From: [email protected]
To: [email protected]
Date: Fri, 6 Jan 2012 10:20:11 -0800
Subject: [meteorite-list] Wanted: Meteorites from Mercury
http://www.skyandtelescope.com/news/Wanted-Meteorites-from-Mercury-136803313.html
Wanted: Meteorites from Mercury
By Kelly Beatty
Sky & Telescope
January 6, 2012
During a recent science conference discussing Messenger's results
from
Mercury, investigator Shoshana Weider (Carnegie Institution of
Washington) commented, "Short of landing on the surface, picking up
a
rock, and bringing it home, the instruments on Messenger that
characterize chemistry are the best we're going to get."
Well, Shoshana, you might still get to hold such a rock someday.
According to a 2008 analysis
<http://arxiv.org/ftp/arxiv/papers/0801/0801.4038.pdf> by Brett
Gladman
and Jaime Coffey (University of British Columbia), chunks of Mercury
should be lying somewhere on Earth right now. The dynamicists
conclude
that 2% to 5% of the debris blasted by impacts off the surface of
Mercury at or above escape velocity (2.6 miles per second) should
reach
Earth within 30 million years.
Their numbers suggest that Mercurian meteorites should be roughly
one
third as common as those from Mars, for which the count now stands
at 60.
Gladman conservatively suggests that at least a half dozen stones
should
be
lying around somewhere on terra firma.
Meteorite collectors would value a Mercurian meteorite above all
others,
likely fetching $5,000 or more per gram, so they've been on the
lookout
for one. A few years ago, prior to Messenger's arrival,
meteoriticists
had speculated that the best existing match to Mercury were a rare
handful of ancient, basalt-rich stones known as angrites
<http://research.jsc.nasa.gov/PDF/Ares-1.pdf>.
But even before Messenger's arrival, ground-based astronomers had
concluded that Mercurian surface rocks contained very little iron -
strange indeed, given that the innermost planet has an iron core
that
takes up 80% of its diameter and more than half of its volume!
"At that time," comments geochemist David Blewett (Applied Physics
Laboratory), "people were expecting Mercury to have a composition
more
like a lower-iron version of the lunar highlands. We now know that
it's
much different than that." After nearly a yearly scrutinizing the
planet
from orbit, Messenger has confirmed that iron is in short supply at
the
surface.
Instead, the compositional clues suggest that a Mercurian meteorite
would
be an igneous rock - or perhaps a fused breccia of different rock
types -
rich in magnesium and volatile elements (especially sulfur and
potassium).
This closely matches the composition of another rare meteorite
group,
the aubrites. Also known as enstatite achondrites, aubrites are
igneous
rocks dominated by the iron-free mineral enstatite (Mg_2 Si_2 O_6 ).
But aubrites aren't from the innermost planet. For one thing,
they're
too reflective - anything coming from Mercury would be much darker,
tinted by some yet-to-be-identified compound that's seen widely
<http://photojournal.jpl.nasa.gov/catalog/PIA14492> in Messenger's
images. It might also smell faintly of sulfur, appear heavily
shocked,
exhibit significant exposure to cosmic rays, and might even be
slightly
magnetic. Such characteristics would certainly have come to the
attention of hunters and collectors, and it's safe to say that none
of
the world's 40,000 well-documented meteorites are from Mercury.
Yet dynamical probabilities argue otherwise, so why haven't such
samples
been found? Gladman and Coffey didn't address how chunks of rock
might
get blasted off the Mercurian surface, only that the high collision
velocities of asteroids and comets should make it easy to do so.
Maybe the launch mechanics aren't understood well enough, suggests
Jay
Melosh, an impact specialist at Purdue University. "Perhaps at the
very
high speeds required for direct transfer, the fragments are simply
too
small," he says. "These ejecta have to be launched from the surface
very close to the impact point - and perhaps our current models do
not
give very good results here." However, Messenger finds that big
impacts
on Mercury are accompanied by clusters of secondary pits, created by
tossed-out debris, that are generally much larger - not smaller -
than
those around comparable lunar craters. "This fact is one of the
current
big puzzles about the Mercurian cratering record," Melosh concedes.
And so the search goes on for what will almost certainly be the most
celebrated meteorite discovery since the finding of stones blasted
from
surfaces of the Moon and Mars a few decades ago.
______________________________________________
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