A New Look at the Martian Landscape
Bernice Wuethrich*

 Mars is 100 million kilometers away, but in at least one respect, we now know it better than our own familiar Earth. On page 1495 of this issue, planetary scientists present a precise map of martian topography, accurate around the planet to within 13 meters of elevation; some parts of Earth are known only to 100 meters or more. "We now have a definitive picture of the shape of the whole planet," says David Smith of the Goddard Space Flight Center in Greenbelt, Maryland, principal investigator of the instrument, called the Mars Orbiter Laser Altimeter (MOLA), that gathered the data from its perch aboard the Mars Global Surveyor spacecraft.

 Thanks to MOLA, a diverse array of martian features has now snapped into sharper focus, including the polar ice caps and the plateaus and lowlands that hint at the processes that shaped the planet. "MOLA's maps allow you to settle issues once and for all that have been contested in Mars geology for 25 years," says Jeff Moore, a planetary geologist with the NASA Ames Research Center in Moffett Field, California. "We're seeing things that nobody had an inkling existed," adds Bruce Jakosky, a geologist at the University of Colorado, Boulder. "In a sense we're seeing the planet for the first time."

 The new map was made by bouncing laser light off the martian surface and using its roundtrip time to determine distance. The map reveals a dramatic landscape of higher highs and lower lows than previously appreciated, with a total range in elevation of 30 kilometers (km), compared to just 20 km for Earth. The data confirm that the southern hemisphere is higher than the northern hemisphere--6 km higher, to be exact. That means that on Mars, "downhill is north," and if the planet had flowing water, the northern lowlands would drain a watershed comprising three-quarters of the planet, says co-author Sean Solomon of the Carnegie Institution of Washington, D.C.

 This MOLA-eye view of Mars may also help resolve the genesis of its split geologic personality. Planetary scientists have long realized that Mars is lopsided--thin-crusted, low, and smooth in the north, and thick-crusted, high, and crater-scarred in the south. Conflicting explanations for the mismatched hemispheres include a huge asteroid impact that blew apart and thinned the crust in the north, or internal processes, such as Earth-like plate tectonics or a huge plume of molten rock rising from the interior that melted northern crust.

 MOLA's data point in one direction. "We favor internal processes," says Maria Zuber, a co-author of the topographic analysis and a geophysicist at the Massachusetts Institute of Technology in Cambridge. Although MOLA found no direct evidence of plate tectonics such as mountain belts or earthquake faults, several features suggest an unprecedented amount of past volcanic activity, signaling a hot interior. For example, the Tharsis rise, a 4000-km-across bulging plateau that straddles the equator, appears to consist of two volcanic domes rather than one. And Olympus Mons, the biggest volcano in the solar system, is not a part of Tharsis as scientists believed, but rises off its western edge. "This argues for a broader mantle heat source for Tharsis than was previously thought," says Zuber. Added to the magnetic stripes recently spotted on Mars's surface (Science, 30 April, pp. 719, 790, and 794)--a possible sign of plate tectonics--the new evidence suggests that an internal, heat-driven process shaped Mars's spectacular topography, says Zuber.

 MOLA's data also tend to refute the idea of a northern impact. The maps show no sign of a giant northern crater, and the north-south boundary is too irregular to be a circular crater wall. Instead, MOLA's team concludes that the boundary is a mosaic of regional effects, shaped by such factors as erosion, volcanism, and debris flung up from a southern impact.

 Indeed, when it comes to impacts, "we've been looking in the wrong hemisphere," says Zuber. MOLA has discovered that the south's Hellas basin, 9 km deep and 2300 km wide, is surrounded by a giant ring of topography 2 km high that stretches 4000 km from the basin's center. These highlands were likely raised by rock blasted out by the impact.

 Other researchers aren't ready to discard the idea that an impact gouged out the north. "It's too soon to jump on a bandwagon," because either an internal mechanism or a megaimpact could produce planetary-scale changes in topograpy, says planetary geologist George McGill of the University of Massachusetts, Amherst. He adds that traces of even a massive impact could have been obliterated over billions of years.

 But McGill and others say they are impressed with the data, which reveal a host of other details, including the size of the polar ice caps. The northern ice cap turned out to be smaller than expected, but MOLA found that the southern polar cap is surprisingly large, because although the visible cap is small, the topography suggests vast layered deposits of ice and dust. Assuming both caps are chiefly water ice, the MOLA team estimated a maximum ice volume of 4.7 million km3--about one-third less than the previous best estimate--suggesting that much of Mars's water has either escaped to space or been sequestered underground.

 MOLA will continue to collect 900,000 elevation measurements daily for the next 2 years, and researchers are now signing up to use the data for questions ranging from the location of ancient water reservoirs to the best places to land spacecraft. "I just can't wait until people have the opportunity to use this map," Zuber says.

Bernice Wuethrich is an exhibit writer at the Smithsonian's National Museum of Natural History.

Related articles in Science:

The Global Topography of Mars and Implications for Surface Evolution.
David E. Smith, Maria T. Zuber, Sean C. Solomon, Roger J. Phillips, James W. Head, James B. Garvin, W. Bruce Banerdt, Duane O. Muhleman, Gordon H. Pettengill, Gregory A. Neumann, Frank G. Lemoine, James B. Abshire, Oded Aharonson, C. David, Brown, Steven A. Hauck, Anton B. Ivanov, Patrick J. McGovern, H. Jay Zwally, and Thomas C. Duxbury

Science 1999 284: 1495-1503. (in Research Articles)
Volume 284, Number 5419 Issue of 28 May 1999, pp. 1441 - 1442 
©1999 by The American Association for the Advancement of Science.