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The Internal Structure of Mars

New observations of Mars reveal that the planet's flat northern lowlands were an early zone of high heat flow that later may have been the site of rapid water accumulation, according to a view of the Martian interior generated using data from Mars Global Surveyor, managed by NASA's Jet Propulsion Laboratory, Pasadena, Calif. Elevation and gravity measurements, which have been used to probe beneath the surface of Mars, indicate a period of rapid cooling early in Martian history, and evidence for large, buried channels that could have formed from the flow of enormous volumes of water.

image of Mars showing the exterior with a 
cut away to the interior mapping By combining highly accurate topographic maps with new plots of the planet's gravitational field, researchers have developed a working draft of what the planet's interior looks like several kilometers below the surface.

This global view of the Martian interior was generated from gravity measurements with the radio science experiment and elevation measurements from the Mars Orbiter Laser Altimeter (MOLA) instruments. Gravity and topography measurements were combined to reveal the structure of the crust on Mars, which preserves the record of melting of the interior and the heat loss from the planet over time.

"The crustal thickness map shows that, as for Earth, Mars has two distinct crustal provinces," explained Dr. Maria Zuber of the Massachusetts Institute of Technology, Cambridge, Mass., and lead author of a study to be published in the March 10 issue of Science. Beneath the rough southern highlands and Tharsis volcanic province the crust, estimated at 80 kilometers (50 miles) thick, thins progressively from the South pole toward the North. In contrast, the northern lowlands and Arabia Terra region of the southern highlands have a crust of uniform thickness, about 35 kilometers (22 miles) deep.

Below is a global slice of the crustal structure of Mars along 0° E longitude as derived from MGS gravity and topography data. In the figure, the south pole is at the far right and the north pole is at the far left. For illustrative purposes the crustal structure is vertically exaggerated and is about 40 km thick under the northern plains and 70 km thick at high southern latitudes. The sloping region under part of the southern highlands (yellow/orange) and the uniform thickness region under the northern lowlands (blue) and Arabia Terra region (green) represent the two distinct crustal provinces. The global dichotomy boundary occurs at the lowlands/Arabia Terra (blue/green) transition. This boundary does not correlate with the crustal structure, which indicates that the geological manifestation of the boundary is primarily due to surfical rather than internal processes.

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The crustal structure accounts for the elevation of the Martian northern lowlands, which controlled the northward flow of water early in Martian history, producing a network of valleys and outflow channels. The new gravity and topography data suggest that the transport of water continued far into the northern plains. The gravity shows features that have been interpreted as either buried channels or relics of an earlier topographic boundary that has been massively eroded. Undoubtedly the Valles Marineris and Kasei Valles outflow regions played a role in forming the present-day gravity lows, but the timing and extent of these channels is the subject of ongoing debate.

Evidence suggests that rapid heat flow in the Northern Hemisphere produced a wide lowland area, encouraging the formation of channels, which could have sluiced water resources into a large basin, even an early ocean. Shown in this image is one such putative channel, draining from the giant Valles Marineris into the wide, flat area of the north.

The features are about 200 kilometers (125 miles) wide and over 1,600 kilometers (1,000 miles) long, with characteristics that can be explained by water flow on the surface or in a submarine environment, later buried by sediments. The large size of these channels implies that any bodies of water in the northern lowlands could have accumulated rapidly. The now-buried channels may represent the means for filling an early ocean.

The gravity and topography also provide information on the cooling of Mars over time, which bears on the early climate and history of water. "The observations suggest that the northern lowlands was a location of high heat loss from the interior early in Martian history, probably due to a period of vigorous convection and possibly plate recycling inside of Mars," said Dr. Sean Solomon, Director of the Department of Terrestrial Magnetism of the Carnegie Institution in Washington, D.C., and a co-author of the study.

The high heat-loss zone corresponds to the part of Mars proposed to have been the site of an ancient ocean. The rapid transport of heat to the surface in this region would have released onto the surface and into the atmosphere gases and water or ice trapped in the interior. The time of rapid interior heat loss may correspond to the period when Mars had a warmer climate, liquid water flowed on the surface, and the planet's surface was shielded from the solar wind by a global magnetic field.

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This text was taken from the Press Release

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See also Internal Structure and Early Thermal Evolution of Mars from Mars Global Surveyor Topography and Gravity, Science, Vol 287, March 10, 2000.

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