Based on what we had seen so far Mars didn't look like the kind of place that would ever be a major tourist destination. The Viking spacecraft in the 1970s sent back some snapshots of what might as well have been dusty fields somewhere in Arizona. Mars Pathfinder's Sojourner, which rambled about a bit in 1997, didn't display anything much more intriguing--sand, some boulders and a boring, flat horizon.
But first impressions can be deceiving. The National Aeronautics and Space Administration's most recent emissary to the red planet, the Mars Global Surveyor, has just completed the first, detailed three-dimensional map of Mars's surface. And it reveals an exotic and extreme landscape that makes our home planet look pretty bland by comparison. The spectacular Martian scenery includes a six-mile-deep crater that could swallow half the U.S. and a volcano that towers to 17 miles.
It's as if a small planet, finding --like New York City--that it couldn't grow outward, simply exaggerated its vertical scale. "The full range of topography on Mars is about 19 miles. That's one and a half times the range of elevations found on Earth," says David Smith of the NASA Goddard Space Flight Center, who headed the mapping effort. The investigators revealed their new look at Mars in the May 28, 1999, issue of Science.
After some setbacks, the Global Surveyor reached Mars in 1997 and began its mapping mission about a year ago. The global maps were created from data obtained by an instrument called the Mars Orbiter Laser Altimeter (MOLA), which bounces an infrared laser beam off the surface and calculates the distance from the spacecraft by measuring the time it takes to detect the reflected pulse. The new high-resolution map represents 27 million elevation readings collected in 1998 and 1999. In a statement not destined to please NATO pilots flying over Yugoslavia, Carl Pilcher, Science Director for Solar System Exploration at NASA, announced at a May 27 press conference that "this incredible database means that we now know the topography of Mars better than many continental regions on Earth."
The new map data are already altering scientists' conceptions of Mars, and they are expected to provide insights that will be the basis for years' worth of further exploration. "The most significant aspect of the data is that it is going to allow us to reconstruct the evolution of the planet," says MOLA team member Maria Zuber of the Massachusetts Institute of Technology. "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."
Indeed, observes Smith, the "most curious aspect of the topographic map is the striking difference between the planet's low, smooth northern hemisphere and the heavily cratered southern hemisphere," which sits, on average, about three miles higher than the north. The difference in elevation between the hemispheres results in a slope from the south pole to north pole that may have had a major influence on the global-scale flow of water early in Martian history. Scientific models of watersheds using the new elevation map show that the northern hemisphere lowlands would have drained three quarters of the Martian surface.
The reason for that slope is apparently the enormous Hellas crater in the crater-pocked southern hemisphere. When an asteroid struck there, the impact created a ring of material that rises more than one mile above the surroundings and stretches out to 2,500 miles from the basin center. The result: a slightly pear-shaped planet with a giant hollow in its bottom. Meanwhile the MOLA data suggest that the elliptical northern hemisphere depression, some of the flattest terrain ever mapped, may have been an ancient seabed shaped by vigorous convection of the planet's mantle or tectonic plate recycling during the planet's formative stages.
On a more regional scale, the new data are also changing the way geologists visualize the flow of water on ancient Mars. The map shows that the eastern part of the vast Valles Marineris canyon slopes away from nearby outflow channels, with part of it lying a half-mile below the level of the outflow channels. "While water flowed south to north in general, the data clearly reveal the localized areas where water may have once formed ponds, " Zuber says.
And what of that water? The researchers say that although the poles appear very different from each other visually, they show a striking similarity in elevation profiles. Based on recent understanding of the north pole, this suggests that the south pole has a significant water ice component, in addition to frozen carbon dioxide. According to recent calculations, the upper limit on the present amount of water on the Martian surface is 800,000 to 1.2 million cubic miles, or about 1.5 times the amount of ice covering Greenland. If both caps are composed completely of water, the combined volumes are equivalent to a global layer 66 to 100 feet deep, about one third the minimum volume for the ancient seas of Mars.
At the high end of the scale, the map is also causing some rethinking. It shows that the volcanic highlands on the equator, known as the Tharsis region, are made up of two gigantic volcanic domes, not one as previously thought. And the grandest volcano of all, Olympus Mons, is a totally separate feature--and not part of the Tharsis rise at all. And there is recent evidence that some Martian volcanos were active very recently.
As superb as these maps are, they are only the beginning--these contain less than 10 percent of the measurements that MOLA can potentially collect during the Global Surveyor mapping mission, and the MOLA instrument is collecting about 900,000 measurements of elevation every day. Already, the new data are being used to refine what is known about the terrain that the next visitor to the surface of Mars--the Mars Polar Lander--will begin surveying on December 3, 1999.
Further observations will be used to refine the global map, to construct local and regional digital elevation models for scientific analysis and aid in assessment of future landing sites, and to attempt to detect and characterize global-scale seasonal variations. "It is going to take some work, but the potential is there to construct and evaluate models of the planet as a global system," Zuber says. "This is an extraordinary opportunity to study how the different parts of a planet influence and are affected by other parts."
So Mars is not Percival Lowell's serene vision of a placid world crossed by canals. It's more the stuff of a next-generation Edmund Hillary--someone ready to take on all 17 miles of Olympus Mons. But maybe, someday, a person could just take in the view.