Washington, DC - Ever since scientists learned
that water once flowed on Mars, they've
wondered whether life might also have flourished on the apparently now-dead planet.
In the 16 August issue of Science, McKay et al.
report the first identification of organic compounds in a Martian meteorite. The
authors further suggest that these compounds,
in conjunction with other mineralogical
features observed in the rock, may be evidence
of ancient Martian microorganisms.
The paper's authors are David S. McKay and Everett K. Gibson, Jr., of NASA's Johnson
Space Center in Houston, TX; Kathie L.
Thomas-Keprta of Lockheed Martin in Houston, TX; Hojatollah Vali of McGill
University in Montreal, Quebec; Christopher S.
Romanek of the University of Georgia's Savannah River Ecology Laboratory in Aiken,
SC; and Simon J. Clemett, Xavier D.F. Chillier,
Claude R. Maechling, and Richard N. Zare of
Stanford University in Stanford, CA.
Organic (complex, carbon-based) molecules
are the requisite building blocks of life on Earth. The authors looked for signs of such
molecules and other mineralogical and
textural indications of past life within the pore space and fractures of meteorite Allan Hills
84001 (ALH84001), one of only 12 meteorites identified as having come from Mars.
ALH84001 is the oldest of the Martian dozen,
having crystallized from molten rock about
4.5 billion years ago, early in the planet's evolution, and it is the only Martian meteorite
to contain significant carbonate minerals. (The carbonates formed some time after the rock,
perhaps about 3.6 billion years ago.)
About 15 million years ago, a major asteroid impact on Mars threw ALH84001 into space,
and about 13,000 years ago it fell onto an ice field in Antarctica. ALH84001, which shows little evidence of terrestrial weathering, was
discovered by meteorite-hunting scientists in
1984 and only recently identified as Martian.
ALH84001 is riven with tiny fractures resulting
primarily from impacts that occurred while the
rock was on Mars. The secondary carbonates
formed along with some of these fractures. The
Science authors prepared thin sample sections
that included these pre-existing fractures, and found on their surfaces a clear and distinct
distribution of polycyclic aromatic hydrocarbons (PAHs), organic molecules containing multiple connected rings of carbon atoms - the first organic molecules ever seen in a Martian rock. A variety of contamination checks and control experiments indicated that the organic material was indigenous to the
rock and was not the result of terrestrial
contamination. For example, the authors noted that the concentration of PAHs
increases inward, whereas terrestrial contamination likely would have resulted in
more PAHs on the exterior of the rock.
The big question is: where did the PAHs come
from?
It is thought that PAHs can form one of two
ways: nonbiologi-cally, during early star formation; or biologically, through the activity
of bacteria or other living organisms, or their
degradation (fossilization). On Earth, PAHs are
abundant as fossil molecules in ancient sedimentary rocks, coal and petroleum, the
result of chemical changes that occurred to the remains of dead marine plankton and early
plant life. They also occur during partial
combustion, such as when a candle burns or
food is grilled.
To address the origin of these PAHs, the
authors examined the chemistry, mineralogy,
and texture of carbonates associated with
PAHs in the Martian meteorite. Under the
transmission electron microscope, the
carbonate globules were seen to contain fine-
grained magnetite and iron-sulfide particles.
From these and other analyses, the authors
developed a list of observations about the
carbonates and PAHs that, taken individually,
could be explained by nonbiological means.
However, "when considered collectively . . . we
conclude that [these phenomena] are evidence
for primitive life on early Mars." Some of their
observations are as follows:
The higher concentrations of PAHs were
found associated with the carbonates.
The carbonates formed within the rock
fissures, about 3.6 billion years ago, and
are younger than the rock itself.
The magnetite and iron-sulfide particles
inside the carbonate globules are
chemically, structurally, and
morphologically similar to magnetosome
particles produced by bacteria on Earth.
High-resolution scanning electron
microscopy revealed on the surface of the
carbonates small (100 nm) ovoids and
elongated features. Similar textures have
been found on the surface of calcite
concretions grown from Pleistocene
groundwater in southern Italy, which have
been interpreted as representing
nanobacteria.
Some earlier reports had suggested that the
temperature at which the ALH84001
carbonates formed was as high as 700
degrees C - much too hot for any kind of
life. However, the isotopic composition of
the carbonates, and the new data on the
magnetite and iron-sulfide particles, imply
a temperature range of 0 to 80 degrees C,
cool enough for life.
The magnetite - a mineral which contains
some ferric (Fe3+) iron, perhaps indicating
formation by oxidation (the addition of
oxygen) - and iron sulfide - a mineral that
can be formed by reduction (the loss of
oxygen) - were found in close proximity in
the Martian meteorite. On Earth, closely
associated mineralogical features involving
both oxidation and reduction are
characteristic of biological activity.
--American Association for the Advancement
of Science News Release
