One of the U.S. robot rovers on Mars is heading for the hills to find out how the planet's mountains are made. The other six-wheeled machine is sensing conditions around a deep crater as ground controllers debate whether to send it in on a drive from which it might not be able to return.

The rover Opportunity's examination of exposed rock layers in the plains of Meridiani led mission geologists to conclude earlier this year that the region was once covered with salt water. The purpose of the twin missions is to determine if Mars could have ever sustained life.

Now, the other rover, Spirit, has seen similar rock layers on the other side of Mars in the 130 meter wide Endurance Crater. Mission manager Matt Wallace says his team is determining whether to enter the crater and is practicing with a test rover to see how well it moves over different rock surfaces on slopes.

"If we go in, there is a possibility, independent of how much testing we do, that we might not come out. So the risk-benefit equation is still being worked. We're spending a lot of time talking about it. Hopefully that will converge and we'll end up making a decision," he said.

Meanwhile, Opportunity is rolling slowly toward the low Columbia Hills and is expected to cover the remaining 400 meter distance in a week to 10 days. Arizona State University scientist James Rice says examining the area, which is littered with boulders, will help determine how the hills formed.

He says they could have been pushed up by a meteor impact or by collisions between moving rocky plates forming the red planet's surface, as is the usual case on Earth. They also could be the edge of a crater whose other sides have eroded.

Surrounding the ridges is evidence that some fluid once flowed down, either lava or water. Mr. Rice says this will be the first close-up probe of martian hills.

"The Columbia Hills are swimming in these plains we have been driving across for the past five months. It's basically a brand new mission starting right now. So, come along with us. We're fixing to go on another great voyage of exploration here and see what we can find," he said.

As winter approaches in Mars' southern hemisphere, dust accumulation on the two rovers' solar panels is reducing the amount of power they generate. This is more serious for Opportunity because its heater is malfunctioning. If temperatures drop too low at night, an instrument called Mini-Tess that helps determine rock composition could fail. University of Nevada researcher Wendy Calvin says that would be a serious loss, but not tragic, since the robots are about two months past their original three month mission and are gathering bonus data.

"I would hate to see us lose the instrument. At the same time, we have a tremendous wealth of Mini-Tess data already to analyze and work with, so I think it's just one of those risks you have to accept," he said.