Further, volcanism that lasted for at least the first billion years of Mars' geologic history might have continually resupplied the martian atmosphere with carbon dioxide that sustained its warm and wet early climate, according to Alfred S. McEwen of The University of Arizona in Tucson and co-authors of the article.

McEwen is director of the Planetary Image Research Lab at the UA Lunar and Planetary Laboratory. Co-authors are Michael C. Malin of Malin Space Science Systems in San Diego, Michael H. Carr of the U.S. Geological Survey and William S. Hartmann of the Planetary Science Institute in Tucson. Theirs is one of four articles in the Feb. 18 Nature analyzing close-up images of Mars taken during the aerobraking phase of the Mars Global Surveyor mission.

The new images from the Mars Orbital Camera show horizontal layers at least as keep as 8 kilometers (5 miles) in the canyons spanning the enire 4,000-kilometer (2,480-mile) Valles Marineris canyon system, McEwen and colleagues report. Earlier studies predicted the photographs would show large chunks of breccia and bedrock fractured by impacts during the heavy bombardment phase of solar system formation, between 3.5 billion and 4.3 billion years ago, and that these coarse layers would be laced with thin lava layers at the planet's uppermost crust.

Contrary to the expected impact-generated disorder, the Mars Orbital Camera captured evidence of layered deposits spanning at least the entire length of Valles Marineris. The chasm walls consist of dark and bright banding that represent layers 5 to 50 meters (16 to 164 feet) thick. The light bands appear to be dust-covered shallower slopes while the dark bands are steeper cliffs. The layers generally follow the topographic contour lines of the great canyon system. That these layers remain intact shows they were deposited after the end of heavy bombardment.

The scientists say that while they do not rule out the possibility that the layers are largely sedimentary rock, the structure and spectral properties of the layers show the strata more likely are the deposits of lava floods.

The scientists estimate that 10 times more lava than previously believed erupted over Mars during 4 billion years of its geologic history. That much lava would cover an area the size of the United States to a depth of 6.4 kilometers (4 miles), McEwen said.

"We conclude that volcanism on early Mars has probably been much more extensive that previously documented, and it must have affected the climate and near-surface environment," the team wrote.

Scientists have theorized that a thick carbon-dioxide atmosphere sustained a warm, wet climate on early Mars. But carbon dioxide would have been lost from the atmosphere as carbonate minerals formed in the presence of liquid water. Some mechanism must have continuously resupplied the air with carbon dioxide. Scientists have proposed two mechanisms for recycling gas into the atmosphere: impacts and volcanism.

"Extensive volcanism on Mars could have maintained a thick atmosphere for a significant period of time after the heavy bombardment," McEwen and his colleagues conclude. "The layers seen by Mars Orbital Camera provide evidence for voluminous volcanism: but a thick atmosphere could have been sustained only if sufficient carbonates exist in the crust of Mars, which has not yet been confirmed."

In another article in the Feb. 18 Nature, Hartmann, Malin, McEwen and others discuss how they used high-resolution images to extend the census of martian craters down to 16 meters (52 feet) across. Planetary scientists use impact craters to map the age of different regions on a planetary surface. Their results indicate a wide range of surface ages. One region -- lava flows within the caldera Arsia Mons, one of the great shield volcanoes of the Tharsis plateau -- is possibly no older than 40 million to 100 million years, they found. This suggests that volcanism is a continuing process on Mars, the scientists said.