Researchers who stunned the world in 1996 with the announcement that a
Martian meteorite contained evidence of ancient life on the red planet
have released new evidence that strengthens their original hypothesis
and allays many of the criticisms leveled at the first paper.

In this latest paper, published in the scientific journal Precambrian
Research Feb. 17, two additional Martian meteorites were examined –
Nakhla and Shergotty, 1.3 billion and 165 to 175 million years old,
respectively. Both younger meteorites showed the same evidence of
microfossils and other remnants of early life as the original meteorite,
the 4.5-billion-year-old ALH84001. “If the features observed in the two
younger Martian meteorites are confirmed to have a biogenic origin, life
may have existed on Mars from 3.9 billion years ago to as recently as
165 to 175 million years ago,” said Everett K. Gibson, a geochemist at
the NASA Johnson Space Center in Houston and the senior author on the

Clusters of very small spheres found in the two younger meteorites are
very similar to those seen in bacteria-containing samples from deep
beneath the Earth’s surface in the Columbia River Basalts in eastern
Washington. Whether or not these sphere-like structures are true
biomarkers has yet to be determined, but the fact that they are embedded
in or coated by clays that are clearly of Martian origin suggests that
they too were formed on Mars.

Studies using a transmission electron microscope have provided further
evidence of fossils in the original Martian meteorite, ALH84001. This
evidence is in the form of tiny magnetite crystals, identical to those
used by aqueous bacteria on Earth as compasses to find food and energy.
Magnetite (Fe3O4) is produced inorganically on Earth, but the magnetite
crystals produced by magnetotactic bacteria are different ñ they are
chemically pure and defect-free, with a distinct size and shape.
Magnetotactic bacteria arrange these magnetite crystals in chains within
their cells.

Additional studies showed that a substantial portion of the hydrocarbons
found in the meteorites were in them when they left Mars and are not the
result of terrestrial contamination. There is also strong evidence that
most of the carbonates in all three meteorites was formed at a time when
Mars was warmer and wetter – an environment much more conducive to life
than the current surface of Mars.

Terrestrial contamination of extraterrestrial samples is an issue not
only with these meteorites, according to the authors, but one that is
being studied in relation to the future return of Martian samples to
Earth. “It’s clear that we need to better understand the biosignatures
of terrestrial and extraterrestrial samples so that when Martian samples
are eventually brought back to Earth, we can determine the presences or
absence of life with certainty,” Gibson said. “However, if water exists
beneath the Martian surface, why shouldnít life be present today on

The other authors of this work, which was funded by NASAís Exobiology
Program and NASA’s Astrobiology Institute, are David S. McKay of JSC;
Kathie L. Thomas-Keprta, Susan J. Wentworth, and Mary Sue Bell of
Lockheed Martin at JSC; Frances Westall, a National Research Council
Fellow at the Lunar & Planetary Institute in Houston; Andrew Steele and
Jan Toporski of the University of Portsmouth, England; and Christopher
S. Romanek of the Savannah River Ecology Laboratory. Of these, Gibson,
McKay, Thomas-Keprta and Romanek were authors of the original paper on
the subject.

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