New pictures of Mars suggest that water gushed from within the red planet billions of years ago and lingered on the surface for some time. The images also indicate that volcanoes belched far more lava onto Mars than suspected, and hot spots of molten rock may still lie beneath the surface, according to reports in tomorrow's Nature.
Mars is cold, dry, and dusty today. But researchers believe the planet's early atmosphere may have been thick enough to sustain a warm and wet climate, creating a hospitable cradle for life (Science, 29 January, p. 648). Volcanoes might have contributed to that climate by spewing greenhouse gases into the martian air. Indeed, robotic surveyors have seen signs of watery flows and major eruptions in the planet's youth, before 3.5 billion years ago. But it wasn't clear whether this water rained upon the surface or burbled up from within, nor did researchers agree about the extent and duration of volcanic activity.
Sharp images from Mars Global Surveyor address both issues. The images, taken about a year ago, resolve features just 4 to 8 meters across. Networks of valleys arise suddenly and disappear in spots, like the sinkholes in limestone country--clear evidence that underground water carved many martian channels, says planetary geologist Michael Carr of the U.S. Geological Survey in Menlo Park, California. And sinuous channels scoured into the bases of canyons indicate that water probably lingered on the surface in some places, Carr notes. Rainfall, on the other hand, would have created far more small tributaries than Surveyor has seen, he says.
Other images of rocky layers in the Valles Marineris canyon point to extensive volcanism during the first billion years on Mars, says planetary geologist Alfred McEwen of the University of Arizona in Tucson. Martian volcanoes may have unleashed enough lava to cover the United States to a depth of more than 6 kilometers, McEwen says--about 10 times more than previously believed. Moreover, few impact craters exist in some regions, suggesting that fresh lava may have erupted onto the surface as recently as 40 to 100 million years ago. "It's likely that there is still magma under the surface of Mars, and it's conceivable that subsurface ice could melt in isolated warm areas," McEwen says.
The evidence for groundwater on early Mars is inarguable, says planetary scientist Maria Zuber of the Massachusetts Institute of Technology in Cambridge. "It will be difficult to argue for precipitation after this," she says. However, Zuber isn't yet convinced that the layers in Valles Marineris are volcanic, because sediments deposited by flowing water could leave similar patterns. Dust also might obscure many impact craters, she notes, making the surface appear younger than it really is.