Is Mars volcanically dead - or could one of the Tharsis or Elysium volcanoes surprise us yet? Recent evidence from Mars Global Surveyor (MGS) data suggests that perhaps they could.

William Hartmann from the Planetary Science Institute in Tucson, Arizona, Alfred McEwen from the University of Arizona and their team of US collaborators have used images from the MGS camera to estimate that lava flows south of Elysium Mons are a mere 20 million years old. "Some individual flow units could be as young as 10 million years or less", says Hartmann. "In geological terms, that's so recent that volcanic activity might start up again at any time somewhere on the red planet."

They came to this conclusion after counting the number of impact craters of various sizes on the lava flows. The older a surface, the more impact craters it will have because it has been around longer to accumulate them (see The ages of Mars). This dating method, however, is uncertain by a factor of up to four and the team's results have yet to gain universal acceptance.

"If I tell you I have an area with an age of 20 million years and I'm out by a factor of four, then the true age could be as much as 80 million years. But that's still within the last few per cent of Martian history," points out Hartmann. "This means volcanism did not shut off in the first 2.5 billion years, as some early scientific papers proposed. Rather, the planet apparently continues to have at least some localised, sporadic activity."

The team have also found evidence for flows less than 100 million years old on the slopes of Olympus Mons and Arsia Mons, one of the Tharsis volcanoes. But the accuracy of these dates will probably not be known for sure until a lander capable of providing absolute ages of rocks in situ, like Mars Express's Beagle 2, or a sample return mission, heads for one of the areas in question.