>Notwithstanding the theological implications of life on other planets,
>or even whether this space chunk actually contains remnants of early
>primitive life, how do we know this meteorite came from Mars? Why not
>planet Earth, for example?
The short answer is the following:
1. This meteorite belongs to a suite of 12 basaltic achondrite meteorites.
Oxygen isotopes show that all twelve of these meteorites are related to each
other, and are clearly distinct from any earth rocks. This rules out a
terrestrial origin.
2. Some of these meteorites show cumulate textures, which means they must
have cooled relatively slowly in a magma chamber in a planetary body with a
relatively large amount of gravity (more than asteroids).
3. Most of the 12 meteorites are significantly younger than the asteroids
in their crystallization ages as determined by radiometric dating. Several
are ~1.3 billion years. The asteroids all cooled > 4.0 billion years ago.
Most cooled very close to beginning of the solar system ~ 4.5 billion years
ago. This piece of evidence also points to a planet rather than an asteroid.
4. At least two of these meteorites trapped significant amounts of gas.
The gas has Ne-Ar-Kr-Xe ratios identical to the Mars atmosphere, as measured
by Viking, and also has nitrogen which is enriched > 30% in 15N/14N relative
to terrestrial air (I made one some of these measurements for my thesis).
This gas composition is very distinct from any other gas reservoir measured
in any other meteorites, or known anywhere else in the solar system, and is
an extremely strong piece of evidence for a Mars origin.
You can now download the Science paper from the Science website. Sorry I
don't know the URL offhand. It has all the references to back this up.
As for the evidence of life, I am truly disappointed after all the hoopla
(and prior rumors). You must remember that some meteorites are loaded with
amino acids which were all produced by chemical reactions in space, and not
by living beings. The real evidence, in my book, would come if they find
racemization (handedness) in amino acids. In laymans' terms, left-handed
amino acids are a characteristic of all life on Earth. Random handedness
indicates non-living chemical processes. A dead-ringer for life on Mars
would be to find a population of right-handed-only amino acids. That would
truly indicate extraterrestrial life.
For the moment, I remain very skeptical that they have found evidence for
life, though the meteorites are definitely from Mars.
--Roger C. Wiens
Old addresses:
wiens@arms.gps.caltech.edu
wiens@legs.gps.caltech.edu
NEW ADDRESS:
wiens@gps.caltech.edu