One rock, called "Pot of Gold" is encrusted with the blueberry nodules. The nodules appear to rest on layered bedding planes, and look well organized. "It's difficult to imagine that this is a random conglomerate of debris," says science team member Larry Soderblom of the US Geological Survey.

Other rocks on Columbia Hills resemble hooded cobras. The scientists have named these 3- centimeter high rocks "Cobra Heads," and think they may be the remnants of hard, encrusted outer material whose softer middles have rotted away. The Cobra Heads and Pot of Gold are part of a jumble of rocks that have cascaded down from a higher rock outcrop. Spirit will attempt to reach this outcrop, provided the rover can navigate the steep slope of the hill.

On the other side of the planet, the rover Opportunity is already five meters down into the large Endurance crater.

"Our early look at the geology in front of us suggests that we will get more insight into the history of water on Mars," says science team member Scott McLennan of the State University of New York in Stony Brook.

Opportunity is currently analyzing an evaporite rock called Tennessee, and scientists believe this will be one of the last measurements of this type of rock as the rover creeps further down into the crater. One future target for Opportunity is a "contact" rock - a type of sedimentary rock that often indicates environmental change.

There appear to be three different sub-units of rocks past the contact rock, even deeper down into the crater. The scientists want to study these rocks but aren't sure if they can descend past the contact rock, since the slope of the crater walls will then approach 30 degrees, which is right at the margin of the rover's ability.

Although the rover missions were planned for three months, they are now in their fifth month of operation. The scientists will continue to operate the rovers as long as possible.

Spirit has experienced some recent difficulties in communication, due to increasingly cold temperatures. The cold affected Spirit's receiver, altering the range of its sweep as it tried to lock on to the radio frequency sent from Earth.

"Spirit's gotten a little hard of hearing, though that's due to temperature, not due to age," says Mark Adler, mission manager at the Jet Propulsion Laboratory. "She's also experienced a little bit of arthritis in one of her joints, and that is due to age."

Spirit's "arthritis" is in the motor for the right front wheel. The motor is taking two to three times the electrical current that the other wheels are taking to do the same job, and that current draw is steadily increasing.

There appears to be some sort of resistance in the gearbox, and the scientists are trying various methods to solve the problem. They may have to drive the rover with only five wheels, turning on the sixth wheel as needed.

The rovers will have to contend with decreased sunlight in the coming weeks, and therefore will not be able to generate as much power from their solar panels.

"We're headed to a confluence of bad things for the rovers around mid-September," says Adler. The sun on Mars is currently in the northern hemisphere, and it will reach its northern-most point in late August.

Mars, with its elliptical orbit, will then be as far away from the sun as its going to go, and the distance between Earth and Mars will be at its greatest at the same time.

The scientist are planning to cut back or suspend communications for a week or two during this time, and get the rovers in position to increase their exposure to sunlight. Because Opportunity is currently on a slope, it is getting more sun now than before.

But Spirit is getting less sun, because Spirit is farther south than Opportunity. Scientists plan to maneuver Spirit onto a slope and point its solar panels north to increase its chance of survival.

Related Links
Mars Rovers at JPL
Mars Rovers at Cornell
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