[meteorite-list] The Other Mars Meteorite - Lafayette Meteorite

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed Sep 15 14:02:00 2004
Message-ID: <200409151801.LAA15720_at_zagami.jpl.nasa.gov>

http://www.astrobio.net/news/modules.php?op=modload&name=News&file=article&sid=1197

The Other Mars Meteorite
Astrobiology Magazine
September 15, 2004

Summary: The most famous Mars meteorite, the Allen Hills
rock with its strange, cylindrical rock segments, may not be the most
intriguing. Consider a rock launched from Mars only 700 million years
ago called Lafayette. Judging by detailed chemical analysis, the outcome
of Lafayette's long journey to Earth points to a past where it might
have been altered at the bottom of a salt-water pool. Or at least that
conclusion is what many meteor scientists propose to describe what might
have landed in North America about three millenia ago.

------------------------------------------------------------------------


The Other Martian Meteorite
by Astrobiology Magazine

Perhaps the most famous of the martian meteorites was discovered at
Allen Hills, Antarctica, and spawned a controversy
about fossil-shapes and whether Mars could once have supported more
favorable conditions than today.

"Most Mars meteorites studied in labs on earth," said Arizona scientist,
painter, and writer -- Dr. William Hartmann, now working with the Mars Global
Surveyor image team, "have clear evidence of having been exposed to
moisture and salty water. One (named Lafayette) has enough weathered
minerals that they could be dated by two labs (California and Arizona)
and the water exposure was found to have happened 670 million years ago."

The Lafayette sample is named after Lafayette, Indiana, where in 1935 it
was identified in a Purdue geological collection. The two pound (800 g)
mass is shaped like a truncated cone that measures between four and five
centimeters (about 2 in) across. The rock's conical shape is consistent
with its melting and solidification as it ablated upon entry through the
Earth's atmosphere. Parts of its surface began to flow into a smooth
veinous crust. At first glance, the meteor might be mistaken for a
mushroom cap [as shown in the banner image].

In 1992, the water found in Lafayette was determined to be
extraterrestrial and still contained the most [0.387 %] water of any
Martian meteor. More interestingly, its composition was enriched in
heavy water, or deuterium, as measured when the rock was step-wise
heated. After it formed initially as an iron-rich, volcanic rock,
Lafayette was apparently altered by water.

The mineral olivine, which generally does not stay around unmodified for
long when water is present, was also found to be the most altered of all
the iron-rich minerals in the sample. When examined in thin sections,
the meteor showed rusty-red grains surrounded by black veins, sometimes
called a fibrous structure.

This volcanic makeup had changed remarkably while Lafayette was still on
Mars--where many had presumed rock changes were not likely to be
identified with water. The conclusion of mineralogists soon
became surprisingly definitive. Many hundreds of millions of years ago,
Lafayette had once resided in salt-water.

And about one part in 300 remained locked up as water still in the meteor.

The timeline proposed
for Lafayette showed a lineage that began around 700 million years ago
on Mars, when some saline began to seep into it and change the rock's
mineral content. About 11 million years ago, the fragment blasted off of
Mars as debris and then landed on Earth [originally in Illinois] about
2,900 years ago. Or put another way, Lafayette arrived relatively
recently on Earth, sometime after the Egyptian pyramids were completed.

In 2000, scientists concluded that the "salts" identified in the
Lafayette alteration (based mainly on the mineral, iddingsite) formed by
fractional evaporation of an acid brine on Mars. When combined with
recent surface evidence for sulfur-rich brines once 'soaking' the
Opportunity rover site, an intriguing picture begins to frame what
earlier seemed to be surprising, but still isolated, Lafayette
discoveries that otherwise might not have had much context to place
it in a martian geological timeline.

These contextual questions will occupy comparisons of the world's
meteorite collections with what the Mars' rovers are chemically
analyzing on the surface this year. If Allen Hills is the oldest and
Lafayette is the wettest, then in this remarkable meteor traffic
between the red planet and Earth, our planetary neighbor seems to
have left behind more than a few intriguing clues about its
geological history.
Received on Wed 15 Sep 2004 02:01:57 PM PDT