NASA's Mars Global Surveyor has snapped photos that suggest horizontal layers extend deep into Mars' canyons, a sign that volcanic activity played a pivotal role in the early geology of the planet, according to a report in the Feb. 18 issue of Nature. One of the researchers, Alfred S. McEwen of the University of Arizona in Tucson, said volcanic activity may have lasted for the planet's first billion years of existence. This theory could explain Mars' warm and wet early climate. The Mars orbital camera captured evidence of layered deposits that span the entire length of one of Mars' most well-known canyons, Valles Marineris. The walls consist of bright and dark banding, layers that follow the topographic contour lines of the canyon system.