Re: New Study Claims Mars Dry for 4 Billion Years

Joseph Lazio wrote:
> >>>>> "m" == msadkins04 <msadkins04@xxxxxxxxx> writes:
>
> m> Joseph Lazio wrote:
> >> >>>>> "m" == msadkins04 <msadkins04@xxxxxxxxx> writes:
> >>
> m> The study is based upon argon decay analysis of the remaining argon
> m> content of seven meteorites "known to have arrived from Mars after
> m> millions of years in space". [...]
>
> m> First, how is it known that the rocks weren't in space for merely
> m> "millions of years" but perhaps for billions of years?
>
> >> Cosmic ray exposure. There is a host of material about this on
> >> the Web, but, simply put, the longer a rock is in space, the more
> >> cosmic rays to which it is exposed, and the more isotopes are
> >> produced in the rock from nuclear reactions.
>
> m> Might they have floated through space for billions of years without
> m> appreciable cosmic ray exposure because they were interior portions
> m> of much larger Martian rocks? By the time a meteorite reaches the
> m> surface of the Earth, it is likely to be much smaller than before
> m> it entered the atmosphere.
>
> Umm, why?

Atmospheric friction, of course. "Meteoroids can lose more than 95
percent of their mass during atmospheric passage." ('Meteoroids' in
space, 'meteors' in the atmosphere, 'meteorites' on the ground.)
Reference:

http://www.ess.ucla.edu/research/cosmochem/meteorite.asp

>
> m> Furthermore, it is not unthinkable that during several billion
> m> years of drifting through the solar system, in belts to which
> m> meteoritic debris is drawn, that collisions may have occurred.
> m> Now, here are two alternative scenarios, which incidentally are not
> m> mutually exclusive: (a) Eleven million years ago the rocks in
> m> question were liberated, in space collisions, from much larger
> m> rocks of which they were interior pieces, and which had previously
> m> limited their cosmic ray exposure; (b) These rocks are the
> m> remaining core pieces of a larger rock (...) whose exterior
> m> portions were burned away (...) during entry into Earth's
> m> atmosphere and/or upon impact, and the appearance of eleven million
> m> years worth of cosmic ray exposure is really the result of a much
> m> longer exposure shielded by the layers of rock surrounding them for
> m> most of their space travel.
>
> I guess in order to demonstrate that either of these are superior to
> the currently stated scenario (of an 11-million year exposure because
> that's how long the rock had been in space)...

That's tautological. And I don't have to demonstrate that my notions
are superior in order to question the standard model: I only have to
ask whether plausible alternative scenarios are reasonable and whether
they have been properly explored. Can you cite references showing that
these scenarios are unreasonable, or even that they are less likely
than the standard model?

>...you have to determine the following:
>
> 1) What's the cross-section for collisions? A collision would be an
> easy way to grind up a rock. However, the likelihood of a rock
> being in a collision depends upon its size. So how big does a rock
> have to be before it is likely to be in an appreciable collision
> during the lifetime of the solar system?

It depends upon a lot of things, including its size, the orbit it
occupies, what else occupies that orbit, how much of that other stuff
there is and what its size is, gravitational interactions, etc.; and
the dynamics of these may vary quite a bit over billions of years in a
solar system. Also, how did those meteoroids get into space to begin
with? It's just possible that might tell you something about the
likelihood of subsequent collisions.

>
> 2) What's the size of rock have to be before cosmic-ray exposure ages
> are affected? Your alternate scenarios depend upon
> "self-shielding," the ability of outer layers of a rock to
> "protect" inner layers. How big does a rock have to be for that to
> be the case?

That also depends upon density and/or composition. Furthermore,
shielding might be a question of degree, since the energy of the cosmic
rays also enters into it, and that varies. Studies for habitation of
the moon suggest that permanent settlements there be protected by at
least two meters of compacted regolith, but that's to render them safe
for human occupation.

Mark Adkins
msadkins04@xxxxxxxxx

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