Science: "It's all a mush in my mind," said Michael Carr, talking about early Mars. That's quite a concession from one of the world's most experienced Mars geologists. After 10 hours of discussion on the state of early Mars, "it's frustrating," he said when asked to sum up a small premeeting workshop.* "Despite the beautiful data we've seen, we're no closer to understanding what [early Mars] climate was. And if it was warm and wet, what caused it to be warm and wet?"

Carr, who has been studying Mars since the early 1970s at the U.S. Geological Survey in Menlo Park, California, wrote the book on Mars. Actually, he wrote two of them: Water on Mars and The Surface of Mars. But that wasn't enough to sort out 40 years of data, much of it still coming from two rovers and two orbiters. The workshop focused on the first period of martian geologic history, called the Noachian--when water flowed on the surface, at least at times--and the transition into the Hesperian, a colder, drier time preceding the bone-dry deep-freeze of the past 3 billion years.