[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> · meteoritemike_at_meteoritecentral.com

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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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