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.

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