Re: your mail

Garry DeWeese (deweese@ucsu.Colorado.EDU)
Thu, 22 Aug 1996 09:16:14 -0600 (MDT)


I really appreciated your concise answer to the qustion of how we know
that ALH 84001 is of Martian origin. But I am puzzled by one part:

On Fri, 9 Aug 1996, Roger C. Wiens wrote:

> 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.

Now as I understand it, the rock was supposedly ejected from Mars due to a
large asteroid/comet impact some 3.6 bya. Why should we assume that the
composition of the Martian atmosphere as measured today by Viking is the
same as it was then? After all, isn't it an assumed condition of
terrestrial evolution that the early earth atmoshpere was quite different
from that today (much higher concentrations of N, CO, CO2)? Why would not
Mars's atmosphere also have changed as it cooled down and became drier?

In other words, it seems that the NASA team is using the present Martian
atmosphere as evidence that the rock is Martian, if the rock is as old as
claimed and contains evidence of early evolutionary stages of life on
Mars, we would certainly expect that the atmosphere when the rock was
ejected from Mars would have been significantly different. This seems
circular reasoning at best.

Garry DeWeese