[meteorite-list] Odyssey of a Moon Rock (SAU 169)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Jul 29 16:53:17 2004
Message-ID: <200407292053.NAA27939_at_zagami.jpl.nasa.gov>

http://www.nature.com/news/2004/040726/full/040726-9.html
    
Odyssey of a Moon rock
Mark Peplow
nature.com
July 29, 2004

Chemical analysis illuminates 4-billion-year history of desert meteorite.

A lucky find in the desert of Oman has allowed scientists to reconstruct
the most detailed ever history of a lunar meteorite. Their results
reveal that the rock has had a violent life, enduring at least four
collisions before it even left the Moon.

The meteorite, called Sayh al Uhaymir 169, was discovered by Edwin Gnos
and colleagues from the University of Bern, Switzerland, during a field
trip to Oman in 2002. It is one of about 30 Moon meteorites that have
been found on Earth since 1979.

The team has worked out that the rock came from the Lalande impact
crater on the Moon, an area just a few kilometres across. It is the
first time that scientists have been able to pinpoint the birthplace of
a lunar meteorite with such precision.

The researchers' best clue came from unusually high levels of the
radioactive element thorium in the meteorite. "The chemistry of this
rock is quite unusual. There's no other rock quite like this, either as
a meteorite or as collected on the Apollo missions," says Gnos.

The team used a detailed map of lunar thorium created by NASA's Lunar
Prospector probe in 1998-99 to work out that the rock must have come
from somewhere in the Mare Imbrium (Sea of Showers), which forms the
right eye of the 'man in the Moon'. Other mineral data pinpointed the
location as Lalande.

Rocky ride

Gnos and his team determined the history of Sayh al Uhaymir 169 by
comparing the ratios of radioactive elements within the rock. As
radioactive elements decay they change into different elements. For
example, over millions of years uranium eventually becomes lead, so the
ratio of uranium to lead helps to reveal how long it is since the rock
crystallised.

Any major impact melts parts of the rock, resetting the radioactive
clock in those areas. So comparing the ages of crystals in different
parts of the rock shows when and how severely it has been battered.
During the meteorite's time in space it was also bombarded with cosmic
rays, which altered its chemical composition, leaving a distinct
signature of its journey to Earth.

The team's analysis suggests that the rock was caught up in the enormous
impact that created Mare Imbrium, which Gnos estimates to have happened
about 3.9 billion years ago. The rock was then bounced through the
Moon's crust by two more impacts, 2.8 billion and 200 million years ago,
that may have been caused by colliding asteroids.

A final impact just 340,000 years ago knocked the rock off the Moon
altogether, and it floated through space before crashing to Earth about
10,000 years ago. It has probably lain undisturbed in the Omani desert
ever since.

Gnos calculates an age for the Imbrium crater that is slightly older
than lunar samples collected by the Apollo missions had suggested. They
dated the impact to about 3.85 billion years ago. The new date is
important for planetary scientists who use the craters of the Moon as a
pictorial calendar that shows how many meteorites there were in our
Solar System at different stages in the last 4 billion years.

Gnos's team will go back to Oman to look for more meteorites at the end
of this year.

References

   1. Gnos E., et al. Science, 305. 657 - 659 (2004).
Received on Thu 29 Jul 2004 04:53:14 PM PDT