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

[Ron Baalke]

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.