains of magnetic minerals within the meteorite, each about one-250th of an inch long. When the rock originally formed 4.5 billion years ago, the grains, behaving like tiny bar magnets, would have lined up in the same direction as Mars' magnetic field. Subsequent geological forces jumbled up the grains and left them with the magnets pointing in all directions.
If the meteorite had melted as it passed through Earth's atmosphere, the magnetic materials would have realigned in the direction of Earth's magnetic field. That occurred with the outer one-tenth of an inch of the meteorite, but the interior retained the jumbled magnetic structure.
Within a magnetically shielded furnace, the researchers found that the grains began to lose their magnetism when heated above 105 degrees. Since the grains are still magnetic, they must have never exceeded that temperature.
''This is the first experimental evidence that a meteorite can be blasted off a planet without being significantly heated,'' said Benjamin Weiss, a graduate student at Caltech and lead author of the Nature paper.
The trip is also two-way. Comets and asteroids have also blasted pieces of Earth into space, some of which made their way to Mars, although not as often, because more energy is required to accelerate rocks out of Earth's stronger gravity and reach Mars' more distant orbit.
While the experiment does not prove that life migrated back and forth between Earth and Mars, ''it makes it a lot more plausible,'' Dr. Melosh said.
A couple of years ago, Dr. Curt Mileikowsky, a physicist associated with the Royal Institute of Technology in Stockholm and former chief executive officer of the Saab automobile company, was curious about the possibility of meteors' spreading life through the solar system and galaxy, so he recruited a team of nine scientists, including Dr. Melosh, to offer some estimates.
In a paper published in June in the journal Icarus, the researchers calculated the total number of rocks shuttling between the two planets, then omitted those that reached bacteria-killing temperatures or spent so much time in space that all of the microbes would have been killed by cosmic rays.
More than five billion Martian rocks capable of carrying living microbes have fallen to Earth in the past four billion years, the researchers calculated. Before that, in the first half-billion years of the solar system's history, when there were a lot more meteors flying back and forth, some 50 billion potential bacteria-carrying Martian rocks tumbled down on Earth.
The smaller Mars cooled off before Earth. ''Therefore Mars had a chance to be an earlier starter,'' Dr. Mileikowsky said.
If life did evolve first on Mars, ''the probability is very, very large for this life to move over to Earth and proliferate there,'' he said.
''And therefore,'' he went on, ''the ancestor for all life on Earth may be a cell on Mars.''
In the opposite direction, according to the calculations, about one billion potential bacteria-carrying Earth rocks have rained down on Mars in the past 4 billion years and another 10 billion rocks in the first half-billion years before that.
At least one species of bacteria, Bacillus subtilis, is capable of surviving the rigors of space travel. In one experiment, 10 percent of the bacteria survived six years in the vacuum of space aboard a satellite orbiting Earth.
In a rock that originally carried a few million microbes, a 10 percent survival rate would still leave a few hundred thousand microbes to populate the new planet. ''Even if a lot more than 90 percent died, it's possible for a couple to be viable and revive,'' Dr. Melosh said.
Other experiments show Bacillus subtilis as well as another common bacteria, Deinococcus radiodurans, can also survive the tremendous jolt of being blasted into space by a meteor -- up to 100,000 times the pull of gravity -- and the bombardment of cosmic radiation during the trip.
The researchers say the findings suggest that NASA should not be overly cautious about Martian rocks brought back to Earth by a future space probe. Some have proposed that all samples be heat sterilized, to protect against unlikely Martian microbes.
If such microbes exist, they ''could already have gotten here anyway'' from meteorites, Dr. Kirschvink said.
Correction: November 2, 2000, Thursday An article in Science Times on Tuesday about the chance bacteria would survive in a meteorite from Mars referred inconsistently to the journal that published the research. It was Science, not Nature.Continue reading the main story