New images of Mars suggest the Red Planet's surface is more active than previously thought, the US space agency (Nasa) reports.
Gullies have appeared in new images taken this year
Photographs from Nasa's orbiting spacecraft Mars Global Surveyor show recently formed craters and gullies.
The agency's scientists also say that deposits of frozen carbon dioxide near the planet's south pole have shrunk for three summers in a row.
They say this is evidence to suggest climate change is in progress.
The new gullies appear in an April 2005 image of a sand-dune slope. A previous shot from July 2002 had no trace of them.
The team operating the Mars Orbiter Camera on MGS has found many sites on the Red Planet with fresh-looking gullies, and checked back at more than 100 gullied sites for possible changes between imaging dates, but this is the first such find.
Such gullies might have formed when frozen carbon dioxide, trapped by windblown sand during winter, vaporised rapidly in spring, releasing gas that made the sand flow as a gully-carving fluid, the team speculate.
"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 Nasa's Michael Meyer, Nasa's Mars Exploration Program chief scientist.
The newly released images also show boulder tracks at another site, which were not there two years ago.
Michael Malin, the principal investigator on the Mars Orbiter Camera, said it was possible strong winds or even some kind of seismic activity had caused them to roll to their new positions.
But some changes may be happening slower than expected, the scientists report.
Studies suggested new impact craters might appear at only about one-fifth the pace assumed previously, Dr Malin said. This had important implications, he added, because crater counts were used to estimate the ages of Martian surfaces.
The Mars Global Surveyor has been orbiting the planet since 1997; Nasa expects it to carry on doing so for several years to come.
"Our prime mission ended in early 2001, but many of the most important findings have come since then, and even bigger ones might lie ahead," said Tom Thorpe, project manager for Mars Global Surveyor.