Further study of a Martian meteorite thought by some scientists to contain signs of ancient life has revealed more evidence that small structures in the rock are in fact of biological origin.
A team of researchers found that crystals of a mineral called magnetite, which had been found previously in the meteorite, is arranged in long chains. The team says these chains could have been formed only by living organisms, though those organisms are long since dead.
Upper figure: Modern magneto tactic bacteria, one showing a chain of magnetite crystals, as seen in the backscattered scanning electron microscope. Lower figure: Magnetite crystals and chains of magnetite crystals in the Martian meteorite ALH 84001 in the same type of electron microscope. One conspicuous chain is indicated by arrows. The diameter of a single crystal is about one-millionth of an inch.
"The chains we discovered are of biological origin," said Mire Friedman, a scientist at NASA's Ames Research Center scientist and leader of the research team.
The finding leads the researchers to suggest that life may have once existed on Mars, when the planet was presumed to be warmer and wetter. Further, researchers increasingly believe that life on Earth could have come from a Mars rock. There is no direct evidence yet to support this possibility, however.
Five-year debate
The Allan Hills meteorite, known to scientists as ALH 84001, is thought to be composed of material that is 4.5 billion years old -- just a bit younger than the entire solar system. The potato-sized object was dug up from a half-mile (1 kilometer) under the Martian surface sometime later when an asteroid or comet hit the Red Planet.
The rock wandered through space for ages before Earth's gravity got hold of it. It fell to our planet sometime between 11,000 and 13,000 years ago, researchers say. Scientists found the meteorite in 1984, in Antarctica's Allan Hills ice field.
A 1996 study of the rock, conducted by researchers at NASA's Johnson Space Center (JSC) in Houston and at Stanford University, claimed to find fossilized trails of microbial life that originated on Mars. The structures being studied are so small that nearly a billion could fit on the head of a pin.
Many scientists doubted the finding, saying the features could have been created on Earth or might not have indicated microbial life at all. Researchers have debated the evidence ever since, and though there is wide agreement that the structures were created on Mars, the debate continues as to their origin.
String of pearls
The newly found chains were formed inside organic material which has a structure that held the crystals together, the researchers say.
"The end result looks somewhat like a string of pearls," Friedman said. Each magnetite crystal in the chain is a tiny magnet, approximately one-millionth of an inch in diameter. Magnetite is an iron oxide, similar to iron rust.
The chains may have served as compasses for bacteria that would navigate with the help of the magnetic crystal chains inside their bodies. The organisms are referred to as magneto tactic bacteria. The chains were preserved in the meteorite long after the bacteria themselves decayed, the researchers say.
The scientists say the magnetite chains probably were flushed into microscopic cracks inside the Martian rock after it was shattered by an asteroid impact approximately 3.9 billion years ago. This cataclysmic event on Mars surface also may have killed the bacteria. The same or a later, asteroid impact ejected the rock, now a meteorite, into space.
Friedman’s team discovered the crystal chains using a technique that enabled them to "see" the tiny chains inside the meteorite without destroying them. Besides the chain-like formation, the team discovered that individual crystals are of similar size and shape, do not touch each other and that the chains themselves are flexible -- further evidence of biological origin.
That a small meteorite from Mars presumably contained large numbers of bacteria suggests that such bacteria were widespread on the surface of Mars, the researchers say. In addition, since magneto tactic bacteria are known to require oxygen, the finding indicates that photosynthetic organisms, the source of oxygen in Earth's atmosphere, must have been present and active on Mars 3.9 billion years ago.
The results are reported in the Feb. 27 Proceedings of the National Academy of Sciences.
More findings
Another research group, led by Kathie Thomas-Keprta of NASA's Johnson Space Center, writes in the same issue of journal that the magnetite crystals inside the meteorite are similar to those formed by modern magneto tactic bacteria now living on Earth.
"This group of magnetite deeply embedded in the Mars meteorite is so similar to the ones produced by the Earth bacteria that they cannot be told apart by any known measurement," said David McKay, a geologist at JSC and a coauthor of the second paper.
"We considered that perhaps Earth bacteria or Earth magnetite had gotten into the Mars meteorite," McKay said, "but extensive examination and testing by both our team and many other investigators eliminated that possibility."
The study by Thomas-Keprta and her colleagues was first reported in the December issue of Geochemical et Cosmochimica Acta, the journal of the international Geochemical Society and Meteoritic Society.
One of many Mars rocks
Experts estimate that roughly a ton of Martian rocks fall on Earth every year. More than a dozen have been found, while others bury themselves in sand or ice, or disappear into the seas. Until late last year, researches had wondered whether a Mars rock might be capable of transporting life to Earth.
A study of the Allan Hills meteorite in October 2000 found that meteorites from Mars remain cool enough on their journey to Earth to sustain embedded microbial life -- if there were any. While no evidence has ever shown that Allan Hills or any other meteorite contained actual life, the October research showed that space rocks could act as vessels for shuttling organisms around the solar system.