In January 2004, two NASA rovers, Spirit and Opportunity, landed on Mars and began to explore the planet's surface. Their mission: To probe the exotic Martian geology for signs of water past or present. NASA scientists explain the challenges of this historic Mars mission and how evidence of water could bolster the chances that life once existed on Mars. SCIENCE BULLETINS are a production of the American Museum of Natural History… http://sciencebulletins.amnh.org/