August 7, 1996
Johnson Space Center
METEORITE YIELDS EVIDENCE OF PRIMITIVE LIFE ON EARLY MARS
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. 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
"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
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 similiar 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 Meterorite 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 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 addtion, 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
similiar 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.
|Curator: Annie Platoff
Responsible NASA Official: Kelly Humphries
Last Updated: 7 August 1996