Digging Mars

The missions come at a time when planetary scientists are conducting a fundamental makeover of their notions regarding the planet's geological history.

"We're tearing down the framework of our earlier understanding of Mars and in its place we have yet to build a complete new structure," McCleese says.

The effort began with the Mariner missions in the mid 1960s and the Viking lander-orbiter missions in the mid-1970s. It blossomed with the arrival of the Mars Global Surveyor (MGS) and Mars Odyssey orbiters in 1999 and 2001. The duo has revealed topography and surface mineral deposits with a stunning level of detail, revealing features like valleys, mesas, gullies, and other signs of erosion that look deceptively Earthlike.

Indeed, data from the Mars Global Surveyor "has turned Mars geology upside down," says Harold McSween, a geologist at the University of Tennessee. "We've always had this view that Mars geology was kind of simple. Mars was a bunch of lava flows, and they just lie there soaking up impacts from meteors."

Yet using the MGS camera they designed, Michael Malin and Kenneth Edgett, with Malin Space Science Systems in San Diego, have "seen layered rock, which they interpret as layers of sediment, everywhere at high resolution," McSween says. "Many of these are deeply eroded," presenting planetary geologists and climatologists with a more complicated story to decipher.

McCleese says most of his colleagues support the notion that early in its history, the planet had a sufficient heat-trapping atmosphere to allow water to collect, flow, and persist on the surface. Many of the large valleys and erosion features they see today probably formed during this period, they argue. Other evidence from remote-sensing studies of the planet's core suggest that, early on, Mars had a strong magnetic field that might have shielded budding life forms on the surface from the bath of space-based radiation enveloping the planet.

Yet many of the features the cameras are capturing also could have formed under more frigid, dry, ice-capped conditions similar to those astronomers see today.

A team led by Marc Kramer, a hydrologist at NASA's Ames Research Center at Moffett Field, Calif., looked again at old Martian crust that in earlier, less detailed images appeared to sport basins and dead-end rivers, which were thought to have formed through groundwater undermining the surface.

Using the more detailed Mars Global Surveyor images and comparing what they saw to hydrological models, team members reported in January that the features actually form drainage networks similar to those found in the Great Lakes region of the US. The origin of such features in the Great Lakes has been traced to glacial melt and gravity rather than large erosive flows of water from rain.

Before he and his colleagues undertook their study, "I was of the mind-set that precipitation may have driven these older features," Dr. Kramer says. "It's still a plausible alternative, but the cold model also can drive the kind of features we're seeing."

These and other competing ideas are providing the intellectual fuel for June's assault on Mars.

The deluge of new data since 1997 is opening "an awful lot of doors to understanding Mars, but they also can leave us scratching our heads," says John Grant, a researcher at NASA-Ames and a member of the MER site selection committee.

"Now we've got the opportunity to actually go to a couple of places where there's been a detailed evaluation of the surface. We can actually start testing hypotheses."

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