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PASADENA — The search for evidence of water on Mars continued this week, as a picture of round pebbles on the Martian surface piqued scientists- interest. The microscopic image of soil was beamed back Wednesday by the rover Opportunity, now in its second week on the red planet.

University of Tennessee planetary geologist Hap McSween, a member of the science team at NASA-s Jet Propulsion Laboratory, calls the shapes -intriguing,- but says they are only tiny pieces in a giant, complex puzzle.

-Discovering these little shapes is a big surprise, but it-s not an `ah-ha- moment where you understand something you didn-t before,- he said. -If anything, it makes you scratch your head even more.-

The pebbles- roundness could be a result of tumbling around an ocean floor or being spewed from a volcano. They are distinctly different from anything that has been seen on the Martian surface before, and McSween says the rover-s movements over the next week will allow scientists to examine them more closely.

Opportunity was scheduled to move about 10 feet on Thursday, dig a trench, and photograph the rocks and soil underground. That plan was scrapped in favor of a move to a rock outcropping, the origin of which may relate to the hematite in the crater.

-We-ve always thought the hematite needed water in some form, but we don-t know for how long, or what the setting was,- McSween said. -Duration matters, because if water was there for only a few days, that-s not a suitable habitat for life. If water was there for centuries, that-s more interesting. In order to understand the duration of water, we have to understand the mechanism by which the hematite was made.-

These questions could take a long time to answer, he said.

-This is incremental science. You do a little bit and a little bit more and sooner or later you can put it all together. That-s the nature of this kind of field work. It-s the slow combination of small bits of information rather than the eureka moment.-

McSween and UT assistant professor Jeff Moersch are co-investigators on the Mars Rover Exploration missions and the THEMIS (Thermal Emission Imaging System) project on a spacecraft orbiting Mars. McSween served on the science team for the Mars Pathfinder mission in 1997. Four of their students are also at JPL working on the rover missions.