Volcanoes belched up to 10 times more lava onto the surface of ancient Mars than previously thought, and the planet may still have some fire left in its belly, scientists say.
In one of two studies published in Thursday's issue of the journal Nature, researchers reported evidence that more than 3.5 billion years ago, Mars' volcanoes filled one canyon alone with enough molten rock to bury the entire United States beneath a fiery lake four miles deep.
The evidence included more than 100 layers of what the researchers believe is volcanic rock jutting from the walls of the 2,400-mile-long Valles Marineris canyon.
"It looks like thick stacks of lava and it goes down as far as we can see," about six miles, said Alfred S. McEwen, a research scientist at the University of Arizona's Lunar and Planetary Laboratory in Tucson.
He and colleagues studied photographs obtained by the Mars Global Surveyor, a spacecraft that has been orbiting Mars since 1997 and begins a two-year mission next month to map the entire planet in fine detail.
Their research supports the growing view that Martian volcanoes were more important than meteor collisions in shaping the planet's surface.
Volcanic activity would also help explain why Mars apparently had a thick, warm atmosphere early in its history. Volcanoes belch carbon dioxide, a greenhouse gas that causes the atmosphere to heat up.
In the same journal, scientists from the Planetary Science Institute of Tucson reported that based on images radioed back by the Mars Global Surveyor, Mars' volcanoes -- one of which looms 14 miles above its dusty, windswept surface -- may not be finished erupting.
They examined photos of a lava flow in a volcanic crater and estimated it formed 40 million to 100 million years ago. The flow was marked with relatively few meteor craters, in contrast with older flows that were heavily pockmarked. That suggests the flow is relatively young.
"We're not saying that there's going to be an eruption in 1999, but there may still be dormant areas on Mars with the potential for future eruptions," said William K. Hartmann, a scientist at the Planetary Science Institute.
He said the research team is analyzing photos that suggest even younger flows.
Steven Squyres, a professor of astronomy at Cornell University helping plan NASA's 2001 rover mission to Mars, said he wouldn't be surprised if Mars were still volcanic. He noted that the Global Surveyor will produce a thermal map showing any surface hot spots.