[meteorite-list] Prospects Dim For Fossils In Martian Meteorites

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:02:29 2004
Message-ID: <200203151818.KAA14242_at_zagami.jpl.nasa.gov>

http://skyandtelescope.com/news/current/article_534_1.asp

Prospects Dim For Fossils In Martian Meteorites
By David L. Chandler
Sky & Telescope
March 14, 2002

Ever since it was trumpeted at a hastily convened press
conference in August 1996, the subject of possible signs of life in the
Martian meteorite ALH 84001 has remained intensely controversial. At the
time, David S. McKay (NASA/Johnson Space Center) and his coauthors presented
four lines of evidence suggesting that the 4-billion-year-old meteorite
contains chemical or fossil traces of microbial life. In the years since,
however, these signs have been disputed by other researchers, who ascribe
them either to terrestrial contamination or to nonbiological processes on
ancient Mars.

To many, the strongest remaining evidence for Martian microbes is the
presence of tiny, uniform, chemically pure crystals of magnetite embedded in
globules of carbonate - crystals that look remarkably similar to those
produced by certain strains of terrestrial bacteria. Kathie L. Thomas-Keprta
(Lockheed Space Systems) and her colleagues have continued to study the
crystals and strengthen their argument that the tiny grains exhibit a suite
of five characteristics found in biogenic crystals and never seen in
magnetite produced by chemical and geophysical processes.

Not anymore, say two teams who will report tomorrow that they have
independently synthesized magnetite crystals in the laboratory showing most
of these same characteristics. The presentations will be made at the 33rd
Lunar and Planetary Science Conference in Houston, Texas. One group, led by
Dadigamuwa C. Golden (Hernandez Engineering) and Douglas W. Ming
(NASA/Johnson), had previously produced perfectly formed, chemically pure
magnetite crystals, thus matching two of the distinctive features of those
in ALH 84001. Now, having completed 3-D examination of their fabricated
crystals, Golden and Ming say these also share the distinctive shapes
(termed truncated hexaoctahedrals).

Using a similar method, Andrea M. Koziol (University of Dayton, Ohio) and
Adrian J. Brearley (University of New Mexico, Albuquerque) have also
produced crystals that match the same nearly uniform size and purity of
those found in the meteorite. But they have not yet been able to study the
3-D shapes, a difficult process. Still, Koziol notes, the fact that both
teams were able to subject carbonate globules to a high-temperature pulse -
470° C. for 5 minutes - without altering them and without introducing other
metals into the magnetite contradicts a key claim made by Thomas-Keprta. "It
doesn't disprove" her overall claims, Koziol says, "but it does show a
pretty nice alternative."

Allan H. Treiman (Lunar and Planetary Institute), who has grown very
skeptical of the McKay group's claims, believes the new work will mark a
turning point in the debate over ALH 84001. Of the original lines of
evidence, he said, "The only one really standing at this point is the
magnetite crystals. This will pretty much be the death knell."

Still, the fossil proponents are far from throwing in the towel. An
assessment led by Joseph L. Kirschvink (Caltech) concludes that up to half
of the meteorite's magnetite grains are indeed truly biogenic. Thomas-Keprta
will also be reporting new results Friday. Everett K. Gibson (NASA/Johnson),
a member of the original McKay team, says that the other groups subjected
their samples to conditions that ALH 84001 could never have experienced,
which undermines the relevance of their results. But Thomas-Keprta "is going
to present some data Friday that's going to knock the socks off some
people," he said. "I think the argument stands stronger now than when we
looked at it in '96."
Received on Fri 15 Mar 2002 01:18:31 PM PST