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 (MGS). 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.

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, MA, 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 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 22 miles deep.

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- field data suggest that the transport of water continued far into the northern plains. The gravity shows features interpreted as channels buried beneath the northern lowlands emanating from Valles Marineris and the Chryse and Kasei Valles outflow regions.

The features are about 125 miles wide and over a thousand 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, DC, 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.

During the ongoing Mars Global Surveyor mapping mission the Radio Science and MOLA experiments will continue to collect data on a near-continuous basis through the end of the mission in February 2001. The MOLA instrument was designed and built by the Laser Remote Sensing Branch of the Laboratory for Terrestrial Physics at NASA's Goddard Space Flight Center, Greenbelt, MD. The Radio Science experiment is implemented from the Center for Radio Astronomy of Stanford University, Palo Alto, CA.