By Paul Rincon
Science reporter, BBC News, The Woodlands, Texas
The structures could be the best place to search for life
Scientists say the possible discovery of mud volcanoes on Mars could boost the search for the planet's past life.
If life ever existed on Mars, the evidence could be buried deep below the surface, where it may be warm enough for water to remain in a liquid state.
Mud volcanoes could transport rocks from depths of several kilometres up to the surface, where robotic explorers could reach them.
Details were presented at the Lunar and Planetary Science Conference in Texas.
Using images taken by Nasa's Mars Odyssey spacecraft, Carlton Allen and Dorothy Oehler of Nasa's Johnson Space Center in Houston identified dozens of mounds in the northern plains of Mars which they say bear a striking resemblance to mud volcanoes.
Infrared data also show the domes cool more quickly at night than the surrounding rock, as one might expect if they were made of sediment.
Together with David Baker from Brown University, the researchers used instruments on Nasa's Mars Reconnaissance Orbiter (MRO) to observe several of these structures in a northern region known as Acidalia Planitia.
Data from the MRO's Crism experiment indicate that the material in the domes is more oxidised than the rock of the surrounding plains. This might suggest the presence of iron oxides, which form in the presence of liquid water.
They also took pictures of the structures with the HiRise camera on MRO; the images show the bright domes standing out against the dark basaltic rock of the surrounding plains.
Dr Allen told BBC News the structures resembled smooth cones with "no breaks", which visibly feathered out towards the margins. The observations, he said, were consistent with material that is "smooth, soft and easily eroded".
Mud volcanoes are relatively common on Earth
On Earth, the largest concentration of mud volcanoes is in Azerbaijan and the adjacent Caspian Sea. But they have been found at more than 40 sites on land and at more than 20 locations beneath the sea.
They are formed when pressurised gas and liquid from as much as several kilometres down, breach the surface. They belch out slurries of fluid, mud and rocks, as well as gases such as methane.
"In Azerbaijan, there is so much methane coming out that they can catch fire," said Dr Allen.
This raises the possibility that mud volcanoes could contribute to the methane observed in the Martian atmosphere.
Methane should last for only a short time in the atmosphere until it is destroyed by sunlight, so its continued presence means it is being replenished by some unknown process.
Dr Allen said the area with the mud volcanoes has not been well surveyed for atmospheric methane.
He said the team had found no evidence that the domes could be active today, as most show clear evidence of erosion. But he suggested they could have formed in the last 10 million years.
Other researchers caution that other processes such as the retreat of glaciers can form similar mound structures.
But they suggest that, if life does exist deep beneath the Martian surface, mud volcanoes could be one of the best ways to get at the evidence.