[meteorite-list] Odds of finding a meteorite on Mars

From: Sterling K. Webb <kelly_at_meteoritecentral.com>
Date: Wed Jan 19 02:24:21 2005
Message-ID: <41EE0B03.98729C5C_at_bhil.com>

Hi, Everybody,

    {Sorry about the defective text editor. Try again.}

    To determine the odds, first you have to calculate
the surface area that has been sampled by both rovers.
The rovers were meant to only move a few hundred
meters, but Spirit had racked up 4030 meters of
odometry in 366 sols and Opportunity has covered 2000
meters or so.
    Assuming that each rover has a useful "sidelong
glance" of 75 to 100 meters to either side of their
route in which they would be able to spot a meteorite
and allowing for some track reversals and turns, the
total area surveyed by the two rovers is probably
about one square kilometer.
    So the find rate is one big iron per square
kilometer!
    Why a "rare" iron instead of a stone? Well, we don't
have a sampling of the meteoroid streams that intersect
with Mars, as we do for the Earth. Perhaps the Martian
orbital environment contains meteoroid streams that
contain more irons. Or it could merely be that an iron
meteorite has a very much longer survival lifetime in
Martian conditions than a stone meteorite.
    But clearly some of the reasons we found this one
are: 1) it's about as big a chunk as could survive to
reach the surface of Mars (size matters), 2) it's
positioned on top of a bare stretch of soil (sand?)
where it sticks out like a sore thumb, 3) this iron
looks like an iron whereas a stone meteorite would
look like, well, a stone and gosh! there sure are
plenty of rocks on Mars, and 4) we discovered it
while we were specifically looking for chunks (of
heat shield, it's true, but we were looking).
    Why does it look "shiny"? Imagine being sand
blasted by Martian dust storms for 500,000 years.
You'd be pretty well cleaned off, my guess...
Yes, the Martian atmosphere is thin and the high
velocity winds do not pack the punch of a wind in
a denser atmosphere and the dust is very fine,
but try putting an iron meteorite in your rock
tumbler with plenty of very fine sand and running
it for 500,000 years. I think it would polish up
nicely...
    The bright appearance of this iron suggests that
environmental alteration by the rare presence of
water or oxygen does not proceed as fast as aeolian
abrasion, if it occurs at all. There is plentiful
evidence for aeolian erosion on Mars. Remember the
"pyramids" of Elysium?
    Why no crust? Iron meteorites do not form a thick
"fusion" crust. They acquire a very thin skin of black
magnetite from thermally forced oxidation. But the
Martian atmosphere does not contains any appreciable
amount of free oxygen, so magnetite probably does not
form on a Martian "iron".
    However, if it has been polished clean by dust
storms, any thin surface alteration would have been
abraded away anyway.
    The actual number of Martian meteorites per square
kilometer is a function of both the fall rate and the
survival rate. At the moment, we are no position to
dis-entangle the two factors. But instinct tells me
the survival rate is high.
    I got an email from Beda Hofmann, who wrote: "I
assume there should be many small meteorites on the
MER rover images.... Besides the new object I have
seen at least 2 candidates."
    Beda, can you post links to those MER images with
candidate meteorites? We can all go meteorite hunting
on Mars! This discovery makes me want to go back over
the images again...
    Since this object is at the large end of the range
we could expect to find (15 cm?), by the power law
there should be in the same statistical area roughly
ten objects 1/3 rd that size (5 cm), and so forth.
    And lastly, Doug, I've got to ask! Why would Martian
tektites be strawberry pink??? Inquiring minds want
to know...


Sterling K. Webb
------------------------------------------------------

"Matson, Robert" wrote:

> Hi Jim/List,
>
> > When you consider infintesimal the odds of finding a meteorite
> > here on Earth after traversing as short a distance as the rovers
> > have, you have to ask whether there are local factors on Mars
> > which dramatically increase the number of meteorites per square
> > kilometer on the surface there.
>
> The odds of finding a meteorite on Earth after travelling a couple
> miles aren't infinitesimal -- they're actually pretty good if you're
> looking in the right kinds of places. (It takes me on average about
> 5 hours of walking to find a meteorite. At an average of 2.5 miles
> per hour, that's a meteorite per linear 20 km.) While the "right
> kinds of places" on earth are few and far between, pretty much the
> entire surface of Mars is the right kind of place: old surfaces,
> dry conditions, no vegetation ;-).
>
> Some areas of Mars (as on earth) are even better: minimal native
> rocks. Opportunity is in such a region. I'm still surprised by
> the SIZE of the find, and that it's an iron rather than a chondrite.
> No telling how many chondrites the rovers have driven by...
>
> --Rob

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Received on Wed 19 Jan 2005 02:23:47 AM PST