Seen from space, glaciers on Mt. Kilimanjaro or young craters nested within older ones in Hawaii's Kilauea volcano are signs of a restless, active planet capable of harboring life.
Now, researchers say they are finding evidence for recent volcanic and glacial activity on Mars - which could establish conditions that could support life on the Red Planet.
Recent is a relative term - in this case 2 million to 4 million years ago. But the evidence implies that some Martian volcanoes may still be active. If so, they could serve as a source of heat, nutrients, and melt water for simple microbial life.
At the least, the new data from the European Space Agency's Mars Express orbiter adds significantly to a growing body of evidence that Mars may be a much more geologically dynamic planet that previously believed.
"These new data show evidence for lots of geological activity during the last 20 percent of Mars' history," says James Head III, a Brown University planetary scientist who is a member of the team reporting the results in the latest edition of the journal Nature. That's been suspected before, but this really fills in the picture, he says.
Indeed, increasing amounts of data from recent Mars missions suggest that the planet has been geologically active in its recent past and is likely to be active today, notes Bruce Jakosky, a planetary geologist at the University of Colorado at Boulder.
"There's no reason to think that these are the last gasps or last burps" of volcanic eruptions, says Dr. Jakosy, who was not part of the group involved in the study.
The team, led by Gerhard Neukum, a geology professor at the Free University of Berlin, used the Mars Express orbiter's high-resolution stereo camera, which produces 3-D images of the surface. The camera records details as small as 32 feet across, far smaller than an Olympic swimming pool.
One image can cover a swath of ground 37 to 62 miles wide and thousands of miles long. The benefits: richer detail than any camera currently orbiting the planet, and a broad context into which specific features fit.
"It was like drinking from a fire hose," Dr. Head says of the images beamed back to Earth between January and July of this year. "But nobody was complaining."
The team derived ages for geologic features in the images by counting craters on the surface, as well as figuring out the relative sequence in which eruptions occurred or crater floors collapsed.
Calderas and vents associated with five volcanoes, including the solar system's granddaddy, Olympus Mons, yielded evidence for eruptions that occurred from 3.6 billion years ago to as recently as 2 million years ago. Much of the activity at these sites is clustered within the past several hundred million years. Lavas on the flanks of Olympus Mons and Hecates Tholus alone range in age from 100 million years to 3.8 billion years - a span of eruptions that far outstrips those of any earthly volcanoes.
One finding the team notes as an "interesting coincidence": Four of the five volcanoes studied appear to have been active between 100 million to 200 million years ago - roughly the same age as volcanically formed Martian meteorites found on Earth.
Over the past year, three independent groups of scientists have reported tantalizing, if inconclusive, evidence for tiny amounts of methane in Mars' atmosphere. Without some means of resupply, the atmosphere should have lost any methane long ago. The leading candidates for providing the gas include volcanic and hydrothermal activity, the presence of microbial life, and some nonbiological geochemical activity, notes Dr. Jakosky.
The new data certainly would be consistent with volcanic sources, and wouldn't rule out microbial life. The Mars Express team also teased from the images what they interpret as previously undetected alpine glaciers along the flanks of a 23,000-foot-high escarpment that forms one border of Olympus Mons, which lies in Mars' tropics.
Ordinarily, ice on the surface would quickly turn to vapor and vanish. But the suspected glaciers are thought to be covered with dust or an ice-dust mixture, which insulates them.
Other evidence etched into the landscape speaks of repeated episodes of flowing water, presumably from volcanic or hydrothermal activity heating ice. Glacial activity has occurred as recently as 4 million years ago. But some of the glaciers appear to be too young to date with existing remote-sensing techniques.
Glacial activity at the tropics, as well as at the poles, also speaks to the wide swings in climate Mars undergoes as it traces its elliptical orbit around the sun and as the angle of its tilt sweeps back and forth over long periods of time, Head says.
The data from Mars Express is expected to add candidates to a growing list of targets for future Mars missions. The US is launching an orbiter next year, a lander to study soil chemistry and ground ice at high latitudes in 2007, and this week selected a mobile "lab" to launch in 2009.
The lab is designed to unravel carbon chemistry on Mars, just as the twin rovers Opportunity and Spirit have been unraveling the history of water on the planet's surface.
The bottom line: "We're beginning to piece together a picture of a planet that is still active," he says.