At http://phys.org/print386572617.html … Curiosity Rover has been looking at samples of sand and at sand dunes and presumably sandstones on Mars. A paper in the journal Science (this week) notes that some of these ripples are sinuous and resemble sand ripples that form under moving water on Earth (as you might encounter paddling in the tidal water at the sea side). Superimposed on these larger ripples are ripples the same size and shape as impact ripples on Earth – caused by the wind etc. Another similarity with water ripples is that one face of each ripple is steeper than the face of the other side. Scientists have decided such ripples are built by the Martian wind dragging sand particles the way flowing water does on Earth – a different mechanism but the same situation is produced. These are now being called 'wind drag ripples'. Now, the surprising finding for the point of view of a lot of people is that they conclude Mars had a thicker atmosphere in the past which formed the wind drag ripples, which would bring it all too close to home (in time). Hence, the caveat is the thick atmosphere probably preceded their formation. This implies wind drag ripples formed on Mars as the atmosphere thinned (whisked away by the solar wind). Such ripples, where found in sandstone on Mars, could be used to date the thinning of Mars atmosphere.