The NASA 'Day in Review' newsletter (jplnewsroom [at] jpl [dot] nasa [dot] gov) 9th October 2015 reports on the discovery that lakes once existed on Mars. This was observed by Curiosity Rover which found 'mudstone' geology – and the existence of mud indicates water. However, the lake itself must have existed long ago (it was found in the Gale Crater feature which is a hole punched in the geology), and wait for it – 3.3 to 3.8 billion years ago.
The same story is at http://phys.org/print363534982.html … which explains that models were involved and this might put a different light on matters. However, it seems that a mountin has to be removed in order to get at the lake bed itself – which is why billions of years are involved. It seems that NASA scientists already had a theory prior to the arrival of Curiosity Rover at the crater – they clearly thought it had been filled with sedimentary layers. The discovery of mudstone appears to be a game changer, in that now the facts fit the theory. See also www.sciencemag.org/lookup/doi/10.1126/science.aac7575
At http://phys.org/print363502441.html … Mars Orbiter has taken images of sand dunes on Mars. Within the space between the dunes a fractered surface was revealed/