Sulfur dioxide may have helped maintain a warm early Mars (Forwarded)
- From: Andrew Yee <ayee@xxxxxxxxxxxxxxxxxxxxxx>
- Date: Fri, 21 Dec 2007 17:54:56 GMT
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December 20, 2007
Sulfur dioxide may have helped maintain a warm early Mars
Hypothesis may aid understanding of early Earth
By Steve Bradt
Sulfur dioxide (SO2) may have played a key role in the climate and
geochemistry of early Mars, geoscientists at Harvard University and the
Massachusetts Institute of Technology (MIT) suggest in the Dec. 21 issue
of the journal Science. Their hypothesis may resolve longstanding
questions about evidence that the climate of the Red Planet was once much
warmer than it is today.
The Science paper's authors are Itay Halevy, a Ph.D. candidate in
Harvard's Department of Earth and Planetary Sciences; Daniel Schrag,
professor of earth and planetary sciences and environmental science and
engineering at Harvard; and Maria Zuber, professor of earth, atmospheric,
and planetary sciences at MIT. A significant portion of the collaboration
occurred during Zuber's 2002-03 year as a Radcliffe Institute fellow.
"There is abundant evidence for a warmer climate, perhaps even a liquid
water ocean, early in Martian history, between 3.5 and 4 billion years
ago," says Schrag, the paper's senior author. "However, scientists have
found it difficult to reconcile this evidence with our understanding of
how the climate system is regulated on Earth."
Over millions of years, the Earth's climate has been controlled by the
carbon cycle and its effect on carbon dioxide, the main greenhouse gas. On
Earth, there is a balance between carbon dioxide vented from volcanoes and
chemical reactions with silicate rocks on the Earth's surface that remove
carbon dioxide from the atmosphere and convert it to calcium carbonate,
commonly known as limestone. Scientists believe that this balance has
helped maintain Earth's habitability over the last 4 billion years.
On Mars, there is not enough volcanic activity today to maintain this
cycle. But this was not true some 4 billion years ago, when a giant
volcanic complex called Tharsis erupted over tens to hundreds of millions
of years -- and also a time when evidence suggests Mars had a much warmer
climate. However, this carbon cycle on early Mars should have produced
vast quantities of limestone like on Earth, and yet almost none has been
found.
The new hypothesis points the finger at sulfur dioxide, another gas
released by volcanoes. Sulfur dioxide is a powerful greenhouse gas, like
carbon dioxide, and it is more reactive with silicate rocks than carbon
dioxide. On Earth, sulfur dioxide is rapidly oxidized to sulfate, and then
removed from the atmosphere. The authors argue that the atmosphere of
early Mars would have lacked oxygen, so sulfur dioxide would remain much,
much longer.
"The sulfur dioxide would essentially pre-empt the role of carbon dioxide
in surface weathering reactions," says Halevy, the first author of the
report. "The presence of even a small amount of sulfur dioxide in the
atmosphere would contribute to the warmer climate, and also prevent
limestone deposits from forming."
In place of limestone, the authors predict that sulfur minerals would form
in any standing water on Mars. This may explain the surprising finding of
the rovers that have identified sulfur minerals as an abundant component
of Martian soils.
"We think we now understand why there is so little carbonate on Mars, and
so much sulfur," Halevy says.
"Our hypothesis may also be important for understanding the early Earth,"
Schrag says. "Before the origin of life, our atmosphere may have looked
much like early Mars. Sulfur dioxide may have had an important role then
as well."
If correct, the hypothesis implies that the oceans in which life evolved
were much more acidic than previously thought. The early Earth may also
provide a test for the hypothesis through the analysis of isotopes of
sulfur in ancient mineral deposits.
Halevy, Schrag, and Zuber's work was funded by the NASA Planetary Geology
and Geophysics program, the George Merck Fund of the New York Community
Trust, and by a Radcliffe Fellowship to Zuber and a Harvard Origins of
Life Initiative Graduate Fellowship to Halevy.
.
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