In the latest study of a 4.5 billion-year-old Martian
meteorite, researchers have presented new evidence confirming
that 25 percent of the magnetic material in the meteorite was
produced by ancient bacteria on Mars. These latest results
were published in the journal Applied and Environmental

The researchers used six physical properties they refer to as
the Magnetite Assay for Biogenicity (MAB) to compare all the
magnetic material found in the ancient meteorite — using the
MAB as a biosignature. A biosignature is a physical and/or
chemical marker of life that does not occur through random
processes or human intervention.

“No non-biologic magnetite population, whether produced by
nature or in the laboratory, has ever met the MAB criteria,”
said Kathie Thomas-Keprta, an astrobiologist at NASA’s
Johnson Space Center (JSC) in Houston and the lead researcher
on the study. “This means that one-quarter of the magnetite
crystals embedded in the carbonates in Martian meteorite
ALH84001 require the intervention of biology to explain their

Magnetotactic bacteria, which occur in aquatic habitats on
Earth, arrange magnetite crystals in chains within their
cells to make compasses, which help the bacteria locate
sources of food and energy. Magnetite (Fe3O4) is produced
inorganically on Earth, but the magnetite crystals produced
by magnetotactic bacteria are very different — they are
chemically pure and defect-free, with distinct sizes and

Four of the MAB biosignature properties relate to the
external physical structure of the magnetite crystals, while
another refers to their internal structure and another to
their chemical composition.

In their earlier studies, the researchers found that
approximately one-quarter of the nanometer-sized magnetite
crystals in ALH84001 had remarkable physical and chemical
similarities to magnetite particles produced by a bacteria
strain on Earth called MV-1. This is the first time, however,
that any researcher has used the full MAB range of
biosignature properties to compare the proposed bacteria-
produced crystals in Mars meteorite ALH84001with the
bacteria-produced crystals from Earth and with the other
magnetites in the meteorite.

The comparison between the proposed bacteria-produced
crystals in the meteorite and crystals known to be produced
by Earth-bacteria MV-1 is striking and provides strong
evidence that these crystals were made by bacteria on Mars.

The fact that Mars Global Surveyor data suggest that early
Mars had a magnetic field is consistent with a reason why
Mars would have magnetotactic bacteria. “Our best working
hypothesis is that early Mars supported the evolution of
bacteria that share several traits with magnetotactic
bacteria on Earth, most notably the MV-1 group,” said Simon
Clemett, a coauthor of the paper at Johnson.

Mars has long been understood to provide the sources of light
and chemical energy sufficient to support life, but in 2001
the Mars Global Surveyor spacecraft observedMednetized
stripes in the crust of Mars, which showed that a strong
magnetic field existed in the planet’s early history, about
the same time as the carbonate containing the unique
magnetites in ALH84001 was formed.

In June, researchers using the Mars Odyssey spacecraft
announced that they had found water ice under the surface of
Mars. These attributes, coupled with a carbon dioxide-rich
atmosphere, would have provided the necessary environment for
the evolution of microbes similar to the fossils found in

“We believe this latest study proves that the magnetites in
ALH84001 can be best explained as the products of multiple
biogenic and inorganic processes that operated on early
Mars,” Thomas-Keprta said.

An international team of nine researchers collaborated on the
three-year study. The team, led by Thomas-Keprta of Lockheed
Martin at Johnson Space Center, was funded by the NASA
Astrobiology Institute. Co-authors of the study are Clemett
and Susan Wentworth of Lockheed Martin at JSC; Dennis
Bazylinski of Iowa State University (funded by the National
Science Foundation); Joseph Kirschvink of the California
Institute of Technology in Pasadena; David McKay and Everett
Gibson of JSC; Hojatollah Vali of McGill University in
Canada; and Christopher Romanek of the Savannah River Ecology

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