"There is a lot more salt down there than we originally thought. That means that there was a lot more water involved in doing this then we originally thought," principal science investigator Steven Squyres told a news conference at NASA's Jet Propulsion Laboratory in Pasadena, California.

"I don't think we've got compelling evidence for what I would call an ocean in the sense of it being a very, very deep body of water. Everything we see is compatible with very shallow water, wetting and drying, a small amount of water to wade around in and then maybe it evaporates away," he said.

Opportunity and its twin rover Spirit, in the fifth month of a scheduled three-month Mars mission, have already exceeded NASA hopes. In March, Opportunity uncovered geologic evidence near its landing site on the barren terrain that the area was once "drenched" in water.

JPL scientists controlling the robots have since sent Opportunity down another crater and are trying to find how far down the traces of sulfate extend.

They then hope to use geochemical calculations to estimate the size of the body of water that was once there.

Squyres stressed that until they had the calculations mission members were working on the hypothesis that materials deeper in the crater "may have been stirred, mixed or blown around by wind" and that the water went through periods of wetting and drying.

On the other side of the planet the rover Spirit has traveled 3.3 km (2 miles) from its landing site and is exploring a knobby rock about the size of a softball that mission scientists have dubbed "Pot of Gold."

The rock contains the mineral hematite, which can signify past water or volcano activity, but puzzled scientists are trying to figure out how it got its strange shape.

"This rock has the shape as if somebody took a potato and stuck toothpicks in it, then put jelly beans on the end of the toothpicks," Squyres said.

"How it got this crazy shape is anyone's guess. I haven't even heard a good theory yet," he said.