Many people venture into the desert for its starkness and simplicity, but I go there for its complexity. The rocks of western Arizona, where I work, reveal one of the most tangled histories on Earth. Layers of carbonate limestones, silty mudstones, quartz sand and solidified lava show that within the past 600 million years, this area was a warm, shallow sea, then a muddy swamp, then a vast desert of shimmering hot dunes, then a glacial ice sheet, then a shallow sea once again. Erupting volcanoes formed islands like Japan, which in turn got shoved 100 miles onto the continent along massive faults, tilting the rock layers on edge and cooking them to create marble and quartzite. Uplift and erosion at last produced the desert landscape we see today.

This kind of detailed historical reconstruction has long been impossible for Mars. Within my lifetime, the Red Planet has been transformed from a point in the night sky into a land of towering volcanoes, dried-up riverbeds, ancient lakes and windswept lava plains. Clearly, Mars has one of the most glorious histories in the solar system. Yet scientists have been able to piece together only the sketchiest outlines of that history. For years, we have debated such sweeping questions as whether Mars was once "warm and wet" and Earth-like or "cold and dry" and barren like the moon, as though the story of an entire world could be reduced to a sound bite.