For years, scientists thought that Mars was a cold, dry planet. But pictures sent back by the Mars Global Surveyor over the past three years have rocked that long-held view. Now, many scientists think that the planet may in fact have once been a warm, wet world-what University of Arizona geologist Victor Baker calls a Blue Mars. The Surveyor images show fissures, gullies, valley networks, and layered rocks that resemble sedimentary deposits-features normally formed by water. But a few dissenters are offering another explanation for the eroded landscape: Carbon dioxide, not water, shaped the Martian surface, they say. According to this view, dubbed White Mars by University of Melbourne geologist Nick Hoffman, Mars was always a frozen wasteland.