The climate on Mars is showing a warming trend and recent images have shown the first evidence of seismic activity on Earth's neighbour planet, scientists said on Tuesday (local time).

New gullies that did not exist three years ago have been pictured on a Mars sand dune - just another of what scientists say are surprising discoveries found by cameras aboard the 8-year-old Mars Global Surveyor that are changing notions about the climate and formation of Mars.

"To see new gullies and other changes in Mars surface features on a time span of a few years presents us with a more active, dynamic planet than many suspected," said Michael Meyer, NASA's Mars Exploration Program chief scientist.

Images taken by the Mars Orbiter Camera on board the Surveyor showed that boulders have fallen down a Martian slope in the past two years.

Michael Malin, principal investigator for the Mars Orbiter Camera, told reporters it was the first evidence scientists had seen of some kind of seismic activity, or possible "marsquake," on the planet.

If so, "it could speak to the planet having warmth in the interior ... which means the interior could be more active than previously thought and there could be a habitable environment in the deeper regions of Mars," said Jack Mustard, geological sciences professor at Brown University in Providence, Rhode Island.

Malin said images of Mars' southern polar cap showed that scarps formed there are retreating at "a prodigious rate" of about 10 feet per Mars year. Mars years are nearly twice as long as Earth years.

The images, documenting changes from 1999 to 2005, suggest the climate on Mars is presently warmer, and perhaps getting warmer still, than it was several decades or centuries ago just as the Earth experienced its own Ice Ages.

Malin said scientists had no explanation yet as to why Mars might be warming.

The Mars Global Surveyor reached orbit in September 1997 for an initial one-martian-year mission. It was subsequently extended and is currently funded through 2006 although Meyer said it was technically capable of extending its mission even further.

In November 2006 the Mars Global Surveyor will be joined by the Mars Reconnaissance Orbiter, which was launched in August on a four-year mission to continue the search for evidence of how long Mars had water, which is the key to sustaining life.