Ever since 19th-century astronomers imagined seeing Martian canals through their telescopes, stories about water on Mars have ebbed and flowed.
Despite an intensive scientific search in recent years, no one has proved the presence of liquid water on the planet. But now the US space agency Nasa has presented the strongest evidence so far.
A camera on the Mars Reconnaissance Orbiter has observed dark, finger-like features that run down some Martian slopes during warm times of the year and fade during the winter.
The best explanation is that these are seasonal flows of salty water or brine, Nasa researchers report in Friday’s edition of the journal Science.
Saltiness lowers the freezing point of water, and brine would be liquid at the pressures and temperatures measured on the Martian surface in summer.
The putative brine streaks are from 0.5 to 5 metres wide and run for hundreds of metres down the slopes of some craters. They are more abundant on the warmer sun-facing sides, as would be expected of water that flows in summer and freezes in winter.
The team discovered the features after Lujendra Ojha, a research student at the University of Arizona, wrote a computer program to detect subtle seasonal changes on the Martian surface.
“I was baffled when I first saw those features in the images after I had run them through my algorithm,” said Mr Ojha. “We soon realised they were different from slope streaks that had been observed before. These are highly seasonal, and we observed some of them had grown by more than 200 metres in a matter of just two Earth months.”
“By comparison with Earth, it’s hard to imagine they are formed by anything other than fluid seeping down slopes,” said Richard Zurek, Mars Reconnaissance Orbiter project scientist. “The question is whether this is happening on Mars and, if so, why just in these particular places.”
However, instruments on the satellite have not detected water directly on the streaks – perhaps because the brine is running in channels beneath the sandy surface.
Although there is still no definite evidence of liquid water on the red planet today, there have been previous hints. There were signs of what might – or might not – have been droplets of brine forming on the struts of Nasa’s Phoenix Mars Lander in 2008.
But Martian geology shows that extensive seas and rivers existed in the past. And there is no doubt that ice and frost still form during the winter in the higher latitudes.
As the authors point out in their Science paper, “liquid water on Mars today would be of great interest for astrobiology”, the search for life beyond Earth.