Release: Evidence of Primitive Life From Mars
August 7, 1996
Johnson Space Center
METEORITE YIELDS EVIDENCE OF PRIMITIVE LIFE ON EARLY
A NASA research team of scientists at the Johnson Space
Center and at Stanford University 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 Aug. 16 issue of the journal Science, presenting the investigation to the
scientific community at large to reach a future consensus that will either
confirm or deny the team's conclusion. The two-year investigation was co-led by
planetary scientists Dr. David McKay, Dr. Everett Gibson and Kathie
Thomas-Keprta of Lockheed-Martin, all from JSC, 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. Wealso found several unusual mineral phases that are known
products of primitive microscopic organisms on Earth. Structures that could be
microscopic 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.
Because 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 may also 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, 15 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 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 1/100th
the diameter of a human hair, and most are about 1/1000th 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
Researchers at Stanford University used a 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 are commonly 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
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.
New England Meteoritical