NASA's Mars (search) rover Spirit has found concentrated salt below the surface of the Red Planet, offering more evidence of past water activity, mission scientists said Tuesday.

The wheeled robot found the salt while analyzing the composition of a trench it had dug in a large crater. Scientists believe the salt may have been deposited after water drained through the soil, dissolving materials in rocks.

Cornell University astronomer Steve Squyres (search), the mission's main scientist, said the findings offered "much more compelling evidence than we have found anywhere else" in the vast Gusev Crater (search) region, which the rover has explored since landing Jan. 3.

NASA announced in March that Spirit's twin rover, Opportunity, had determined that rocks at a site on the other side of the planet were once soaked with liquid water and conditions would have been suitable for life.

Evidence at that location included a large amount of crystalized salt inside rock, indicating it was dissolved in water and then left behind when the water evaporated.

The findings were announced as Opportunity was being readied to enter a deep crater that could offer clues to the planet's history of water, despite the risk that the craft may not be able to get out.

Mission scientists have said the potential scientific value of exploring Endurance Crater outweighs the risk that the rover may not be able to drive back up its steep inner slope.