[meteorite-list] EETA 79001 and the Martian Atmosphere

From: Walter Branch <waltbranch_at_meteoritecentral.com>
Date: Tue, 8 Feb 2011 22:31:17 -0500
Message-ID: <1C31886EA28F4BDE87CC8D75044F4CE3_at_acer330bb84976>

Hello Everyone,

I feel like an idiot.

I know that trapped atmospheric gases have been found in some martian
meteorites but for some unexplained reason, I had always thought that the
gases had been incorporated in the rock at the time of formation. I always
wondered how gases from the martian atmosphere could have been trapped in a
rock at the time it formed, particularly an igneous rock in an underground
magma chamber.

Well, thanks to William Cassidy, the man who founded ANSMET, I now realize
my assumption was wrong. Cassidy makes it clear that gasses became trapped
in martian rocks at the time of the impact which launched the rock from the
surface. The rather lengthy quote below is from Cassidy's book "Meteorites,
Ice and Antarctica" an excellent book (see more after the quote).

>From pages 119-121

EETA 79001 was an important find for another reason. It contained proof that
SNC meteorites come from Mars. This meteorite had been highly shocked during
the impact that ejected it from the martian surface, and one of the shock
effects was to produce pods of glass that had been melted from the
constituent minerals of the meteorite by the transient heating generated by
shock pressures. The melts that were formed were partly injected along
cracks in the rock and partly retained as molten beads at the sites where
they formed. Cooling occurred immediately behind the shock wave and the
beads were chilled to glass before they could crystalize. Apparently the
crater forming asteroid had built up a lense off compressed atmosphere in
front of it during it's lengthy trajectory toward the surface of the planet.
When it struck the surface it injected highly compressed air into the target
rock, and some of this was trapped in the shock-melted inclusions. We know
the composition of the martian atmosphere from measurements made by Viking
Landers I and II. When some of the glass inclusions were picked out of EETA
79001 and remelted, the gave up their dissolved gases. These gasses when
analyzed and corrected for slight terrestrial contamination, contained
nitrogen and carbon dioxide in the same abundances as the atmosphere of
Mars; they also had isotopes of argon, neon, , krypton, and xenon in the
same abundances as does the martian atmosphere. This neat bit of detective
work by a number of workers, for the first time tied a SNC meteorite
directly to the planet Mars and, through this meteorite, to all the other
meteorites.

I love Cassidy's book. If you like meteorites in general, martian and lunar
meteorites (like me) and are curious about the ANSMET program and you don't
have a copy of this book, you are really missing out. The book is a gold
mine of information regarding ANSMET. It is very readable, technical in some
places, humorous in others and poignant in others. Some books I love holding
and reading and this is one of those books. It is hardbound with glossy
pages and nicely illustrated. I like the physical proportion of the book and
I even like the dust jacket (I usually abhor dust jackets).

Anyway, many thanks to Dr. Cassidy for clearing that up with me and thanks
for writing such a wonderful book.

-Walter Branch
Received on Tue 08 Feb 2011 10:31:17 PM PST