Ancient life on Mars?
Lynn_Rothschild at QMGATE.ARC.NASA.GOV
Wed Aug 7 18:01:44 EST 1996
Subject: Time: 2:57 PM
OFFICE MEMO Ancient life on Mars? Date: 8/7/96
In response to a barrage of phone calls and e-mails, I am passing on what I
know about the "life on Mars" excitement.
In 10 days an article will be published in Science by researchers at Johnson
Space Center (led by David McKay), with collaborators at McGill, Savannah
River and Stanford University. These workers have been studying a meteorite,
ALH84001, which was found in the Antarctic but is thought to be from Mars.
The rock is thought to have crystallized 4.5 billion years ago (Ga). It
contains carbonate globules that are thought to have formed 3.6 Ga. The
meteorite arrived in the Antarctic about 13,000 years ago. There are several
aspects of the meteorite which suggest that they contain the remains of a
microscopic Martian biota.
1. It contains abundant polycyclic aromatic hydrocarbons (PAHs) that are
indigenous to the meteorite. PAHs are not, of themselves, indicative of life.
However, ice and other meteorites from the Antarctic show far lower levels of
PAHs, and are different chemically. PAHs are abundant as fossil molecules in
ancent sedimentary rocks, coal and petroleum where they are derived from
organisms such as marine plankton.
2. Areas of the meteorite rich in PAHs also typically contain partially
dissolved carbonate globules which are orange in visible light. Many of the
globules display alternating black and white rims. The Fe-rich rims consist
of an aggregate of tiny ovoids intermixed with irregularly shaped objects
ranging from 20 to 100 nm. These objects include elongated structures that
look like nannobacteria. (I saw SEM's of these this morning and one can
easily imagine that some of the elongated structures are nannobacteria in the
process of cell division.)
3. There are magnetite and iron sulfide particles in the 10 to 100 nanometer
size range. Simple abiogenic explanations of their formation do not allow
them to coexist with partially dissolved carbonates. However, bacteria are
capable of leaving this suite of biomarkers. There are terrestrial magnetite
particles known as magnetofossils which are the fossil remains of bacterial
magnetosomes, and are found in a variety of sediments and soils.
This infomation was presented on NASA select television this morning, and I
also have the page proofs of the Science article. Bill Schopf (UCLA) was at
the press conference as an outside member of the scientific community. Bill
is, of course, a specialist in the oldest terrestrial fossils. He is fairly
convinced that the meteorite is from Mars, but is somewhat less convinced, on
the basis of the evidence thus far, that the fossils are biologic in origin.
One of the issues that is troubling is that the fossils are an order of
magnitude or two smaller than the earliest terrestrial fossils. However, they
are not out of range of modern nannobacteria.
My opinion?. I certainly hope that this is the first evidence of a Martian
biota because it would be the most exciting discovery ever for evolutionary
biology - that is, that there was more than one origin of life. What is
inevitable about evolution vs what is contingent? As Gould points out, we are
not inevitable, but on a planet with a chemistry such as ours, is life based
on carbon inevitable? I often make the point that, given carbon-based life
and an atmosphere rich is inorganic rather than organic carbon, eventually
there would be strong selection for the evolution of autotrophy and probably
light-driven carbon fixation (i.e., photosynthesis). The current models
suggest early Earth and early Mars were similar physically and chemically. If
life is inevitable given a certain set of physical and chemical constraints,
it should have arisen on early Mars as well. If not, we are left with
explaining why not. But, to sound a note of caution (and paraphrasing Carl
Sagan), extraordinary science requires extraordinary proof. This will
certainly be a fascinating story to follow.
Mail Stop 239-12
NASA/Ames Research Center
Moffett Field, CA 94035-1000
Lynn_Rothschild at qmgate.arc.nasa.gov
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