[meteorite-list] The First-Ever Meteorite from Mercury? (NWA 7325)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 1 Feb 2013 11:47:33 -0800 (PST)
Message-ID: <201302011947.r11JlXf5019911_at_zagami.jpl.nasa.gov>

http://www.skyandtelescope.com/news/home/The-First-Ever-Meteorite-from-Mercury-189374981.html

The First-Ever Meteorite from Mercury?
by Kelly Beatty
Sky & Telescope
February 1, 2013

NWA 7325 is a meteorite like no other. Found in Morocco last year,
this clutch of small stones looks to be a near-perfect geochemical match
to the surface of the innermost planet.

When dynamicists run the numbers, it's at least statistically possible
that meteorites should fall to Earth from all over the inner solar
system - even from Mercury

Spurred by the Messenger orbiter's close scrutiny of Mercury's surface,
the hunt has been on to find meteorites from the innermost planet. All
previous candidates (called angrites and aubrites)
are close but imperfect matches to the unique composition found by
Messenger on Mercury's surface: dark igneous rock enriched in magnesium
but virtually free of iron.

Now, finally, they might have one in hand.

[Image]
Weighing just a bit more than 100 g, this is the largest fragment of the
meteorite NWA 7325. Note the amazing light-green color of its fusion
crust - one of many characteristics hinting that it might be from the
planet Mercury. The cube at right is 1 cm square. Click here
<http://www.sr-meteorites.de/NWA7325/nwa7325-1.JPG> for a larger view.
Stefan Ralew sr-meteorites.de

Last April, German meteorite dealer Stefan Ralew bought a clutch of 35 small
meteorites that had been found a few months earlier in the Moroccan desert. The
fragments from a single fall totaled about 12 ounces (354 g). Right away
he could see that they were unusual: Their fusion crust, created by
flash heating as they decelerated in Earth's atmosphere, was greenish.
This was especially evidence in the largest, golfball-size piece,
weighing just over 100 g. "Green and glassy fusion crusts are known from
a few lunar meteorites," Ralew explains, "but they all don't have an
extreme color as this one."

The new Moroccan find is now officially known as Northwest Africa 7325
<http://www.lpi.usra.edu/meteor/metbull.php?code=55627>. Ralew sent
samples to the laboratory of Anthony Irving (University of Washington),
well known for his expertise with unusual meteorites from the Moon,
Mars, and elsewhere.Once cut open, the stones revealed interiors full of
a stunning emerald-green silicate mineral infused with chromium. The
crystals were relatively large and obvious, suggesting that the magma
from which they solidified cooled slowly. Irving and his team found lots
of magnesium and calcium in the suite of silicate mineral, but even more
important is what didn't find: they contain virtually no iron.

Irving, who'll present his team's findings
<http://www.lpi.usra.edu/meetings/lpsc2013/pdf/2164.pdf> at a
planetary-science conference next month, is trying to keep his
enthusiasm in check. "NWA 7325 is tantalizing, and certainly more
consistent with the Messenger results than either angrites or aubrites,"
he explains, "but we need a [spacecraft-returned sample] for 'ground
truth'."


[Interior of meteorite NWA 7325]
A polished cut surface of the meteorite NWA 7325 reveals green crystals
of the silicate mineral diopside (colored by chromium ions). This
meteorite contains abundant magnesium and calcium yet almost no iron -
hallmarks of what geochemists believe rocks from Mercury should be like.
The cube is 1cm tall. Click here
<http://www.sr-meteorites.de/NWA7325/nwa7325-3.JPG> for a larger view.
Stefan Ralew / sr-meteorites.de

Shoshana Weider (Carnegie Institution of Washington), who's spent years
studying Messenger's spectra of Mercury
<http://www.lpi.usra.edu/meetings/lpsc2013/pdf/2189.pdf>, likewise
offers a cautionary note. The planet's surface seems to be rich in the
silicate mineral enstatite, which is not obvious in NWA 7325. Also,
there shouldn't be so much calcium. To explain these discrepancies, she
and Irving agree that meteorite might have been a deeply buried rock -
well below the surface - before a powerful collision sent it flying off
into interplanetary space.

There are still many unknowns about these weirdly green space rocks.
Tests are under way to determine how long ago they crystallized and how
long they were exposed to cosmic rays as they drifted in space before
reaching Earth.

One way to zero in on planetary paternity would be to see if NWA 7325's
ratios of three magnesium isotopes match what Messenger's gamma-ray
spectrometer is seeing on Mercury. It's a challenging observation,
explains Patrick Peplowski (Applied Physics Laboratory), because there's
magnesium in part of the GRS's housing. "There does exist the potential
to detect different magnesium isotopes, but I expect that the errors on
any resulting isotopic ratios would be at the ~5% level," Peplowski
says. "I suspect that this is larger than would be needed to compare to
NWA 7325, but I'm not sure."

Another idea is to melt one of the NWA 7325 stones and then let it cool
and recrystallize under controlled conditions, to see how closely the
result mimics Mercury's surface composition. "A lot of scientists will
want to get their hands on this," Weider notes.

However, any researchers wanting a piece big enough to melt down will
likely have to get in line. So far Ralew has donated less than 1 ounce
of NWA 7325 for scientific analysis, and he's got the rest. It's not
inconceivable that bits of this unique find could fetch $5,000 per gram
on the sometimes-frenzied meteorite market.
Received on Fri 01 Feb 2013 02:47:33 PM PST