Tell me more about meteorites from Mars.
By Donald Goldsmith
In August 1996, NASA scientists announced that they had
discovered indications of tiny fossil cells within a rocky
meteorite that had been found on the Antarctic ice, and
whose scientific designation is ALH 84001.
Geologists conclude that this meteorite came from Mars from
the fact that the details of its chemical composition,
especially the relative amounts of different isotopes of the
elements contained in the rock, are unlike those on Earth,
but exactly match the measurements of Martian soil made by
the Viking spacecraft that landed on Mars in 1976.
ALH 84001 is by far the oldest of the dozen meteorites from
Mars that have been identified -- well over 4 billion years
in age. After billions of years on Mars, the rock was
blasted from the Martian surface by an impact of a much
larger meteorite about 16 million years ago. The rock
orbited the sun for millions of years, eventually collided
with the Earth many thousand years ago, and then remained in
Antarctica until its discovery in 1984. Similar meteorites
almost certainly await discovery in the Arctic (and in fact
all over the world); the trick is to recognize them as
extraterrestrial. This is made easier in certain regions of
Antarctica that have relatively few Earth rocks, and where
the slow flow of ice tends to concentrate the rocks that do
exist into areas that can be more easily examined.
The rock from Mars contains chemical and mineral compounds
similar to those produced by some bacteria on Earth, as well
as tiny cylindrical objects, with sizes less than one
one-thousandth the thickness of a human hair, that resemble
the shapes of living cells. This evidence that life once
existed in a 4-billion-year-old Martian rock is intriguing
but far from conclusive. The compounds can be produced by
processes that do not involve life, and the cylindrical
objects could also arise from non-biological chemical
Scientists are now deeply engaged in further studies of
pieces of ALH 84001 in an attempt to discover more evidence
for or against ancient life on Mars. In those long-vanished
eras, Mars had liquid water on its surface; today it has
none whatsoever. If liquid water is essential for life, as
some scientists believe, it seems quite possible that Mars
once did have life on its surface but no longer does.
Martian life might have become completely extinct, or
perhaps some of its life forms might have managed to survive
beneath the surface of Mars, in soil similar to that of the
frozen Arctic tundra, where small amounts of ice in the soil
occasionally become liquid.
If a rock from Mars contains ancient fossils--or possibly
even tiny living organisms--could it infect living creatures
on Earth? This seems highly unlikely, because we would
expect differences between Martian life and Earthlife to be
so large that organisms from one planet could not affect the
other planet's life forms. But even a tiny possibility of
such infection should not be ignored: When we do manage to
collect samples of the Martian surface, we must be as
careful as we can be to analyze them in a way that does not
risk contamination, either of the Martian samples with
Earthlife (which would make our results useless) or of
Earthlife with the samples, which could possibly have
dangerous results for life on Earth.
Most experts estimate the chances of infection from
organisms that might exist in Martian meteorites as
vanishingly small. These rocks have been striking Earth for
millions of years without producing any noticeable effect on
Earthlife. In addition, it is easy to theorize that since
any Martian organisms would not have evolved to succeed in
Earthlike conditions, but would still have to compete with
our own bacteria, the danger from terrestrial organisms will
always be much greater than those from Martian forms of
life, if they exist and reach our planet.
Donald Goldsmith is an astronomer and science writer whose
most recent books are The Hunt for Life on Mars and Worlds
Unnumbered: The Search for Extrasolar Planets.
Return to Episode 2
An Access Excellence Science Mystery sponsored by Genentech, Inc.
Copyright © 1997 Genentech, Inc.; all rights reserved.