Crystals of the mineral magnetite, embedded within a famous Martian meteorite, have provided scientists with the latest evidence that primitive life once existed on Mars.

In a pair of papers published in the February 27 issue of the journal Proceedings of the National Academy of Sciences (PNAS), two teams of scientists presented evidence that magnetite, either as individual crystals or in long complex chains, could only have been created in the Martian meteorite ALH84001 by ancient microbial life.

Top: Modern magnetotactic bacteria, one showing a chain of magnetite crystals, as seen in the backscattered scanning electron microscope. Bottom: Magnetite crystals and chains of magnetite crystals in the Martian meteorite ALH84001 in the backscattered scanning electron microscope. One conspicuous chain indicated by arrows. The diameter of a single crystal is approximately one-millionth of an inch. Photo: NASA

One group, led by Kathie Thomas-Keprta of NASA's Johnson Space Center, examined the individual crystals of the mineral found in the meteorite. They found that a quarter of the magnetite crystals in ALH84001 are identical to a type of magnetite that on Earth is created only by a specific strain of bacteria designated MV-1: evidence, they claim, that the magnetite crystals in ALH84001 were formed by similar bacteria of a Martian, not terrestrial, origin.

"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 JSC geologist and a coauthor of the paper. "We considered that perhaps Earth bacteria or Earth magnetite had gotten into the Mars meteorite, but extensive examination and testing by both our team and many other investigators eliminated that possibility."

Magnetite crystals can be made by a variety of organic and inorganic methods. However, the crystals identified in ALH84001 are unusual in that they are chemically pure and free of defects: properties common to magnetite created by magnetotactic bacteria on Earth but not to inorganic formation processes. "There is currently no known inorganic chemical means of producing these magnetite crystals with their unique morphologies," said Dennis Bazylinski of Iowa State University.

"The process of evolution has driven these bacteria to make perfect little bar magnets, which differ strikingly from anything found outside of biology," explained Joe Kirschvink, a Caltech geobiologist and a coauthor of the paper. "In fact, an entire industry devoted to making small magnetic particles for magnetic tapes and computer disk drives has tried and failed for the past 50 years to find a way to make similar particles."

A separate group led by Imre Friedmann of NASA's Ames Research Center focused on the long chains of magnetite crystals found in the meteorite. Those chains, which vaguely resemble a string of pearls, could only have been formed by organisms, they argued in their paper.

"The chains we discovered are of biological origin," said Friedmann. "Such a chain of magnets outside an organism would immediately collapse into a clump due to magnetic forces."

The chains likely served as a kind of compass for magnetotactic bacteria that lived within the meteorite when it was a rock on the surface of Mars, allowing the bacteria to orient themselves by using the planet's magnetic field. The flexible chains consist of tiny crystals of magnetite, each as small as 25 nanometers (one millionth of an inch) across, spaced far enough apart so that they do not touch: all additional evidence of a biogenic origin for the chains, Friedmann's group argued.

The two papers are the latest salvos in a scientific debate on the possible evidence of life in the Martian meteorite. The debate started in August 1996 when a team that included McKay and Thomas-Keprta published a paper in the journal Science that argued that magnetite crystals in ALH84001, along with carbonate deposits, traces of complex organic compounds, and features that strongly resembled microfossils, were all evidence of primitive life that existed on Mars early in the history of the solar system.

That paper ignited the interest the general public worldwide when first announced. However, in the months and years that followed other scientists argued against the conclusion that the meteorite was evidence of ancient life on Mars. While the meteorite's origin was widely accepted -- it is one of over a dozen such meteorites conclusively linked to Mars -- scientists debated the merits of other, inorganic methods that could create the features originally ascribed to biological processes.

One team of scientists argued as early as 1996 that the magnetite structures seen in the meteorite had whisker-like shapes inconsistent with a biological origin. Instead, they concluded that the magnetite crystals were made by inorganic processes at temperatures as high as 800 deg C (1470 deg F).

However, the crystals reported in the PNAS paper have a different shape: a "truncated hexaoctahedral", an elongated version of an octahedral crystal created by putting two four-sided pyramids together at their bases. "This is the first report of any crystal having the truncated hexaoctahedral habit," said Thomas-Keprta, "and as such, these crystals are interpreted as Martian magnetofossils and constitute evidence of the oldest life yet found."

The "oldest life" statement comes from separate work that dated carbonates in the meteorite believed to have an organic origin at 3.9 billion years old, older than even primitive life on Earth. "Mars is smaller than Earth and it developed faster," explained coauthor Simon Clemett. "Consequently, bacteria able to produce tiny magnets could have evolved much earlier on Mars."

These papers are unlikely to be the last word in the debate about life on Mars, but they may represent an important step forward in the search for life. "If the conclusions of this paper are correct, then this will be one of the most important papers ever published by PNAS," said one anonymous reviewer of the Thomas-Keprta paper quoted in a NASA press release. "What can be of greater scientific interest than evidence for extraterrestrial life?"