The right front wheel of Spirit is continuing to draw too much electric current because of some internal resistance, so mission operators have turned off the wheel motor and are driving on five wheels instead of six. With a lame front wheel, they've found it's more effective to navigate by driving backward.

"Driving may take us a little longer than it used to; it's kind of like driving with an anchor on one side," says Joe Melko, a rover engineer at NASA's Jet Propulsion Laboratory (JPL).

Spirit is not on its last legs, even though it has traveled six times its designed capacity. Mission planners believe Spirit will be able to navigate a long, roundabout route up the foothills. The sixth wheel will still be used on difficult terrain, such as a steep slope or an area filled with jagged rocks.

As Spirit drove along the base of the Columbia Hills on its way to a less steep path leading upward, it ran over an interesting outcrop of layered rock.

"The good news is we don't need to go very much farther because, 'eureka,' we have found it," says Matt Golombek, a rover science team member from JPL. "For the first time in Spirit history, we have outcrop underneath rover wheels."

Golombek says the outcrop could help explain how the Columbia Hills formed. Because the hills are believed to be older than the lower plains, the outcrop could provide clues about the early history of Mars.

The rover continued past the outcrop in order to be in position to acquire sunlight for its solar panels, but it will soon take a look at the outcrop in detail. Spirit will then climb backwards up the Columbia Hills, following the less steep, north-facing slope. This type of slope will provide the rover with greater sunlight, giving Spirit more energy to operate during the martian winter.

On the other side of the planet, the Opportunity rover is continuing its slow descent into the large Endurance Crater. Data from Opportunity's alpha particle X-ray spectrometer show that there is more chlorine the deeper you go into the crater. Jutta Zipfel, a rover science-team member from the Max Planck Institute for Chemistry in Mainz, Germany, says that the Endurance Crater rocks have the highest concentrations of chlorine than any other rocks on Mars so far.

Chlorine could be an indication of sodium chloride, also known as table salt. Or it could be part of a more complex chlorine compound. Zipfel says they do not know which element is bound to the chlorine. (Chlorine is never found free in nature because it binds with nearly every element).

On Earth, when water evaporates, minerals like sulfate, chlorides, and bromides condense out. Zipfel says it's not unexpected to find a chlorine-rich layer in evaporite sediments on Mars, but they do not know why the chlorine increases the further you go down into the crater.

"One possibility is that the residual fluids or brines that led to the formation of evaporite rocks changed their composition over time," she says.

Jack Farmer, a rover science-team member from Arizona State University, Phoenix, says that pyroxene also increases further down into the crater. Pyroxene is a dark, iron-rich indicator mineral of basalt. Basalt, a volcanic rock, is the most common type of rock found on Mars.

Opportunity also has spotted ridged areas on rocks that may be the result of minerals transported by water. These sharp, thin features have been named "razorback," and Opportunity will attempt to study them in detail to determine their origin. On Earth, such features form when water migrates through cracks in rocks and deposits minerals. Then, if the deposited mineral is denser or harder than the surrounding rock, it resists wind erosion that will wear the rest of the rock away. Finally, only the mineral is left, forming narrow ridges similar to the ones seen on Mars.

If the martian ridges formed this way, they would reflect a younger history than the crater itself. The cracks in the rocks would have been generated by the powerful asteroid impact that formed the crater.

"The outcrop rocks are sitting there, and they get completely fractured by this impact," says Farmer. "Those fractures become conduits for fluids that can migrate through later. How much later is uncertain - it could have been shortly after impact, or it could have been a long time after the impact."

Farmer says that other than water-deposited minerals, the ridges could have formed from sediment that somehow got cemented into the cracks.

At the bottom of Endurance crater are rippled dunes, and mission scientists hope Opportunity will be able to drive down low enough to reach the outer margins of these dunes.

The martian winter peaks on September 20. The rovers will undergo extended periods of rest and "deep sleep" while they recharge their batteries, allowing them to keep working despite the reduced sunlight of winter.

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