Scientists have known for decades that Mars, at least in its ancient past, has had a considerable amount of water. But when Mars Global Surveyor began mapping the Red Planet in sharp detail early in 1999, it disclosed startling evidence that water has shaped martian landforms within the past 10 million years. The discovery challenges the prevailing view that Mars’ surface has remained extremely cold and dry — much as it is today — for the past 3.9 billion years. It confirms the idea that internal heat periodically triggers short-term warmer and wetter conditions — conditions conducive to life — in the global martian hydrological cycle, University of Arizona Regents’ Professor Victor R. Baker says in a review article, “Water and the martian landscape,” published in Nature July 12. Baker is head of the UA department of hydrology and water resources.