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

From: Sterling K. Webb <kelly_at_meteoritecentral.com>
Date: Wed Jan 19 02:13:02 2005
Message-ID: <41EE085E.551EC3FC_at_bhil.com>

Hi, Everybody,

    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 sandblasted
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
Received on Wed 19 Jan 2005 02:12:30 AM PST