6th July 16:53
Sulfur dioxide may have helped maintain a warm early Mars (Forwarded)
Office of News and Public Affairs
Steve Bradt, 617-496-8070
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
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
"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
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 ****ysis 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.