Mars appears to have been volcanically active more recently than previously supposed, according to growing evidence from Europe's Mars Express orbiter.
By Paul Rincon
BBC News Online science staff
New estimates suggest volcanoes could have been active between one million years ago and 20 million years ago, but more work is needed to refine the dates
Previous spacecraft data suggested that volcanism on Mars ceased some time around 600-500 million years ago.
Some researchers even speculate Mars could be volcanically active today.
Mars Express project scientist Agustin Chicarro said some volcanoes were "extremely young" based on current data.
"For volcanic phenomena, we may be talking about a few million years, meaning in the order of one to 20 million years. But this depends very much on the data we have and in a few months we may have much better data," he told BBC News Online.
The ages were estimated by counting the number of impact craters associated with volcanoes in images from the High-Resolution Stereo Camera (HRSC) aboard Mars Express.
This is a method widely used by planetary scientists to estimate the ages of planetary surfaces. However, these estimates are relative, Dr Chicarro cautioned, and involve a certain amount of guesswork.
More cratered surfaces are deemed older, while smoother surfaces are considered younger. This assumes a constant cratering rate since the heavy bombardment that terrestrial planets underwent about four billion years ago.
The three volcanoes known as Tharsis Montes are each about 15km tall
But there are complications on Mars, according to Dr John Murray of the Open University, a co-investigator on the HRSC.
"You can get dust storms and ice near the poles, so you find that the smaller craters are gone and the larger ones are still there," he said.
Those volcanoes with the most recent activity appear to be the massive Olympus Mons and three other giant volcanoes in Mars' Tharsis region: Arsia Mons, Pavonis Mons and Ascraeus Mons (collectively known as the Tharsis Montes).
Dr Chicarro is open-minded on the question of present-day volcanism on Mars.
"It would have to be quite coincidental that just in our terrestrial space age, volcanism on another planet would cease," he observed.
"If [volcanism] does exist, it doesn't mean we have to have major volcanic flows. If, let's say, the last big volcanic explosion happened a few million years ago, it could happen again - or not.
"There may also be much smaller domes elsewhere that are still active. But at the moment, things like hydrothermal activity are perhaps a bit more likely."
Hydrothermal, or hot water, systems similar to terrestrial hot springs may once have existed on Mars.
The existence of hydrothermal activity on the Red Planet today might make the existence of Martian life that little bit more plausible, Dr Chicarro suggested.
On our own planet, hydrothermal systems are havens for thermophilic (heat-loving) microbes and may have given life a foothold on the early Earth.
Nick Hoffman of the University of Melbourne, Australia, has suggested that ice towers might form next to steaming hydrothermal vents on Mars. Microbial life forms live on chemical energy in similar ice towers in Antarctica.
Previous estimates of the last volcanic activity on Mars were largely based on data from Nasa's Viking missions in the 1970s.