Dark streaks on crater and valley walls may indicate that brackish water currently flows across the surface of Mars.
By Dr David Whitehouse
BBC News Online science editor
New images and analysis suggest the slopes around the Red Planet's largest extinct volcano, Olympus Mons, contain dark stains caused by brine flowing down hill.
The images show streaks on the surface of Mars
The discovery indicates that the substantial underground ice deposits on Mars can sometimes melt and flow across the surface.
It is bound to increase speculation that life may exist near to the surface of the planet.
The study has been done by Tahirih Motazedian, of the University of Oregon, US. It supports work done by others.
She told BBC News Online that she had examined images of Mars taken at different times and had seen new streaks form within time intervals of months.
She speculates that geothermal activity driven by volcanic heat may be causing the melting of subsurface ice.
Briny water flows downhill
The water dissolves surrounding minerals to form a super-saline brine which, because it contains salts, can remain liquid at lower temperatures and pressures than pure water can.
When the brine trickles on to the surface, it flows downhill staining the surface.
"The streaks originate from distinct geologic horizons below the Martian surface, where the water-ice table has been intersected by crater and valley walls," she said.
'Dynamic fluid flow'
Significantly, the dark streaks are never overlain or cut by other features like craters or sand dunes, just as if they were made by water marking the surface.
"They passively overlay existing features except where they are forced to flow around obstacles," she said.
The dark streaks always begin upslope as a point and widen downslope, just like flowing water.
The streaks have the same dispersive patterns that liquid water has when it flows downhill, "highly indicative of dynamic fluid flow", says Tahirih Motazedian.
Stains said to be caused by the water
Images taken of the Mangala Valles region show that the dark streaks are being formed at the present time.
Two images taken a few months apart show new streaks have appeared.
"This demonstrates the existence of a currently active, short-term process of surface change on Mars," the researcher said.