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Meteorite Yields Evidence of Primitive Life on Early Mars
Donald L. Savage
Headquarters, Washington, DC August 7, 1996
Johnson Space Center, Houston, TX
Stanford University, Palo Alto, CA
METEORITE YIELDS EVIDENCE OF PRIMITIVE LIFE ON EARLY MARS
A NASA research team of scientists at the Johnson Space
Center (JSC), Houston, TX, and at Stanford University, Palo
Alto, CA, has found evidence that strongly suggests primitive
life may have existed on Mars more than 3.6 billion years ago.
The NASA-funded team found the first organic molecules
thought to be of Martian origin; several mineral features
characteristic of biological activity; and possible
microscopic fossils of primitive, bacteria-like organisms
inside of an ancient Martian rock that fell to Earth as a
meteorite. This array of indirect evidence of past life will
be reported in the August 16 issue of the journal Science,
presenting the investigation to the scientific community at
large for further study.
The two-year investigation was co-led by JSC planetary scientists
Dr. David McKay, Dr. Everett Gibson and Kathie Thomas-Keprta
of Lockheed-Martin, with the major collaboration of a Stanford
team headed by Professor of Chemistry Dr. Richard Zare, as
well as six other NASA and university research partners.
"There is not any one finding that leads us to believe
that this is evidence of past life on Mars. Rather, it is a
combination of many things that we have found," McKay said.
"They include Stanford's detection of an apparently unique
pattern of organic molecules, carbon compounds that are the
basis of life. We also found several unusual mineral phases
that are known products of primitive microscopic organisms on
Earth. Structures that could be microsopic fossils seem to
support all of this. The relationship of all of these things
in terms of location - within a few hundred thousandths of an
inch of one another - is the most compelling evidence."
"It is very difficult to prove life existed 3.6 billion
years ago on Earth, let alone on Mars," Zare said. "The
existing standard of proof, which we think we have met,
includes having an accurately dated sample that contains
native microfossils, mineralogical features characteristic of
life, and evidence of complex organic chemistry."
"For two years, we have applied state-of-the-art
technology to perform these analyses, and we believe we have
found quite reasonable evidence of past life on Mars," Gibson
added. "We don't claim that we have conclusively proven it.
We are putting this evidence out to the scientific community
for other investigators to verify, enhance, attack -- disprove
if they can -- as part of the scientific process. Then,
within a year or two, we hope to resolve the question one way
or the other."
"What we have found to be the most reasonable
interpretation is of such radical nature that it will only be
accepted or rejected after other groups either confirm our
findings or overturn them," McKay added.
The igneous rock in the 4.2-pound, potato-sized
meteorite has been age-dated to about 4.5 billion years, the
period when the planet Mars formed. The rock is believed to
have originated underneath the Martian surface and to have
been extensively fractured by impacts as meteorites bombarded
the planets in the early inner solar system. Between 3.6
billion and 4 billion years ago, a time when it is generally
thought that the planet was warmer and wetter, water is
believed to have penetrated fractures in the subsurface rock,
possibly forming an underground water system.
Since the water was saturated with carbon dioxide from
the Martian atmosphere, carbonate minerals were deposited in
the fractures. The team's findings indicate living organisms
also may have assisted in the formation of the carbonate, and
some remains of the microscopic organisms may have become
fossilized, in a fashion similar to the formation of fossils
in limestone on Earth. Then, 16 million years ago, a huge
comet or asteroid struck Mars, ejecting a piece of the rock
from its subsurface location with enough force to escape the
planet. For millions of years, the chunk of rock floated
through space. It encountered Earth's atmosphere 13,000 years
ago and fell in Antarctica as a meteorite.
It is in the tiny globs of carbonate that the
researchers found a number of features that can be interpreted
as suggesting past life. Stanford researchers found easily
detectable amounts of organic molecules called polycyclic
aromatic hydrocarbons (PAHs) concentrated in the vicinity of
the carbonate. Researchers at JSC found mineral compounds
commonly associated with microscopic organisms and the
possible microscopic fossil structures.
The largest of the possible fossils are less than 1/100
the diameter of a human hair, and most are about 1/1000 the
diameter of a human hair - small enough that it would take
about a thousand laid end-to-end to span the dot at the end of
this sentence. Some are egg-shaped while others are tubular.
In appearance and size, the structures are strikingly similar
to microscopic fossils of the tiniest bacteria found on Earth.
