An excerpt from an OUPblog article published on Friday 9th October 2015, written by David A Rothery, Professor of Planetary Geosciences at The Open University and author of Planets: A Very Short Introduction, and the forthcoming Moons: A Very Short Introduction.
"The discovery of water on Mars has been claimed so often that I’d forgive anyone for being sceptical about the latest announcement. Frozen water, ice, has been proven on Mars in many places, there are lots ancient canyons hundreds of km long that must have been carved by rivers, and much smaller gullies that are evidently much younger including a few where changes have been seen.
In 2008 the Phoenix lander actually saw water on Mars. When it scraped away at the dirt, it found water-ice a few centimetres down, but more excitingly droplets that could hardly be anything other than water were seen to form on the lander’s legs. It was suggested that the water had condensed around wind-blown grains of calcium perchlorate, a salt mineral whose properties enable it to scavenge water from the air and then dissolve it. Moreover, whereas pure water would freeze at the local temperature at that site (between -10°C and -80°C), water containing enough dissolved salts could stay liquid."
Discover more: Read the rest of the article on the OUPblog.