The sunlight is seasonal and also power-limiting as the rover's age and get covered by dust. Among the failure models for eventually retiring the rovers, an electronic glitch or dust accumulation are most likely than a mechanical breakage.

Both rovers have been coated by some dust falling out of the atmosphere during that time, with estimates of the dust thickness ranging from 1 to 10 micrometers, or between 1/100th and 1/10th the width of a single human hair.

Of the two, NASA's Mars Exploration Rover Spirit is definitely the more dust-laden. The Opportunity rover appears to be collecting less dust, perhaps because of a cleaning by wind or even "scavenging" of dust by frost that forms on the rover some nights during the martian winter.

In imagining the texture of the rocks found by the Opportunity rover, the mission team has compared them to spongy sandstone. They are pockmarked, porous, dried and cracked. The voids and holes in these spongy rocks may have arisen from repeated cycles of evaporation to harden the surfaces followed by a washing away to dissolve the more soluble interior portions.

NASA's Mars Exploration Rover Spirit is definitely the more dust-laden. As a result, Spirit has gradually experienced a decline in power as the thin layer of dust has accumulated on the solar panels, blocking some of the sunlight that is converted to electricity.

The panoramic camera team's analysis indicates that the layer of dust on Spirit's calibration target is about 70 percent thicker than that on Opportunity's.

Prior to this mission, the Meridiani plains were compared to the Rust Belt states, those in the middle north of America (Michigan, Ohio, Pennsylvania). The other comparison was to the red dirt found in Oklahoma and northern Texas--the so-called Red River region.

In addition to red dirt, the rovers have found bedrock. On earth, bedrock is common in northern New England, particularly Maine and New Hampshire, the Granite state. But the wind blows around enough dry dust on Mars to cover what might be exposed bedrock. This debris layer blankets most of the rest of the planet. Additionally, meteors have pulverized the martian surface leaving a thick crushed layer.

A portion of Mars' water vapor is moving from the north pole toward the south pole during the current northern-summer and southern-winter period. The transient increase in atmospheric water at Meridiani, just south of the equator, plus low temperatures near the surface, contribute to appearance of the clouds and frost.

Frost shows up some mornings on the rover itself. The possibility that it has a clumping effect on the accumulated dust on solar panels is under consideration as a factor in unexpected boosts of electric output from the panels.

The atmosphere of Mars contains water, but in miniscule amounts. "Even though we are currently seeing frequent clouds with Opportunity, if you squeezed all of the water out of the atmosphere, it would only be less than 100 microns deep, about the thickness of a human hair," said Mark Lemmon of Texas A&M University's College of Geosciences.

Because of the lack of water, weather on Mars has a lot to do with dust in the atmosphere. A small dust storm one month before the rovers landed spread small amounts of dust around the planet.

"Both rovers saw very dusty skies at first. It was only after the dust settled after a few months that Spirit could see the rim of the crater it was in, Gusev Crater, about 40 miles away," Lemmon said.

British scientists have speculated that the British Mars Lander, Beagle 2, crashed because the atmosphere was thinner than usual as a result of heating caused by atmospheric dust from the December storm.

"I can think of at least three things could kill us," said Cornell's principal investigator for the Mars rovers, Steve Squyres, when discussing the mission lifetime with the Astrobiology Magazine. "The first is dust build-up on the solar arrays. But the dust build-up is not that bad, especially for Opportunity, and with spring approaching both vehicles should do ok for awhile."

"The second thing is if something mechanical goes wrong," said Squyres. "The rovers have a lot of moving parts, and we've seen a few mechanical funnies on Spirit. Nothing serious, but enough to catch your attention. Stuff could just wear out."

"The third thing is, we've got a lot of single-string electronics in these vehicles," said Squyres. "There's not a lot of redundancy. Now, we have the ultimate redundancy in that there are two vehicles. But within each rover there are a lot of electrical parts that, if they just flat-out fail on us, the rover's dead. Bang! It just dies overnight and never talks to us again. That could happen."

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