The Mars Meteorite
The discovery of the unique martian meteorite
On August 7, 1996, NASA claimed to have found fossil life
inside of one of the known Mars meteorites.
The meteorite is known as ALH84001
and weighs 1.9 kg. Scientists are fairly confident that it comes from Mars because its abundances of oxygen-16, oxygen-17, and oxygen-18 are very different from Earth rocks. It was found in the Antarctic. The Antarctic
is a particularly good place to find meteorites because they are relatively
easy to identify as they fall on vast sheets of ice which flow up against
ALH84001 is distinct from the other known martian meteorites.
The other martian meteorites are composed of young volcanic basalt.
ALH84001 is composed of almost pure pryoxene which is one of the first
materials to solidify from an originally molten planet.
ALH84001 is from the ancient martian highlands.
from molten rock
by an impact, but not ejected from surface
1.8-3.6(?) b.y.a------- Carbonate
minerals depositied by water flowing through the rock
to space by an asteroid impact
in Antarctica and classified as a diogenite meteorite
as a martian meteorite
from Jakosky, 1998, pg. 145
Events in the history of the martian meteorite ALH84001.
The occurence of each of the geological events is indicated by geochemical
evidence, and the dates are determined using various isotopic indicators.
Here is a video
showing the infered history of ALH84001.
Evidence for fossilized life in ALH84001
NASA scientists, David McKay and Everett Gibson, examined
ALH84001 for evidence of fossilized life.
Evidence for fossils would be contained in the carbonate
material that was deposited within the rock some 1.8-3.6 b.y.a.
McKay and his co-workers describe three kinds of features
in ALH84001 that they interpret as evidence for ancient martian life (features
are all in and near carbonate mineral globules).
1. Microscopic shapes
that resemble living and fossil bacteria on Earth.
2. Elliptical, rope-like,
and tubular structures in fractures in the carbonate mineral globules.
3. Presence of PAHs, which are complex organic molecules. While these can form in both biological and nonbiological processes, their concentration is much higher in ALH84001.
for more images of ALH84001, including an image of the orange carbonate
To show that shapes could really be fossil bacteria, McKay
and co-workers have tried to show that:
the bacteria shapes are actually the sizes and shapes of
known living organisms.
the bacteria shapes are really part of the rock and were
not produced accidentally while they prepared the sample for study.
the bacteria shapes are not Earth bacteria that somehow wiggled
into ALH84001 while it was in Antarctica.
Microscopic mineral grains are present in ALH84001
like some produced by living and fossil bacteria on Earth. McKay
and co-workers suggest that these mineral grains in ALH84001 may have been
produced by martian bacteria by arguing that:
the mineral grains formed on Mars.
the mineral grains have chemical compositions, crystal structures,
sizes, and shapes like biologically produced grains on Earth.
the different microscopic mineral grains, and the larger
mineral crystals they occur in are not likely to have formed by inorganic
processes, without assistance from life.
Organic chemical compounds, or polycyclic
aromatic hydrocarbons (PAHs), that resemble the decay products of
bacteria on Earth found in ALH84001.
PAHs in ALH84001 include phenanthrene, pyrene, chrysene,
perylene or benzopyrene, and anthanthrene.
Even more complicated PAH molecules are common on the Earth,
but are very rare in ALH84001.
McKay and co-workers argue that PAHs in ALH84001 are
derived from ancient martian bacteria by trying to show that they are not
contamination from laboratory procedures. Further, they contend:
they are not Earth PAHs that entered the meteorite while
it was in Antarctica
they are not like PAHs in other meteorites (which have nothing
to do with life)
they are consistent with decomposition of simple bacteria
they have a higher concentration in the inside of the meteorite than on the outside
However, scientists later found terrestrial bacteria living inside the meteorite! At least some contamination took place.
Still, it doesn't prove (nor
disprove) that there was past life on Mars.
Traces of amino acids found in this and other meteorites support the
idea that building blocks of life are common in Universe.
This leads to an interesting corollary --
Did life on Earth come from Mars or a comet or a Leonid