In the evolving debate over water on Mars, the Hesperian epoch is a dividing line. Planetary scientists agree that before the Hesperian, in the first billion years of the planet, water flowed on the surface, cutting valleys and eroding impact craters. Since the end of the Hesperian, what water remained has almost always been locked up as ice. But a paper in the 2 July issue of Science adds a new twist: Researchers have discovered dense networks of rain-carved valleys dating from the end of the Hesperian, when geologists believe that Mars was completely iced up.
The unmistakable signs of flowing water show up in images from the Thermal Emission Imaging System on the orbiting Mars Odyssey spacecraft. Planetary geologist Nicolas Mangold of the University of Paris South in Orsay and his colleagues now describe two areas of treelike networks of valleys carved into the high ground above and draining into the great canyons of Valles Marineris. The branching valleys are as closely packed as those of typical drainage systems on Earth, they note, in contrast to the sparse valley networks usually reported on Mars. Some valleys run right up to ridge crests, which rules out spring-fed streams, because not enough groundwater could have accumulated there to create springs. And the valley networks are continuous and leave no blank areas, suggesting that the water was not flowing beneath snow or ice. "They're really best explained by rainfall," says Mangold.
This rain was still falling on Mars--episodically, at least--late in the Hesperian, the team argues, despite the supposed long-term chilling. They point out that the surface cut by the valleys has been dated to the late Hesperian by a count of the craters formed by the steady drizzle of impacters since the valleys were cut. And the northern end of one drainage area was later covered by lava dated by crater-counting to the early part of the next epoch, the Amazonian.
Other planetary geologists have no doubts about the water, but they are less sure about when it was there. Mangold and colleagues based their age estimates in part on a 1992 paper that reported crater counts over many regions of Mars, says planetary geologist Gerhard Neukum of the Free University Berlin. "It may not be so late [in the] Hesperian as they think," he says. Kenneth Tanaka of the U.S. Geological Survey in Flagstaff, Arizona, the first author on the 1992 crater-counting paper, agrees. Additional "crater counts would make me feel more comfortable" about a late Hesperian age, he says.