NASA's Mars rover Spirit has found concentrated salt below the surface of the Red Planet, offering more evidence of past water activity, mission scientists said on 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, the mission's main scientist, said the findings offer "much more compelling evidence than we have found anywhere else" in the vast Gusev Crater region, which the rover has explored since landing on 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 that conditions would have been suitable for life.

Evidence found at that location included a large amount of crystallized 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 for entry -- scheduled for Wednesday -- into a deep crater that could offer clues to the planet's water history, despite the risk that the craft may not be able to get out. Scientists have said the potential scientific value of exploring Endurance Crater outweighs the risk that the rover may not be able to drive out.