Strongest evidence yet for water on Mars

 
By David Batty

Stories of water on Mars have been disputed since 19th-century astronomers mistook an optical illusion for canals built by intelligent life. But new images of dark finger-like channels on the red planet may be the strongest evidence yet of water flowing on its surface.

Images from the US space agency Nasa's Mars Reconnaissance Orbiter show many long, dark "tendrils" a few metres wide, which scientists claim could be streams of water running down the sides of craters.

More than a thousand dark trails were observed running down some slopes in Mars's southern hemisphere during warm periods of the year, fading in the autumn.

There are more trails on the warmer, sun-facing parts of the planet, which would be consistent with water that flows in summer and freezes in winter.

Researchers from the University of Arizona said that salty water was the "best explanation" for the markings, which are between half a metre and five metres wide and run for hundreds of metres down some craters.

Although the images do not provide definitive proof of salt water on Mars, scientists claim that temperatures on the sun-facing areas of the planet's surface would be too warm for frozen carbon dioxide and too cold for pure water.

Salt lowers water's freezing temperature meaning that despite the coldness of Mars, water as salty as Earth's oceans could melt in the planet's warmer season.

"The best explanation for these observations so far is the flow of briny water," said Alfred McEwen of the University of Arizona.

"It's a mystery now, but I think it's a solvable mystery with further observations and laboratory experiments."

The trails are thought to contain a few litres or tens of litres of water each, according to the research paper published in the Science journal.

Scientists have long believed water probably flowed on Mars in its distant past but have so far only detected samples of frozen water near the surface.

"Nasa's Mars Exploration Program keeps bringing us closer to determining whether the planet could harbour life in some form and it reaffirms Mars as an important future destination for human exploration," said Nasa spokesman Charles Bolden.