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(meteorobs) Marsbugs- V.3. N.7. A1 (fwd)
---------- Forwarded message ----------
Date: 7 Aug 1996 17:15:29 -0500
From: Julian Hiscox <julian_hiscox@micro.microbio.uabdot edu>
To: "Dr. Julian and Melissa Hiscox" <marsgene@aoldot com>
Subject: Marsbugs- V.3. N.7. A1
Subject: Time: 5:13 PM
OFFICE MEMO Marsbugs: V.3. N.7. A1 Date: 8/7/96
This message was passed along the internet, author unknown, and is of interest
to the current debate about the possible origin and evolution of life on Mars:
Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian
Meteorite ALH84001
David S. McKay, Everett K. Gibson, Kathe L. Thomas-Keprta, Hojatollah Vail,
Christopher S. Romanek, Simon J. Clement, Xavier D.F. Chillier, Claude R.
Maechling, and Richard N. Zare.
ALH84001 is a martian meteorite, a coarse-grained orthopyroxene containing
relatively large amounts of carbonate, with a crystallization age of 4.5 Gyr.
Carbonate globules within fractures in the rock are dated at 3.6 Gyr.
Fractionation of carbon has taken place to enhance C-13 consistent with
terrestrial biogenic process (but other processes not excluded). PAHs also
appear on interior fracture surfaces in excess of 1 ppm. They present
extensive tests and discussion to show that they are confident that these are
all indigenous to the meteorite and do not represent contamination. Mass spec
studies show these PAHs are complex not simple and suggest (to the authors) a
biogenic source. They then discuss TEM studies of the Fe/S fraction in the
meteorite. Nanometer sized magnetite and Fe-sulfide phases are associated
with Mg-Fe-rich carbonate. These observed structures and concentrations can
be explained by either inorganic or biogenic processes. However, they argue
that the range of conditions (PH) for inorganic precipitation is unlikely to
have occurred on Mars, whereas biogenic processes seem to offer a more natural
explanation for the detailed structures observed, and they are apparently
similar to terrestrial magnetofossiles (remains of bacterial magnetosomas).
SEM studies of carbonate globules are then discussed (typically ovoid and 100
nm across). Origin of the ovoids and other observed textures is unclear, but
they may be related to terrestrial microfossils, or they may be erosional
features due to partial dissolution of the carbonate. They do not believe
these structures result from contamination. They suggest possible
microbiological activity for both the ovoid carbonate structures and the
Fe-sulfides. On the basis of a number of circumstantial arguments, they
"interpret that the carbonate globules have a biogenic origin and were likely
formed at low temperatures". (The temperature of formation of the carbonates
in this meteorite has been controversial). "It is possible that all of the
described features can be explained by inorganic processes, but these
explanations appear to require restricted conditions." The evidence
consistent with life includes: (1) penetration of the igneous rock by fluid
leading to possible organic deposits of minerals along veins (2) formation of
the carbonate globules much later than the formation of the rock itself, (3)
SEM and TEM images of the globules that resemble terrestrial biogenic
structures, (4) magnetite and iron sulfide particles that could have been
formed biogenically. The authors feel that the cumulative effect of these
points is to provide "evidence for primitive life on early Mars"