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Pole-to-Pole Slope and Tharsis Control
of the Shape of Mars

One of the discoveries made with MOLA made is that the surface of Mars slopes upward going from the North Pole to the South Pole. The image below is a pole-to-pole view of Martian topography from the first MOLA global topographic model [Smith et al., Science, 1999].

see caption

This slice runs from the north pole (left) to the south pole (right) along the 0° longitude line. It shows the pole-to-pole slope of 0.036°, such that the south pole has a higher elevation than the north pole by ~6 km. This global-scale slope was likely present for most of Mars' history and controlled the surface and subsurface transport of water indicated by images of outflow channels and valley networks. The regional high (in orange, to the right) in mid-southern hemisphere latitudes corresponds to the western edge of the topographic annulus that encircles the massive Hellas impact basin. In the figure warm colors correspond to high elevations and cold colors correspond to low elevations. Note the exceedingly flat northern hemisphere in blue. (Image credit: MOLA Science Team).

But this pole-to-pole slope isn't the only factor in the flow of water on Mars. The formation of the massive volcanic rise, called the Tharis rise, may have affected the early climate and flow of water on the Martian surface.

see caption
The Tharsis Region

Tharsis is a BIG load on the surface of Mars and when it was emplaced it deformed the whole surface of the planet, just like what would happen if if a beach ball was punched. By developing models, the MOLA Team showed that loading of Mars by Tharsis explains much of the global shape and long-wavelength gravity field of the planet. Loading by Tharsis also explains a ring of negative gravity anomalies and a topographic trough around Tharsis, as well as gravity-anomaly and topographic highs centered in the Arabia Terra region and extending northward toward the Utopia basin. The Team showed that the Tharsis-induced trough and antipodal high were largely in place by the end of the Noachian Epoch, the earliest stratigraphic epoch in Martian history (about 4 billion years ago).

Tilts on the planet caused by Tharsis loading also exerted control on the location and orientation of valley networks. The MOLA Team determined this by showing that most valley networks have azimuths that on average match the tilt induced by the combination of the response to Tharsis loading and the pole-to-pole slope of topography.

Why this is important? The release of carbon dioxide and water accompanying the emplacement of approximately 300 million cubic kilometers of Tharsis magmas (about 10,000 Mauna Loas!) may have sustained a warmer climate than at present, enabling the formation of ancient valley networks and causing massive erosion. This is because the magmas released enough CO to produce a 1.5-bar atmosphere (producing a Greenhouse effect) and enough water to produce a global layer 120-m deep. This analysis shows that Tharsis may be the factor responsible for Mars's early wetter, warmer climate.

Links & Resources

Text and Figures on the Ancient Geodynamics and Global-scale Hydrology on Mars.

See also Ancient Geodynamics and Global-Scale Hydrology on Mars, Science, Vol 291, March 30, 2001.

More information about the pole-to-pole slope.

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NASA Official: Greg Neumann
Last Updated: 1/19/2007