[meteorite-list] sher got a mystery on their hands...

From: Darren Garrison <cynapse_at_meteoritecentral.com>
Date: Wed, 21 Nov 2007 23:57:33 -0400
Message-ID: <pev9k3l747d9vktpr4l25j6uftod6sfv22_at_4ax.com>

http://www.sciencedaily.com/releases/2007/11/071121220939.htm

New Light On Early Formation Of Earth And Mars

ScienceDaily (Nov. 21, 2007) ? A team of scientists from NASA's Johnson Space
Center (JSC) and the Lunar and Planetary Institute and the University of
California, Davis (UCD) has found that terrestrial planets such as the Earth and
Mars may have remained molten in their early histories for tens of millions of
years. The findings indicate that the two planets cooled slower than scientists
thought and a mechanism to keep the planet interiors warm is required.

These new data reveal that the early histories of the inner planets in the solar
system are complex and involve processes no longer observed. Evidence of these
processes has been preserved in Mars, while it has been erased in Earth. So Mars
is probably the best opportunity to understand how Earth formed.

The formation of the solar system can be dated quite accurately to 4,567,000,000
years ago, said Qing-Zhu Yin, assistant professor of geology at UC Davis and an
author on a new paper. Mars' metallic core formed a few million years after
that. Previous estimates for how long the surface remained molten ranged from
thousands of years to several hundred million years.

The persistence of a magma ocean on Mars for 100 million years is "surprisingly
long," Yin said. It implies that at the time, Mars must have had a thick enough
atmosphere to insulate the planet and slow down cooling, he said.

Scientists think that early crust formation alone cannot account for the slow
cooling magma ocean seen in large planets. This new evidence instead implies
that Mars, at one time, had a primitive atmosphere that acted as the insulator.
?The primitive atmosphere was composed mostly of hydrogen left over from
accretion into a rocky planet, but was removed, probably by impacts, about 100
million years after the planet formed,? said Debaille.

Debaille and her colleagues performed precise measurements of neodymium isotope
compositions of nine rare Martian meteorites called shergottites using mass
spectrometers at JSC and UCD. Shergottites, named after the first-identified
meteorite specimen that fell at Shergotty, India, in 1865, are a group of
related meteorites from Mars composed primarily of pyroxene and feldspar.

The scientists examined shergottites because their large range in chemical
compositions is thought to be a fingerprint of the formation of their deep
sources very early in the history of Mars.

?These rocks were lavas that were made by melting deep in Mars and then erupted
on the surface," said Brandon. ?They were delivered to Earth as meteorites
following impacts on Mars that exhumed them and launched them into space." Mars
meteorites are a treasure chest of information about that planet and have been
the focus of extensive research by scientists.

The metallic element samarium has two radioactive isotopes that decay at a known
rate to two daughter neodymium isotopes. By precisely measuring the quantities
of neodymium isotopes, Debaille was able to use these two radiometric clocks to
derive the times of formation of the different shergottite sources in the
Martian interior.

?We expected to find that their sources all formed at the same time,? said
Debaille. ?But what we found instead was that the shergottite sources formed at
two different times. The oldest formed at 35 million years after the solar
system began to condense from ice and dust into large planets about 4,567
million years ago. The youngest formed about 110 million years after the solar
system began to condense.?

Debaille and her colleagues found that the scenario that best fits the data is
one where a global-scale magma ocean formed from melting in Mars during the
final stages of accretion and then slowly solidified over this time period.

?The most recent physical models for magma oceans suggest they solidify on
timescales of a few million years or less, so this result is surprising,? said
Brandon. ?Some type of insulating blanket, either as a rocky crust or a thick
atmosphere, is needed as an insulator to have kept the Martian interior hot.?

Vinciane Debaille (LPI), Alan Brandon (JSC), Qing-zhu Yin and Ben Jacobsen (UCD)
present these new findings in a paper published in the Nov. 22 issue of Nature.

Adapted from materials provided by NASA, Johnson Space Center.
Received on Wed 21 Nov 2007 10:57:33 PM PST