[meteorite-list] Are Martian 'Pearl Chains' Signs of Life?

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:44:36 2004
Message-ID: <200103090105.RAA20354_at_zagami.jpl.nasa.gov>

http://www.sciencemag.org/cgi/content/full/291/5510/1875a

ASTROBIOLOGY:
Are Martian 'Pearl Chains' Signs of Life?

Richard A. Kerr
Science Magazine
March 9, 2001

Life on Mars jumped back into the headlines last week with the
publication of two papers claiming that nanoscale mineral grains in
the famous martian meteorite ALH84001 were left by ancient martian
bacteria. One paper was old news to researchers (Science,
22 December 2000, p. 2242). The other got a generally cautious
reception when it was reported in the media, but now many
experts are turning downright incredulous as they get a chance to
inspect the published images. One of the two papers "defines a
new low in the great ALH84001 debate," says microscopist John Bradley
of MVA Inc. in Norcross, Georgia, a longtime critic of
martian microbe claims. Even the fence sitters are unimpressed:
"There's a lot of subjectivity" in the analysis, says geologist Allan
Treiman of the Lunar and Planetary Institute in Houston. "They've
gone too far in interpreting the images" as signs of life.

Meteorite ALH84001 first made headlines in 1996, when a group of
researchers claimed that the chemical, mineralogical, and isotopic
makeup of the meteorite--and some buggy-looking microscopic
features--spoke of ancient life back on Mars. All but one of those
lines of evidence have been withdrawn or discounted as not
definitive, singly or collectively. The remaining evidence is grains
of the iron-oxide mineral magnetite a few tens of nanometers in size,
the same sort of particles that some earthly bacteria form, stringing
them into long chains to make magnetic compasses.

In one of the 27 February Proceedings of the National Academy of
Sciences (PNAS) papers, microscopist Kathie L. Thomas-Keprta of
Lockheed Martin in Houston and colleagues argue that about
one-quarter of ALH84001's magnetite is indistinguishable from the
magnetite of a particular terrestrial magnetotactic bacterium, and
therefore the martian magnetite probably has a bacterial origin, too.
Thomas-Keprta made the same argument in another paper late last year.
Other researchers agreed about the resemblance but concluded that the
evidence was not extraordinary enough to prove such an extraordinary
claim.

Now comes the claim that some of ALH84001's magnetite is arranged in
chains like pearls on a string, just the way some bacteria form
magnetite on Earth. In the second PNAS paper, Imre Friedmann of
NASA's Ames Research Center at Moffett Field, California, and
colleagues present scanning electron microscopy (SEM)images of what
they believe are chains of magnetite grains produced by bacteria. In
a mode of SEM operation that highlights heavy elements such as iron,
images show bright blobs of presumably iron-rich material lined up
across the surface. "The chains we discovered are of biological
origin," says Friedmann, because the fuzzy blobs have a uniform size
and shape within a chain, have consistent gaps between them, are
aligned end to end when elongated, and can bend in curved chains,
just like magnetite chains of earthly bacteria.

Initial news reports quoted vague reactions from experts who had yet
to see the images or had seen them in faxed versions only, but the
real McCoys are getting a decidedly cool reception. Microscopist
Peter Buseck of Arizona State University in Tempe is among the most
receptive. "It's an interesting paper," he says. "I have no problem
dismissing some of the [chains]. There are others that seem to come
close to a real [bacterial magnetite chain]. It's a matter of taste."
Buseck can't recall anyone finding anything like these chains
preserved for so long on Earth. Here they seem to fall apart on the
death of the bacterium, not be preserved for billions of years as
required for any martian examples.

Meteoriticist Ralph Harvey of Case Western Reserve University in
Cleveland is less understanding. "We've seen this before" with
ALH84001, he says. "Someone says, 'Let's take a novel technique and
turn it on a very complex rock.' Who knows what the inorganic
magnetite in rock may look like with this technique? They're just
interpreting things in a narrow way." Some nonbiological process
might just as well produce magnetite in such arrangements, he says,
given that magnetite very much like Thomas-Keprta's has been made in
the laboratory (Science, 31 March 2000, p. 2402). An equally
intensive search of other rocks--both extraterrestrial and earthly
--is in order, says Harvey. If these "chains" are going to change
anyone's mind, adds Buseck, "we're going to need better chemistry and
images [of the chains], perhaps better than is available now."
Received on Thu 08 Mar 2001 08:05:38 PM PST