[meteorite-list] SNC Trapped Gases/Martian Atmosphere

From: Sterling K. Webb <kelly_at_meteoritecentral.com>
Date: Thu Apr 22 09:50:24 2004
Message-ID: <3CB13CDB.B4327252_at_bhil.com>

Hi, Mark and List,

    The early classic SNC's all have CRE (cosmic ray exposure)
ages of around 10,000,000 years. The CRE age dates the length of
time a smallish rock wanders the solar system, being exposed to
cosmic rays, before bumping into a planet, in the case of SNC's,
the Earth. I say a smallish rock, because if the piece that
became a meteorite was buried deep inside a much larger chunk of
rock cosmic rays wouldn't get to it. However, blasting a big
chunk off a planet is much, much harder than knocking off a small
chunk, so it's generally assumed that the CRE ages date the
transit time from Mars to Earth in the case of the SNC's. So most
likely, they left Mars 8 to 12 million years ago.
    The significance of isotopic ratios of "noble" gasses is that
these gasses are heavy enough not to escape easily from their
planet of origin by energetic processes at the top of the
atmosphere and snooty enough not to engage in any chemical
reactions that would remove them from the atmosphere. Therefore,
it is thought that the isotope ratios of noble gasses do not vary
much over a planet's history and can be regarded as a unique
signature for that planetary body. Like a Martian "fingerprint."

Sterling K. Webb
----------------------------------------------------------------
Mark Fox wrote:

> April 7, 2002
>
> Also, are there any good clues as to approximately
> when most of the SNCs so far studied were blasted off
> the Martian surface, or left home as so to speak?
>
> I have gotten the impression from what I have read
> that the impacts that brought the Martian rocks to us
> occurred quite early in the history of the red planet
> (millions to a few billion years ago).
>
> Mark Fox
> Newaygo, MI USA
>
Received on Mon 08 Apr 2002 02:46:52 AM PDT