Book excerpt: In January of 2004, twin robotic explorers, Spirit and Opportunity, landed on Mars. Expected to last for 90 days, the two rovers explored the Martian surface for nearly two years. Their objective was to search for evidence of ancient water on Mars, and to determine if Mars ever had conditions that would have been suitable for life. Opportunity, landed on Meridiani Planum, a smooth plateau near the Martian equator, coming to rest in a small impact feature named Eagle Crater. After leaving Eagle Crater, Opportunity drove eastward to Endurance Crater, a much larger carter that allowed access to deeper and older rocks. At these locations, Opportunity, found strong evidence for ancient water on Mars. This evidence includes "blueberries,” small concretions rich in hematite that precipitated from water. Other evidence includes rocks that ware made largely of sulphate salts, deposited when water evaporated, and rocks that preserve ancient ripples that formed billions of years ago as water flowed over sand on Mars. The conditions long ago at Meridiani Planum could have been suitable for some simple forms of life. Whether life could have developed there, however, is a more difficult question.