Researchers pinpoint event that led to Mars' heyday

March 14, 2001

Planetary scientists at Washington University in St. Louis and various collaborators have concluded that the Tharsis rise in Mars' Western Hemisphere is key to many of the Red Planet's mysteries, including its large-scale shape and gravity field, and its early climate and water distribution. Roger J. Phillips, Ph.D., Professor of Earth and Planetary Sciences, and Director of the McDonnell Center for the Space Sciences, and his colleagues suggest that an enormous load of volcanic material emplaced at the Tharsis rise caused global changes to the planet's outer strong layer, or lithosphere, creating many key features in the landscape. Two major ones, the Tharsis trough, surrounding Tharsis, and the Arabia bulge, on the planet's opposite side, are the result of this deformation. These features, in turn, are essential in the development of the Martian valley networks, the most common type of drainage system on Mars. Additionally, water and carbon dioxide released by Tharsis volcanism may have created an atmospheric greenhouse sufficient to warm the surface to above freezing, thus enabling running water to form the valleys.

The research was published as part of the March 16, 2001 issue of Science magazine.

The Tharsis rise dominates the western hemisphere of Mars. It is a broad, elevated region rising up to 10 kilometers above its surroundings and encompassing over 30 million square kilometers. The rise is the site of large-scale volcanism and extensive fracturing of the crust. Phillips and colleagues analyzed gravity and topography fields obtained from the Mars Global Surveyor spacecraft and compared them to a model of how these fields would behave when the heavy Tharsis volcanic load is dumped onto the spherical lithospheric shell.

"Imagine that Mars is a beach ball and that the Tharsis load is your fist," Phillips said. "As your fist pushes into the beach ball, there is a bulge created on the opposite side of the ball, and a depression or trough surrounds your fist. It is that simple."

The model correctly predicts many of Mars' broad-scale gravity and topography features. The researchers believe that these features were in place at the end of Mars' oldest time period, the Noachian epoch. This period extends back to near the planet's origin, estimated at about 4.6 billion years ago, and it ended between 3.8 and 3.5 billion years ago.

According to Phillips, the Tharsis magmas could have produced enough carbon dioxide and water to induce a climate that was sufficiently warm for liquid water to be stable on the planet's surface. "The total release of gases from Tharsis magmas could produce the integrated equivalent of a 1.5-bar carbon dioxide atmosphere and a global layer of water that is 120- meters thick," Phillips reported. "Much of the water would have been lost to space, but even so, these quantities of volatiles are sufficient to warm the atmosphere to the point at which the surface temperature is above freezing. The accumulation of atmospheric carbon dioxide from Tharsis may have made the latter part of the Noachian the most favorable time for this condition."

The researchers said that the development of the Tharsis trough and Arabia bulge was a major influence on the location and orientation of Martian valley networks and outflow channels. They pointed out that nearly all of the large Martian outflow channels originate in or flow into the Tharsis trough. The model tested the role of Tharsis loading in valley network orientations and the researchers concluded that many of these systems, similar to Earth's river systems, had to have formed after a "significant fraction" of the Tharsis load was in place. This is because the valleys follow the downhill direction of the topography induced by the deformation of Mars in response to the Tharsis load. As these valleys were formed near the end of the construction of Tharsis, in the latter part of the Noachian epoch, they may reflect the clement conditions induced by large amounts of atmospheric carbon dioxide that accumulated from the volcanism that developed Tharsis.

At the very end of the Noachian epoch, volcanism declined and carbon dioxide and water were removed from the atmosphere by a combination of factors, including stripping by the solar wind and thermal escape, among others. The removal of carbon dioxide and water would have driven surface temperatures below freezing, and Phillips estimates that this could have occurred in less than a few hundred million years. The preceding few hundred millions years, in the latter part of the Noachian, could well be considered Mars' "brief, shining moment."

"It is possible that during the Noachian Epoch, the structural and volcanic events associated with Tharsis evolution were the sine qua non that linked fluvial, geodynamical and climate activity on Mars, " Phillips concluded.

Washington University in St. Louis

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