[meteorite-list] LPSC 2013: Do we have a meteorite from Mercury?

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 22 Mar 2013 12:03:21 -0700 (PDT)
Message-ID: <201303221903.r2MJ3LfL015093_at_zagami.jpl.nasa.gov>

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http://www.planetary.org/blogs/emily-lakdawalla/2013/03211549-lpsc-hermean-meteorite.html

LPSC 2013: Do we have a meteorite from Mercury?
By Emily Lakdawalla
Planetary Soceity
March 21, 2013

Before yesterday, my answer to this question would be "no." Now my
answer is "probably." But it's not clear if we know which of the
meteorites in our collections is from the innermost planet.

Yesterday at the Lunar and Planetary Science Conference, in the middle
of a session about MESSENGER's mission to Mercury, there was a
presentation by Anthony Irving
<http://www.lpi.usra.edu/meetings/lpsc2013/pdf/2164.pdf> that posed an
interesting question: do we have in our hands any meteorites from Mercury?

Dynamically speaking, it's very, very hard to get a rock from Mercury to
Earth. Mercury is located so close to the Sun that you have to impart a
tremendous amount of energy to blast a rock off of it and into a high
enough orbit to reach Earth. It helps a little bit that Mercury is such
a small planet (so its gravity is relatively low). And there's also a
way in which its proximity to the Sun helps: any asteroids that hit
Mercury are traveling insanely fast (between 20 and 50 kilometers per
second). That would make a big enough bang to spew a lot of rocks into
space. Still, it's never been considered too likely that one of them
could get all the way to Earth, and then be found.

Irving said that recent work has suggested that it is more likely than
we used to think. (I didn't catch who it was who did this work.) That
work suggests it's about 23 times harder to get a rock from Mercury to
Earth than it is from Mars to Earth. Given that we've got more than 70
known Mars meteorites in our collections, that means we ought to have
found 3 (give or take a couple) Mercury meteorites by now. So which ones
are they?

He thinks he may have found one: NWA 7325. (The "NWA" bit means it was
found in northwest Africa.) Here is what it looks like. Isn't that green
rind lovely?

[Photo]
NWA 7325, a unique meteorite
<http://planetary.s3.amazonaws.com/assets/images/z-misc/20130321_nwa_7325_ralew.jpg>
Stefan Ralew - sr-meteorites.de

Irving explained its composition in detail. Since most of that detail
will mean as little to you as it did to me, I won't try to repeat it,
but there were a couple of key points. It is an enstatite meteorite,
meaning that it's made of the kinds of minerals that occur in the rocky
mantles of rocky planets. That's important because it means it is a
piece of a world that was large enough to have differentiated into a
rocky mantle and a metal core. But a lot of meteorites are from places
like that.

NWA 7325 has olivine in it that is insanely magnesium-rich. Iron and
magnesium are two elements that are almost always found together in
rocks; the ions they make have the same size and charge so they happily
occupy the same positions in crystal lattices. It's weird to have a rock
that is so dominantly magnesium-rich. Mercury's surface rocks are known
(thanks to MESSENGER) to be unusually low in iron. But that's hardly
conclusive.

Irving talked about the oxygen isotope ratios. Oxygen occurs in three
main flavors, with either 16, 17, or 18 neutrons in the nucleus. The
16-neutron one is the most common. The relative amounts of the different
isotopes present in rocks got set when the planets formed and differ
from planet to planet depending on when, where, and how they formed. If
you measure the amounts of oxygen-18 and oxygen-17 in proportion to
oxygen-16 in rocks, each planet's rocks bear a reasonably unique
fingerprint. NWA 7325's oxygen isotope ratios do not match any known
meteorites from any other planet-size body. In fact, they're not
particularly similar to much of anything that we've measured oxygen
isotope ratios for.

NWA 7325 is massively enriched in europium. This happens when you make a
global magma ocean in which feldspar crystallizes and floats to the top
of the ocean, like it did on the Moon.

Essentially, Irving's argument boiled down to: this meteorite is from a
planet-sized body, but not from any of the planet-sized bodies that we
know we have meteorites from. Mercury is the only planet-sized rocky
body left in the solar system that we don't know of any meteorites from
yet. So either NWA 7325 is from Mercury or it's from some
since-destroyed biggish protoplanet from which we don't have any other
meteorites.

The chemistry is not a perfect match. Irving said that the differences
between NWA 7325's bulk chemistry and what MESSENGER has shown us about
Mercury can be explained away. That was the weakest part of his
argument, for sure. Questioners in the session clearly weren't totally
convinced, arguing that if you can make NWA 7325 from Mercury, you could
just as well make it from other meteorites of known composition and
provenance. I have no way of evaluating the validity of either side of
that argument!

For me, the biggest news from his talk was that regardless of whether
NWA 7325 is from Mercury, we ought to have a couple of pieces of Mercury
in our existing meteorite collections. Which ones are they?

One other note about this talk. At the opening, Irving talked about how
northwest Africa is in many ways a more productive place to search for
meteorites than Antarctica. You can search year-round, he said, and you
don't get much help from the penguins in Antarctica in looking for
meteorites. The story is different in northwest Africa, he said, and
showed a slide of several Berbers in ethnic garb reading a meteoritics
journal. The audience (including me) laughed, because it was a
surprising image. But there was no mockery in his intent. He said that
many Moroccan residents of the deserts that preserve meteorites are
learning a great deal about the value of these rocks not just
financially but also scientifically, and are interested and willing
participants in the scientific enterprise of finding and interpreting
space rocks. They're citizen scientists. And are much more helpful than
the penguins!
Received on Fri 22 Mar 2013 03:03:21 PM PDT