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Cobblestones on Mars?

Researchers analyzing the latest close-up images of rocks from the Mars Pathfinder rover announced this week that they provide new evidence that the red planet had a warm, wet--and possibly life-supporting--climate early in its history. The images show pebbles that could have been rounded by tumbling in flowing water, along with what appears to be a conglomerate, a rock made of sand and water-worn pebbles.

Mars today is so dry, cold, and nearly airless that liquid water cannot exist on its surface. But images of the planet's oldest terrains taken 20 years ago by the orbiting Viking spacecraft traced networks of valleys, presumably cut by running water 4 billion years ago. Geologists have debated whether the water fell as rain or simply oozed out of ground warmed by internal heat, which would imply a more severe climate.

The new images imply rainfall and river flow by showing what look like stream-rounded "cobbles" several centimeters across on the Martian surface, along with similar-looking lumps protruding from other rocks. One rock has lumps, as well as pockets where pebbles may have popped out. Pebbles in a matrix are the hallmarks of conglomerates, points out Pathfinder team member Henry Moore of the U.S. Geological Survey in Menlo Park, California. And conglomerates form when loose pebbles rolling along in a stream or flood are cemented together with smaller debris and sand.

Moore notes that other processes not involving water could have made the rounded cobbles. And some planetary scientists aren't convinced yet that the lumpy rocks really are conglomerates. But if they are, says Pathfinder project scientist Matthew Golombek of the Jet Propulsion Laboratory in Pasadena, California, the new images imply "long eras of liquid water flowing by." He adds: "That is one requirement for life."