Crystals in Meteorite May Indicate Long-Ago Life on Mars, Scientists Say
Two scientific teams announced Monday that they have discovered the most compelling evidence yet from a Martian meteorite that life once existed on Mars. But the findings have not convinced many critics, who say such claims continue to be overblown.
The evidence comes from the controversial Martian meteorite dubbed ALH840001. Claims made about finding traces of fossil life in the four-pound, potato-shaped meteorite in 1996 were met with skepticism and even scorn by some scientists.
Some of those dramatic claims--including one that small worm-shaped objects in the rock were fossilized Martian creatures--have since been retracted. Another claim, that carbon compounds in the rock were the decayed bodies of ancient Martian organisms, has been stymied by arguments that such compounds could be created inorganically, or without life.
No one is suggesting that the 4.5-billion-year-old rock is evidence of current life on Mars, now a cold, dry and apparently lifeless place. But many scientists suggest that the Mars of a few billion years ago was more hospitable: warmer, wetter and possibly a home to life. In those meteorites, the teams are studying evidence of what could be fossilized traces of life.
The strongest evidence for ancient life within the meteorite now rests with tiny magnetic crystals, called magnetite, in the space rock. The purity, shape, size and orientation of the crystals suggest that they were created by a special type of bacteria that use magnetite crystals as compasses, said Kathie Thomas-Keprta, a microscopist with Lockheed Martin who has studied the rock intensively.
“People say, ‘Magnetite, that’s not too sexy.’ But it might be as useful as dinosaur bones,” said Thomas-Keprta. Thomas-Keprta’s team is reporting in today’s issue of the Proceedings of the National Academy of Sciences that the shape of the crystals found in the meteorite is identical to that of crystals created by a bacterial strain on Earth called MV-1. That bacterium lives in water, can survive without oxygen and can gain its energy from carbon dioxide.
“That fits just perfect with what we would see on Mars,” said Thomas-Keprta, who said she believes that the 3.9-billion-year-old crystals represent the “oldest life form ever found.” Because of their configuration within the space rock, the crystals would had to have formed on Mars, rather than on Earth after impact, scientists said.
The teams suggest that the magnetotactic bacteria lived in Martian mud billions of years ago and that remnants of the bacteria were forced into harder rock by the impact of a meteorite on the Martian surface. A subsequent impact 13 million to 16 million years ago then sent the rock into space for a long journey that ended on Earth about 13,000 years ago when the rock landed on the Allen Hills Antarctic ice sheet, where it lay until its discovery in 1984.
Another report, by an international team led by biologist Imre Friedmann of Florida State University and the NASA Ames Research Center, showed that the crystals exist in chains. The chains are surrounded by halos that appear to be remnants of bacterial membranes that once surrounded the crystals.
“Chains cannot be produced inorganically. A chain of magnets would immediately collapse into a clump because of the magnetic field. What keeps them from collapsing is the organism itself,” said Friedmann. “There’s no way it’s not a biological scenario.”
Other evidence that the magnetic crystals are biological in origin comes from earlier work by Thomas-Keprta indicating that the crystals are not touching each other, as inorganic magnets would, but were separated, possibly by a thin biological membrane.
The crystals--so small that a billion would fit on the head of a pin--are very uniform, and are in strange orientations not dictated by the forces of magnetism. This suggests that biological forces might have influenced them, said Simon Clemett, a cosmochemist at Johnson Space Center in Houston. Clemett began studying the rock in the mid-1990s as a graduate student at Stanford.
“That sense of control is something that couldn’t happen randomly. It’s very directed,” said Clemett. “I think of all the lines of evidence, this is by far the best. I can’t see it being disproven.”
But the theory is still far from being proved, according to other scientists.
“I’m not convinced they’re wrong--but I’m not persuaded they’re right,” said Peter Buseck, a mineralogist at Arizona State University and expert on magnetite crystals. “Not everyone shares their optimism.”
Buseck, who had not yet seen the new research, said evidence for chains of crystals and membrane sheaths around crystals would be very intriguing. But he added that the scientists had a long way to go before they proved that the crystal structures were indeed traces of life.
Added Richard Frankel, an expert on magnetotactic bacteria at Cal Poly San Luis Obispo: “In my view, it’s going to require confirmatory evidence from another source, a sample return [from Mars] or another meteorite.”
A large part of the argument that the crystals are traces of life is that such crystals had never been created inorganically. But that is no longer true.
In a paper to be published in American Minerologist next month, Doug Ming and colleagues at the Johnson Space Center--where most of the work looking for life in the meteorite has been done--show that the crystals can be produced inorganically, with no life involved.
“There is a feasible way to make these in the lab,” Ming said. The process involves mixing the right chemical constituents and creating a shock event, or short-term heating, like the one that a meteor impact might cause.
The experiment, Ming said, created crystals that were basically the same size, shape and chemistry as the crystals in the meteorite. Scientists are still examining whether their lab-grown crystals share other specific characteristics that Thomas-Keprta says are indicators of life.
Even if they find crystals that are exactly the same, he said, that won’t disprove that the meteorite’s crystal have a biological origin. But it should hold up a big red caution flag.
Whether there was life on an ancient Mars, he said, remains “an open question” that won’t be answered until scientists “drill and go down deep” beneath the Martian surface and possible return samples of Martian rock to Earth for study.
