[meteorite-list] Mojave Crater: Source of Martian Meteorites?

From: Galactic Stone & Ironworks <meteoritemike_at_meteoritecentral.com>
Date: Tue, 11 Mar 2014 20:07:30 -0400
Message-ID: <CAKBPJW-2V3MUqg2QtLoPsF=MgqnYd99JOPk5zwGXsNrkd5uWQA_at_mail.gmail.com>

Hi Ron and List,

What an interesting little piece of synchronicity that Mojave crater
on Mars is the source of meteorites found in the Mojave desert on
Earth (Los Angeles 001 and 002). That's pretty cool. :)

Best regards,

MikeG

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On 3/11/14, Ron Baalke <baalke at zagami.jpl.nasa.gov> wrote:
>
>
> http://www.skyandtelescope.com/news/Mojave-Crater-Source-of-Martian-Meteorites-249584481.html
>
> Mojave Crater: Source of Martian Meteorites?
>
> Where did the Martian meteorites called shergottites come from? A team
> of European researchers believe their launch pad was a fresh-looking
> 55-km-wide
> crater punched into ancient terrain about 3 million years ago.
>
> Imagine walking into a room full of geologists, plunking a box full of
> rocks on a table, and asking them to figure out where on Earth your samples
>
> came from.
>
> [Image]
> The Los Angeles meteorite, found in California's Mojave Desert, has proven
> to be a chunk of Mars known as a shergottite. The scale cube is 1 cm on
> a side.
> Copyright 2000 Ron Baalke.
>
> That's the challenge facing the researchers who study meteorites from
> the planet Mars. The count of Martian stones now totals about 150,
> representing
> 69 discrete falls on Earth. They're all igneous rocks and fall into three
> compositional clans known as shergottites, nakhlites, and chassignites
> - named for an archetype within each group. (There's one oddball, an ancient
>
> rock known as ALH 84001, that has gotten a lot of attention in past years.)
>
> The rocks themselves are old. However, they were blasted from the Martian
> surface in the geologically recent past, based on how long they were exposed
>
> to cosmic rays in space before reaching Earth: 11 million years ago for
> the nakhlites and chassignites, and just 1 to 5 million years ago for
> the shergottites.
>
> So where'd they come from? That question has dogged planetary geologists
> for decades. But they've now got powerful new tools - three heavily
> instrumented
> orbiters around Mars - to try to identify the interplanetary launch pads.
> Several researchers have suggested young-looking Martian craters as
> possibilities
> in the past.
>
> [Image]
> Mojave crater, 36 miles (58 km) across, is a fresh-looking crater on the
> ancient Xanthe Terra plain of Mars.
> NASA / JPL / Arizona State Univ.
>
> Now a trio led by Stephanie Werner (University of Oslo) has put forth
> a candidate site as the source of the shergottites. As the researchers
> note in the March 6th online edition of Science, the crater is surrounded
> by a broad apron of dusty, ejected material that bears few impact craters;
> this, together with other evidence, suggests to them that Mojave formed
> within the past 5 million years. Also, spectra obtained with the European
> Space Agency's Mars Express orbiter shows patches within the crater enriched
>
> in the minerals pyroxene and olivine, which are common in shergottite
> meteorites.
>
> But the case for Mojave as the source crater isn't ironclad. For starters,
> it's a big impact - 34 miles (55 km) across. Werner and her team admit
> that craters of this size should occur on Mars only once every 35 to 50
> million years, so a geologically recent blast of that size is statistically
>
> unlikely.
>
> Moreover, the crater's rim and deposits have been crosscut by multiple
> sets of intersecting streambeds that end in broad fans of sediment - very
> reminiscent of landforms in the Mojave Desert of California and the
> Southwest.
> Geologists believe this sort of erosion occurred on Mars very early in
> its history - but not within the past 5 million years. Moreover, the craters
>
> flanks show several episodes of runoff; it apparently didn't happen all
> at once.
>
> Werner isn't concerned, though. "It can be multiple stages even if it
> happened only five million years ago. "It can be multiple stages even
> if it happened only five million years ago," she explains. "Hydrothermal
> systems in such a crater can easily exist for a few hundred years, and
> also seasonal variations may produce several episodes."
>
> [Image]
> An oblique view of the rim of Mojave crater on Mars.
> NASA / JPL / Univ. of Arizona
>
> Another issue is that Mojave punched into some of the oldest rocks on
> Mars, a region called Xanthe Terra that dates at least 4.1 billion years
> ago. Further, Mojave lies at the confluence of two giant flood channels,
> Simud Vallis and Tiu Vallis. But most researchers consider the source
> rock for the shergottite meteorites to have solidified between 150 and
> 600 million years ago. Nakhlites and chassignites aren't dramatically
> older, probably 1.3 billion years. "The consensus was (and still is) that
> the shergottites are young," comments Alfred McEwen (Arizona State
> University).
>
> Not surprisingly, previous searches for the source of the Martian meteorites
>
> didn't single out Mojave but instead focused on craters in much younger
> terrains. "There are plenty of other young craters on Mars capable of
> delivering rocks to Earth with a wide range of ages," McEwen says.
> "Shergottites
> have a range of exposure ages, so they originated from multiple impacts.
> The claim that it must have been Mojave is very weak."
>
> Yet Werner and her colleagues think a case can be made that shergottites
> really came from ancient Xanthe Terra. They note that the isotopic "clocks"
>
> used to date very old rocks are concentrated in an iron-rich mineral called
>
> pyrrhotite, which would have been vulnerable to alteration whenever the
> ground heated up (say, during nearby impacts). And they add that all that
> water flowing through the area could have altered the rocks' apparent
> ages as well.
>
> This is one of those scientific "whodunits" in which we might never know
> who the "perp" really is without sending rovers to visit the most promising
>
> candidates or get Martian samples shipped back to Earth that were gathered
> from known locations. Until then, the search goes on.
>
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Received on Tue 11 Mar 2014 08:07:30 PM PDT


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