[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Latest Research Casts New Doubt On Evidence For Fossil Life In Martian Meteorite

Stanford University

1/21/98

CONTACT: David F. Salisbury, News Service
(650) 725-1944; e-mail: david.salisbury@stanford.edu

Latest research casts new doubt on evidence for fossil life in Martian
meteorite

New analyses of the famous Martian meteorite, ALH84001, have cast
additional doubt on the likelihood that it contains the fossilized remains
of ancient Martian microbes.

Two studies published last week find that much of the organic material in
the meteorite appears to be terrestrial, rather than extraterrestrial, in
origin. Richard Zare, the Marguerite Blake Wilbur Professor of Chemistry
at Stanford who headed the team that discovered organic material of
possible Martian origin in the potato-sized rock, says that the new findings
do not directly refute the original research. One of the analyses, however,
does suggest that the meteorite contains considerably more terrestrial
contamination than he had thought, Zare acknowledges.

ALH84001 was thrust into the limelight in August 1996 when a team of
scientists published a controversial analysis in the journal Science. They
argued that they had discovered organic material, unusual mineralogical
features and electron microscope images showing tiny oval and worm-
shaped features that, when taken together, provided compelling
circumstantial evidence that the meteorite was inhabited by Martian
microorganisms more than three billion years ago.

Scientists at NASA's Johnson Space Center in Houston provided the
electron microscope images of the putative nanofossils. Researchers from
the University of Georgia and McGill University contributed the
mineralogical evidence. Zare's research group produced data showing
that the meteorite contained a family of organic compounds called
polycyclic aromatic hydrocarbons (PAHs) that could have been produced
by the decomposition of alien microorganisms.

In the 17 months since the research was announced, other scientists have
published dozens of independent analyses that have both supported and
attacked the Martian microbe hypothesis. In the last month, however, the
weight of new research appears to be stacking up against the pro-life
position.

In December, John Bradley of MVA Inc. and Ralph Harvey of Case
Western Reserve University published a paper in the journal Nature that
attacked the NASA group's interpretation that the oval and worm-like
shapes that it reported could be the fossils of microorganisms. Duplicating
the NASA researchers' methods, Bradley and Harvey reported that all the
shapes that they could find in the meteorite are non-biological in nature
and consist of the fractured surfaces of common crystals. In the same
issue of the journal, the NASA team strongly contested this interpretation.

The two analyses of the organic material in the meteorite appeared in the
Jan. 16 issue of the journal Science.

A team from Scripps Institution of Oceanography, headed by Jeffrey Bada,
analyzed the meteorite for amino acids, the building blocks of life. They
found amino acids present in very low concentrations (between 7 parts
per million and 100 parts per billion). "What we found was that, yes, there
are amino acids in the meteorite at very low levels, but they are clearly
terrestrial and they look similar to amino acids we see in the surrounding
Antarctic ice," Bada said in a Scripps news release. (The meteorite spent
an estimated 13,000 years in the Antarctic ice before it was discovered.)

Bada based his conclusion that the amino acids were due to terrestrial
contamination on the results of an analytic technique called liquid
chromatography. The method determines the "handedness" of the amino
acids. Terrestrial organisms produce only left-handed amino acids, where
non-biological processes produce a mixture of left- and right-handed
molecules. Bada found that the amino acids in the meteorite were left-
handed and so concludes that they must be terrestrial. There is at least a
50 percent chance, however, that Martian life (if it exists) would also favor
left-handed molecules. So the experiment is by no means conclusive,
Zare said.

Even if the amino acids in the meteorite come from terrestrial
contamination, Zare says this does not prove that the PAHs which his
group found are also terrestrial in origin. "Amino acids are soluble in
water. So water provides a mechanism for carrying them into the interior
of the meteorite. But PAHs are highly insoluble and I don't know of any
mechanism that would transport them into the rock's interior where we
found them," he said.

Zare finds the second analysis, performed by a University of Arizona
research team headed by A. J. Timothy Jull, much more interesting and
compelling. "It is state-of-the-art and an extremely valuable study of the
degree of contamination in the meteorite," he said.

Jull's group burned samples of the meteorite at two different temperatures
to separate the organic carbon from the carbon contained in inorganic
minerals, which burn off at higher temperatures. They then analyzed the
isotopic ratios of the carbon from the two sources.

In previous work, Jull had determined that the carbonates in ALH84001
are substantially enriched in the isotope carbon-13 compared to those on
Earth. He and his colleagues interpret this as an indication that the carbon
dioxide in the early Martian atmosphere was also enriched in carbon-13.
If that is the case, then the tissue of Martian organisms would also have
elevated levels of carbon-13. When the team analyzed the ratio of carbon
isotopes in the organic carbon, however, it found that fully four-fifths of the
material had the same isotopic signature as terrestrial carbon. The other
20 percent appears to have a preterrestrial origin, they found.

"It looks like regular terrestrial organic material, with the exception of one
small component in ALH84001," Jull said in a University of Arizona news
release.

The analysis "indicates a much greater degree of terrestrial contamination
in the meteorite than I suspected was present two years ago," Zare said.
"In that sense, Jull's study does cast new doubt on our hypothesis that the
meteorite contains evidence of past Martian life."

On the other hand, the Stanford chemist does not believe that the study
completely rules out an extraterrestrial origin for the PAHs. "Jull's work is
for the whole rock. As in real estate, location is everything. His study does
not give any indication of the locations from which these different carbon
isotope fractions are coming. So I cannot tell where the PAHs, which are
concentrated around carbonate spheroids in the meteorite's interior, fall
in the terrestrial or preterrestrial fraction."

The saga of the provocative rock is far from over. Last summer NASA and
the National Science Foundation awarded grants for 23 new
investigations of AHL84001 as part of a coordinated program designed to
determine whether it contains traces of alien life. These studies will be
producing results in the next two to three years.

Although it may be decades before the significance of the meteorite is
determined conclusively, Zare sees several beneficial effects that are
independent of the debate's ultimate outcome. These include a
revitalization of research on meteorites, increased efforts to extend the
boundaries of the scientific ability to measure trace quantities of chemical
compounds in materials, and its illustration of the critical importance of
multidisciplinary research.

Most important, he says, it has given a major new impetus to research that
addresses the closely related questions of "How did life begin on Earth?"
and "Is there life beyond Earth?" A concrete example of this is NASA's
decision to found a new $7 million to $10 million-per-year Astrobiology
Institute specifically for this purpose. Zare is serving as chair of the search
committee for the institute's first director.