Streams of salty water could be coursing their way down the steep sides of craters on Mars, scientists believe after identifying a series of dark lines in photographs.
Hopes the streaks may indicate flowing water were raised because they appear in the Martian spring and disappear in winter.
"The best explanation we have for these observations so far is flow of briny water, although this study does not prove that," said Dr Alfred McEwen, of the University of Arizona as the findings were published in the journal Science.
If it is water, it could be flowing just beneath the surface and is likely to be salty because salt lowers the temperature at which water freezes, making it just warm enough to be liquid during the spring.
The lines are between around half a metre to five metres wide but extend for hundreds of metres.
In some of the sites scientists identified more than 1,000 individual flows reaching down steep slopes such as the rims of impact craters.
University of Arizona student Lujendra Ojha, who was the first to spot the features after identifying subtle seasonal changes in the orbiter images, said: "I was baffled when I first saw those features... we soon realised they were different from slope streaks that had been observed before. These were highly seasonal, and we observed some of them had grown by more than 200 metres in a matter of just two Earth months."
The lines appear to lengthen and darken on rocky equator-facing slopes from late spring to early autumn. They therefore seem to favour warm areas and times, suggesting that they consist of volatile material. However, the sites are too warm for frozen carbon dioxide, and in some regions too cold for pure, non-salty water.
When the scientists turned the orbiter's imaging spectrometer on to the flow-marked slopes, they got a surprise – the instrument, which remotely analyses the chemical composition of materials, failed to confirm the presence of water.
Dr McEwen said this could be because the water quickly dries on the surface and did not happen to be there when the analysis was carried out. Alternatively, it might exist at shallow depths below ground.
A subterranean flow of briny water could rearrange sand grains or alter surface roughness in a way that darkens its appearance.
Mars has not yet yielded definitive evidence of liquid water active on its surface today.
Fresh-looking "gullies" suggest activity in geologically recent times that might involve water, and subterranean frozen water has been detected near the surface in some regions.
Possible droplets of brine have also appeared on the struts of the Phoenix Mars Lander probe which touched down near Mars' north pole in 2008.