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Early Mars: Warm Enough to Melt Water?
February 14, 2003
Denver - While some researchers believe that only asteroid
collisions made Mars warm enough to have running rivers,
a Penn State researcher believes the planet had to be
continuously warmer to form Mars' deep valleys, but
he does not know how the planet warmed up.
Some recent research suggests that early Mars was
cold most of the time and warmed up only when objects
impacted the planet. The impacts would warm the atmosphere
and melt water trapped in underground and surface ice,
causing rivers to flow and cutting the valleys that
rival Arizona's Grand Canyon.
"I do not think this is right," said Dr.
James F. Kasting, distinguished professor of geosciences
and meteorology.
"I do not think there was enough time involved
to form the types of features that we see on the Martian
landscape."
Kasting believes that a greenhouse effect warmed the
planet. However, he has calculated that a carbon dioxide
and water greenhouse would not have warmed the planet
above the freezing point of water. On Mars, before enough
carbon dioxide accumulated in the atmosphere to warm
things up, the carbon dioxide would condense into dry
ice clouds and eventually there would be ice caps.
"It does not seem possible to get above freezing
with gaseous carbon dioxide and water," he told
attendees at the annual meeting of the American
Association for the Advancement of Science today
(Feb. 14) in Denver.
Which is why some researchers think the planet was never
warm. But, according to Kasting, features like Nanedi
Vallis, which is a half-mile to over a mile wide in
places and over a half mile deep, could not be made
during the short time rivers would run after an impact.
"The channel at the bottom of Nanedi Vallis is
only about 100 feet across," says Kasting. "It
took millions of years of constant running water to
carve the Grand Canyon. It would take a similar time
on Mars."
One possible solution is that other greenhouse gases
were in play in the Martian atmosphere. Methane would
be a good candidate, but most sources of methane on
Earth are biological. Today's sources of methane are
methanogenic bacteria in ruminant animals and rice paddies,
but in the pre-oxygen atmosphere of the past, methanobacteria
could have lived in many places.
"Hillary Justh, a graduate student in geosciences,
ran a model of Mars with three atmospheres of carbon
dioxide and a tenth of a percent of methane in the atmosphere,"
adds the Penn State researcher. "Because 3.8 billion
years ago the solar luminosity was only 75 percent of
what it is today, the model returned an average temperature
of minus 13 degrees Fahrenheit."
This by itself would not have been enough to allow
widespread liquid water. However, the Martian surface
could have received 20-30 degrees Fahrenheit additional
warming from the greenhouse effect of carbon dioxide
ice clouds. This might have allowed at least the tropics
to remain above freezing.
One problem with methane-producing bacteria is that
the ones we know here on Earth, both from the fossil
record and today, prefer warm environments.
Some geologic processes generate methane, but only
in small amounts. These process require water and ultramafic
rocks to form serpentine rocks with methane as a by-product.
This process occurs at mid-ocean ridges on Earth and
can also occur during asteroid collisions that excavate
large amounts of mantle material.
"We do not really know much about how plate tectonics
works on Mars, and even if we did, it is doubtful that
enough methane could be generated to create the necessary
greenhouse," says Kasting. "Mars probably
did need a biological source of methane to form a planet-warming
greenhouse."
Researchers think that Mars has a supply of water,
which is required for all terrestrial life. They also
think that volcanic activity on Mars produced a tenth
of a percent or so of hydrogen and substantial amounts
of carbon dioxide, the two compounds that methanobacteria
on Earth need to produce methane. Evidence of these
methanobacteria could be found in subsurface fossils
or, the bacteria could still be there today.
"What we need to do is go and take samples,"
said Kasting, a member of Penn
State's NASA-sponsored Astrobiology Research Center.
NASA's Mars Exploration Rover Mission, scheduled to
launch later this year, will almost do that. Scheduled
to have two fully capable robotic vehicles like the
Sojourner, it will sample soils looking for signs of
life. However, while the science objectives of the rover
missions are to determine if water was present on Mars
and whether there are conditions favorable to the preservation
of evidence for ancient life, the mission will not return
samples to Earth. The first NASA sample-return mission
is scheduled for 2020 or later.
**aem**
EDITORS: EDITORS: Dr. Kasting may is at 814-865-3207
or at kasting@essc.psu.edu
by
e-mail.
Contacts:
A'ndrea Elyse Messer (814) 865-9481 aem1@psu.edu
Vicki Fong (814) 865-9481 vfong@psu.edu
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