A U.S. robotic rover on Mars is finding evidence that the shallow salt water sea that once covered part of the red planet was deeper than suspected. The signs come from a crater that allows mission geologists to peer beneath the surface of Mars.

The Opportunity rover has been slowly rolling down the slope of Endurance crater and finding the telltale signs of evaporated salt water: sulfates, or salt deposits left over when the water evaporated and the ground hardened into rock.

This is the same evidence the robot discovered last March in bedrock sticking through the nearby surface. That finding caused mission scientists to conclude at the time that a shallow salt water sea had covered the Meridiani Plains region of Mars at some time in the past.

Cornell University geologist Stephen Squyres says the newest salt findings in the crater indicate the sea was deeper than the March results indicated.

"There's a lot more salt here than we originally thought," he said. "That means there was a lot more water than we originally thought. What we hope to do here is work our way down the stack [slope] and try to understand just how many meters of this stuff is here. Once we have that number, then by doing geochemical calculations, we can start to attach some numbers as to how much water was involved."

On the other side of Mars, the Spirit rover has analyzed a strangely shaped small rock and found signs that also suggest ancient water. Its instruments have found the mineral hematite, which usually forms in water, but can also form by volcanic heating. Scientists are looking for other chemical signs of water in the rock, but in the meantime are puzzled about what its unusual form means. Most peculiar of all are the rocks' short projections, like worms with bulbous heads sticking out of it.

Mr. Squyres says he has never seen anything like it on Earth.

"They're only a few millimeters in size. They are kind of dangling out there. They are not round," he said. "I don't know how these things formed and they're driving me nuts, to be perfectly honest! But we're having a lot of fun trying to figure them out."

Spirit found the weird rock at the foot of Columbia Hills, more than three kilometers from where it landed in January. Mission engineers say this is a longer drive than the six-wheeled vehicle was designed for and it is showing signs of age. The gears in its front right wheel are overheating, suggesting that its lubricant has thickened in the extremely frigid Martian temperatures and is not coating the gear teeth.

Ground controllers plan to turn the rover's heaters on and park it at an angle to allow the lubricant to flow back to the gears. At the same time, technicians are using an identical laboratory model to practice driving it with five wheels, just in case the relubrication effort fails and the sixth wheel with it.