In the other study, led by Michael C. Malin, features at the south pole were observed to retreat by up to 10 feet (3 meters) from one Martian year to the next.

The odd shapes -- circular pits, ridges and mounds -- were first photographed in 1999. Since then, the features have eroded away by up to 50 percent.

The pits are growing, the ridges between them shrinking.

Caplinger and Malin caution that a year's worth of data does not reveal when this erosion began or how long it will continue. Yet they speculate that the features could have been created in a Mars' decade and may erode away completely within one to two decades.

"We know that the pits we see at the surface today are not very old, and that they will not last very long," Malin said.

The rate of erosion suggests the features are made of moderately dense but solid carbon dioxide, rather than water ice, the scientists conclude. But that does not preclude the possibility of water ice at the south pole.

He said the only way to find out is to go there and drill down.

Caplinger said no one knows for sure what effect the extra carbon dioxide might have on the climate. "Not much," he figures.

But he said many scientists assume that Mars undergoes climate change. Photos of the surface suggest water may once have flowed on Mars, implying that it would have been warmer. And Earth's ice ages offer the lesson that change is inherent in a climate.

Despite more than three decades of Red Planet exploration, scientists are still relatively clueless about the climate of Mars, said Paige, the UCLA researcher. Continuous or recurring observations have typically been confined to short time periods.

The two new studies herald a change, Paige said. And expect more.

"We're moving toward a situation where we'll have a permanent spacecraft presence on Mars," Paige said.