[meteorite-list] Wanted: Meteorites from Mercury

From: Pete Pete <rsvp321_at_meteoritecentral.com>
Date: Sun, 8 Jan 2012 15:12:58 -0500
Message-ID: <BAY153-W27421C9BC3DD1309C00673F89B0_at_phx.gbl>

Hi, All,

 

I know there's been only scattered remarks about the Messenger mission, but is the current consensus that angrites do not originate from Mercury?

 

Best,
Pete
 


> From: baalke at zagami.jpl.nasa.gov
> To: meteorite-list at meteoritecentral.com
> 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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Received on Sun 08 Jan 2012 03:12:58 PM PST