The meteorite, called ALH84001, was found in 1984 in
Allan Hills ice field, Antarctica, by an annual expedition of
the National Science Foundation's Antarctic Meteorite Program.
It was preserved for study in JSC's Meteorite Processing
Laboratory and its possible Martian origin was not recognized
until 1993. It is one of only 12 meteorites identified so far
that match the unique Martian chemistry measured by the Viking
spacecraft that landed on Mars in 1976. ALH84001 is by far
the oldest of the 12 Martian meteorites, more than three times
as old as any other.
Many of the team's findings were made possible only
because of very recent technological advances in high-
resolution scanning electron microscopy and laser mass
spectrometry. Only a few years ago, many of the features that
they report were undetectable. Although past studies of this
meteorite and others of Martian origin failed to detect
evidence of past life, they were generally performed using
lower levels of magnification, without the benefit of the
technology used in this research. The recent discovery of
extremely small bacteria on Earth, called nanobacteria,
prompted the team to perform this work at a much finer scale
than past efforts.
The nine authors of the Science report include McKay,
Gibson and Thomas-Keprta of JSC; Christopher Romanek, formerly
a National Research Council post-doctoral fellow at JSC who is
now a staff scientist at the Savannah River Ecology Laboratory
at the University of Georgia; Hojatollah Vali, a National
Research Council post-doctoral fellow at JSC and a staff
scientist at McGill University, Montreal, Quebec, Canada; and
Zare, graduate students Simon J. Clemett and Claude R. Maechling
and post-doctoral student Xavier Chillier of the Stanford
University Department of Chemistry.
The team of researchers includes a wide variety of
expertise, including microbiology, mineralogy, analytical
techniques, geochemistry and organic chemistry, and the
analysis crossed all of these disciplines. Further details on
the findings presented in the Science article include:
* Researchers at Stanford University used a dual laser mass
spectrometer -- the most sensitive instrument of its type in
the world -- to look for the presence of the common family of
organic molecules called PAHs. When microorganisms die, the
complex organic molecules that they contain frequently degrade
into PAHs. PAHs are often associated with ancient sedimentary
rocks, coals and petroleum on Earth and can be common air
pollutants. Not only did the scientists find PAHs in easily
detectable amounts in ALH84001, but they found that these molecules were
concentrated in the vicinity of the carbonate globules. This
finding appears consistent with the proposition that they are
a result of the fossilization process. In addition, the
unique composition of the meteorite's PAHs is consistent with
what the scientists expect from the fossilization of very
primitive microorganisms. On Earth, PAHs virtually always
occur in thousands of forms, but, in the meteorite, they are
dominated by only about a half-dozen different compounds. The
simplicity of this mixture, combined with the lack of light-
weight PAHs like napthalene, also differs substantially from
that of PAHs previously measured in non-Martian meteorites.
* The team found unusual compounds -- iron sulfides and
magnetite -- that can be produced by anaerobic bacteria and
other microscopic organisms on Earth. The compounds were
found in locations directly associated with the fossil-like
structures and carbonate globules in the meteorite. Extreme
conditions -- conditions very unlikely to have been
encountered by the meteorite -- would have been required to
produce these compounds in close proximity to one another if
life were not involved. The carbonate also contained tiny
grains of magnetite that are almost identical to magnetic
fossil remnants often left by certain bacteria found on Earth.
Other minerals commonly associated with biological activity on
Earth were found in the carbonate as well.
* The formation of the carbonate or fossils by living
organisms while the meteorite was in the Antarctic was deemed
unlikely for several reasons. The carbonate was age dated
using a parent-daughter isotope method and found to be 3.6
billion years old, and the organic molecules were first
detected well within the ancient carbonate. In addition, the
team analyzed representative samples of other meteorites from
Antarctica and found no evidence of fossil-like structures,
organic molecules or possible biologically produced compounds
and minerals similar to those in the ALH84001 meteorite. The
composition and location of PAHs organic molecules found in
the meteorite also appeared to confirm that the possible
evidence of life was extraterrestrial. No PAHs were found in
the meteorite's exterior crust, but the concentration of PAHs
increased in the meteorite's interior to levels higher than
ever found in Antarctica. Higher concentrations of PAHs would
have likely been found on the exterior of the meteorite,
decreasing toward the interior, if the organic molecules are
the result of contamination of the meteorite on Earth.
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