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The avalanche on Mars, February 19th
2008
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Washington, DC — A team of
scientists, including Carnegie's Conel Alexander and Jianhua Wang,
studied the hydrogen in water from the Martian interior and found
that Mars formed from similar building blocks to that of Earth, but
that there were differences in the later evolution of the two
planets. This implies that terrestrial planets, including Earth, have
similar water sources--chondritic meteorites. However, unlike on
Earth, Martian rocks that contain atmospheric volatiles such as
water, do not get recycled into the planet's deep interior. Their
work will be published in the December 1 issue of Earth and
Planetary Science Letters. It is available online.
Debate in the scientific community
focuses on how the interior and crust of Mars formed, and how they
differ from those of Earth. To investigate the history of Martian
water and other volatiles, scientists at NASA's Johnson Space Center
in Houston, Carnegie, and the Lunar and Planetary Institute in
Houston studied water concentrations and hydrogen isotopic
compositions trapped inside crystals within two Martian meteorites.
The meteorites, called shergottites, were of the same primitive
nature, but one was rich in elements such as hydrogen, whereas the
other was depleted.
The meteorites used in the study
contain trapped basaltic liquids, and are pristine samples that
sampled various Martian volatile element environments. One meteorite
appears to have changed little on its way from the Martian mantle up
to the surface of Mars. It has a hydrogen isotopic composition
similar to that of Earth. The other meteorite appears to have sampled
Martian crust that had been in contact with the Martian atmosphere.
Thus, the meteorites represent two very different sources of water.
One sampled water from the deep interior and represents the water
that existed when Mars formed as a planet, whereas the other sampled
the shallow crust and atmosphere.
"There are competing theories that
account for the diverse compositions of Martian meteorites,"
said lead Tomohiro Usui. "Until this study there was no direct
evidence that primitive Martian lavas contained material from the
surface of Mars."
Because the hydrogen isotopic
compositions of the two meteorites differ, the team inferred that
martian surface water has had a different geologic history than
Martian interior water. Most likely, atmospheric water has
preferentially lost the lighter hydrogen isotope to space, and has
preferentially retained the heavier hydrogen isotope (deuterium).
That the enriched meteorite has
incorporated crustal and atmospheric water could help to solve an
important mystery. Are Martian meteorites that are enriched in
components, such as water, coming from an enriched, deep mantle, or
have they been overprinted by interaction with the Martian crust?
"The hydrogen isotopic composition
of the water in the enriched meteorite clearly indicates that they
have been overprinted, so this meteorite tells scientists more about
the Martian crust than about the Martian mantle," Alexander
said. "Conversely, the other meteorite yields more information
about the Martian interior."
The concentrations of water in the
meteorites are also very different. One has a rather low water
concentration and that means that the interior of Mars is rather dry.
Conversely, the enriched basalt has 10 times more water than the
other one, suggesting that the surface of Mars could have been very
wet at one time. Therefore, scientists are now starting to learn
which meteorites tell us about the Martian interior and which samples
tell us about the Martian surface.
"To understand the geologic
history of Mars, more information about both of these environments is
needed," Alexander said.
This work was supported by a NASA Mars
Fundamental Research Program grant, a NASA Cosmo chemistry Program
grant, and by a Astrobiology Institute grant.
The Carnegie Institution for
Science is a private, nonprofit organization headquartered in
Washington, D.C., with six research departments throughout the U.S.
Since its founding in 1902, the Carnegie Institution has been a
pioneering force in basic scientific research. Carnegie scientists
are leaders in plant biology, developmental biology, astronomy,
materials science, global ecology, and Earth and planetary science.
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