CAMBRIDGE, Mass. -- Researchers have known for some time that Mars has a deep dent in its southern hemisphere. Until recent measurements yielded a highly accurate, global map of the red planet's topography, they didn't know that the Hellas basin could swallow Mt. Everest, or that the asteroid that caused the crater hurtled debris as far as 2,500 miles across the planet's surface.
"Probably the most unexpected finding was how much the Hellas impact basin dominates the topography of the southern hemisphere," said Maria Zuber, Griswold Professor of Planetary Sciences at the Massachusetts Institute of Technology. Another major finding is the planet's dramatic slope from its south to north pole. This would have significantly affected water transport and cycles on the planet, she said, leading to ponding of water in certain places.
Zuber and three other MIT researchers are part of a team of 20 scientists who created a global grid of Mars based on data from the Mars Orbiter Laser Altimeter (MOLA), an instrument on the Mars Global Surveyor spacecraft. Their work, published in today's (May 28) issue of the journal Science, is expected to lead to new insights about how Mars evolved over the past 4 billion years.
The map is expected to give scientists the basis for years' worth of further exploration and definition of the red planet. "The most significant aspect of the data is that it is going to allow us to reconstruct the evolution of the planet," Zuber said. "From the shape of the surface and information on gravity, magnetics and surface composition, we can model the internal structure and the planet's thermal evolution. These models bear closely on the history of climate and water.
"It is going to take some work, but the potential is there to construct and evaluate models of the planet as a global system. This is an extraordinary opportunity to study how the different parts of a planet influence and are affected by other parts," she said.
Dramatic shifts in terrain
Mars has the highest mountains and deepest craters of any of the terrestrial planets. There is a big contrast in elevation between the mostly flat northern hemisphere and the heavily cratered southern hemisphere, which sits on average three miles higher than the north. The topography of Mars has a 30-kilometer (19-mile) dynamic range, one and a half times that found on Earth, said David Smith of NASA's Goddard Space Flight Center, principal investigator for MOLA and lead author of the study.
Researchers from NASA's Goddard Space Flight Center, the Carnegie Institute of Washington (D.C.), Washington and Brown universities, the Jet Propulsion Laboratory and the California Institute of Technology, both in Pasadena, worked on the map. MIT researchers Gordon Pettengill, professor emeritus of Earth, Atmospheric and Planetary Sciences (EAPS); EAPS research scientist Gregory Neumann and EAPS graduate student Oded Aharonson are part of the team.
The high-resolution map represents 27 million measurements gathered during March and April this year and summer 1998. Each elevation point is known with an accuracy of about six feet in the flat northern hemisphere to 42 feet elsewhere. The level of detail is made possible by MOLA, which works by sending and receiving infrared laser pulses.
Zuber said that even though the researchers designed MOLA to be very accurate, they were nonetheless "rather startled at how well we have been able to measure the elevations. The topography of Mars is now known in a global sense better than many areas of Earth's continents. And we have not yet implemented the most advanced post-processing techniques, so the values are going to get better yet." MOLA continues to collect about 900,000 elevation measurements daily during the ongoing Mars Global Surveyor mission.
The data will be used, among other things, to assess where NASA's Mars Polar Lander mission will set down this winter.
A deep impact
"Before our map, it was known that Hellas was a big hole. But we believe that we have identified evidence that the material excavated from the basin was deposited in a mile-high ring that extends out to 2,500 miles from the center of the basin," Zuber said. "This material contributes significantly to the high elevation of the southern hemisphere and underscores the role of major impacts in shaping the early surfaces of the solid planets."
The volcanic Tharsis province includes the Olympus Mons volcanic shield, which shoots up nearly 17 miles. This area is in sharp contrast with the Hellas impact basin -- six miles deep and 1,400 miles across -- that was probably caused by an asteroid. The basin is surrounded by a ring of material that was ejected from it that would cover the United States with a 2-mile-deep layer.
The MOLA data suggest that the elliptical northern hemisphere depression was likely shaped by vigorous convection of the planet's mantle or tectonic plate recycling during the planet's formative stages.
The dramatic slope from Mars' south to north poles would have influenced the predominant way water flowed early in Martian history. The northern hemisphere's lowlands would have drained three-quarters of the Martian surface, but the map shows there are local areas, such as within the massive Valles Marineris canyon system, where Mars' limited water supply may have formed ponds, Zuber said.
MOLA was designed and built by the Laser Remote Sensing Branch of the Laboratory for Terrestrial Physics at Goddard. The Mars Global Surveyor mission is managed for NASA's Office of Space Science, Washington, DC, by the Jet Propulsion Laboratory (JPL), Pasadena, CA, a division of the California Institute of Technology. JPL's industrial partner is Lockheed Martin Astronautics, Denver, CO, which developed and operates the spacecraft.