PLANETARY GEOLOGY Earth has its high-standing continents and low-lying ocean basins, thanks to plate tectonics. And Mars has its smooth northern lowlands and its cratered highlands. But there's no credible sign that plate tectonics ever operated on Mars, so how did a third of the planet come to be as much as 4 kilometers lower than the rest? For the past quarter-century, a leading theory has held that a giant impact battered the young planet and excavated the northern lowlands, but that idea seemed to have serious problems. Now, two new studies purport to ease the difficulties with a giant impact. In one study, researchers reveal the true dimensions of the huge “Borealis basin,” making it look much more like the crater of a giant impact. And a second group has run simulations that suggest how an impactor could have blasted out an 8000-kilometer-wide crater without melting it into an unrecognizable puddle of magma. “I think there's much to recommend [a giant impact] now with all this new work,” says Sean Solomon, a planetary geophysicist at the Carnegie Institution of Washington's Department of Terrestrial Magnetism in Washington, D.C. ![Figure][1] Striptease. Removing the Tharsis volcanoes of Mars (reds and yellows, left panel) from part of the northern lowlands (light blue, right panel, top ) uncovered an elliptical basin that looks like an impact crater. CREDIT: MARS ORBITER LASER ALTIMETER (MOLA) TEAM/GODDARD SPACE FLIGHT CENTER/NASA Last month, planetary geophysicist Jeffrey Andrews-Hanna of the Massachusetts Institute of Technology in Cambridge and colleagues presented their test of the giant-impact hypothesis at the Lunar and Planetary Science Conference (LPSC) in Houston, Texas. In 1984, planetary scientists Donald Wilhelms, now retired from the U.S. Geological Survey, and Steven Squyres of Cornell University first proposed that a huge impactor had blasted out the Borealis basin. Fitting a circle to the “dichotomy boundary” between the basin and the highlands, they suggested that the circle could mark the outer edge of a huge crater. But the fit was too rough to win many converts. So Andrews-Hanna and his colleagues looked for a better way to trace out the dichotomy boundary. The great Tharsis volcanic complex had obscured much of the boundary when it smothered one-quarter of the planet with lava hundreds of millions of years after the lowlands formed. To “remove” Tharsis, they drew on measurements of martian gravity and surface height from the past decade of Mars orbiters. Subtle variations in the pull of gravity—evidenced in variations of a spacecraft's orbit—reflect the added mass of Tharsis lavas as well as the extent of the deep, less-dense crustal rock buoying up the highlands. The height of the surface constrains the volume of added lavas. By combining the data in a model, the researchers erased Tharsis's contribution to the present surface and traced the topographic edge of the Borealis basin right under Tharsis. Rather than a circle, the best shape for the basin turns out to be a 10,650-kilometer-long ellipse, they reported at the LPSC meeting. That's a familiar look for big impact basins. The 2300-kilometer Hellas impact basin in the southern highlands, for example, is also elliptical and also underlain by a uniformly thin crust. And there's no particular reason, Andrews-Hanna said, why the giant-impact theory's only serious rival—a peculiar sort of churning deep within the planet—would produce an elliptical basin or the observed sharp boundary between thin and thick crust. The other long-standing objection to impact excavation was the mere survival of a craterlike feature of any sort. Any object hundreds of kilometers across that hit Mars at tens of thousands of kilometers per hour, the thinking went, would heat the planet so dramatically that any crater would disappear in a sea of globe-girdling melted rock. Now that computer models are up to the task of simulating giant impacts in detail, that thinking is changing. As most recently reported at last December's American Geophysical Union meeting, planetary scientist Margarita Marinova of the California Institute of Technology in Pasadena and colleagues have simulated a range of giant impacts on Mars, all producing 8000-kilometer craters. They found that the melt from vertical impacts does in fact obliterate the crater but that faster, low-angle impacts do not. Simulated glancing blows at angles below 30° produce less melt overall and splash much of it into space. The giant-impact mechanism “was always thought dynamically impossible,” said Andrews-Hanna at LPSC. “Now you can't dismiss the possibility.” The two studies are convincing researchers that “the giant impact lives (after all)!” as planetary geophysicist Roger Phillips of Southwest Research Institute in Boulder, Colorado, playfully writes in an e-mail. Few are entirely won over yet, but a giant impact worked out as the origin of Earth's moon. Perhaps it will also serve to explain the deepest mystery of martian geology. [1]: pending:yes