By Paul Rincon
BBC News science reporter, Cambridge
Fields of volcanic cones discovered at the North Pole of Mars suggest the Red Planet could still be geologically active, scientists have said.
Mars Express looks across the surface with a cone in the foreground and top-left
The cones, seen in images from Europe's Mars Express probe, have no blemishes from impact craters.
This suggests the volcanoes erupted very recently and that the sites could have ongoing volcanism.
Mars Express scientist Gerhard Neukum presented the results at a conference in Cambridge.
"Mars is a planet that was very recently active - maybe one, or two, or three million years ago. And in some areas, I have the impression it is really ongoing," said Dr Neukum, of the Free University in Berlin, Germany.
But what cannot be determined is when, if at all, some of these volcanoes might erupt again: "It could be a million years from now, it could be tomorrow," he added.
Dr Neukum acts as the principal investigator for the High Resolution Stereo Camera (HRSC) on Mars Express, which took the images in which the cones were discovered.
There may be 50-100 of the volcanic cones covering a flank of the North Pole about one million square kilometres in area. They are between 300m (980ft) and 600m (1,970ft) tall, said Dr Neukum.
In addition to the North Pole, other regions with recent - and possibly ongoing - activity on Mars include parts of Tharsis - home to the volcano Olympus Mons - parts of Elysium and the so-called highland-lowland boundary.
By counting the number of craters on the surfaces of Solar System objects, scientists can estimate the age of those surfaces.
If they are heavily cratered, they 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 cones appear to be fresh with no discernible evidence of cratering. Dr Neukum admitted it was possible the cones could be ancient features that have been eroded by wind, but added that this was unlikely.
"I don't see any wind-related features in the region. We should see it and we should see the remains of craters somewhere. But we don't," he told the BBC News website.
Volcanic activity appeared to have peaked on Mars at around 1.5 billion years ago, Dr Neukum said, adding: "Mars is still active within certain limits; it's still not dead."
Dr Neukum thinks that volcanic activity strongly influences glacial activity on Mars. This is because on the Red Planet, eruptions also mobilise water.
In some cases, this water freezes and forms glaciers, says Dr Neukum. But other scientists believe glacial activity on the planet is more strongly influenced by the inclination of Mars in its orbit around the Sun.
The Mars Express results were presented at the American Astronomical Society Division of Planetary Sciences meeting in Cambridge, UK.