Ancient ripples?
Fossil dunes
August 8, 2016Erosional remnants
August 22, 2016
A Piece of Mars: Potential signs of wind activity are everywhere on Mars. Take this 0.96×0.54 km (0.6×0.34 mi) scene, which is on bedrock dated to be several billion years old. There’s a fabric of ridges trending from the upper right to lower left. The smaller and smoother ones are clearly windblown bedforms. The larger, bright ones are shedding boulders, so if they’re old bedforms then they’ve been lithified. How old are they? Billions of years old? Or did they form sometime in the intervening years? (HiRISE ESP_046389_1695, NASA/JPL/Univ. of Arizona)
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“The smaller and smoother ones are clearly windblown bedforms.” Agreed that very likely the wind has some role in how these form, but should these big ripples be fit into any of the categories of windblown bedforms that we know from Earth? (Three cheers for the rippunes on Namib. We now know there’s at least one type of bedform on Mars that has no Terran equivalent.) These big ripples in Tyrrhena Terra have about the size range of the plains ripples at Meridiani. Taking the big Meridiani ripples as a possible equivalent, I’ve got a paper in front of me from LPSC 2010 – Constraints on Ripple Migration at Meridiani Planum From Observations of Fresh Craters by Opportunity and Hirise. – Golombek, Robinson, McEwen, Bridges, Ivanov, Tornabee and Sullivan. Along the Meridiani traverse many smaller craters and some medium sized ones clearly post-date the last movement of the ripples. These authors come up with a range of 100,000 to 300,000 years since the big ripples were active enough to create visible alterations to small craters in Hirise images. The surface “armour” grains are just 1 to 2 mm in diameter and just one to a few grains deep. There is no sign of blowout at any of the many places where impact has damaged the armour, exposing much finer sand beneath. These authors call these granule ripples, implying that when they are active, they move in the familiar way, with sand saltation and granule creep? If we’re going to wait for direct confirmation, by seeing it happen, this might take many human lifetimes, but the internal structure of these ripples should show evidence of how they formed. Do we see mixed sand and granules with some crude layering marking earlier surfaces? How about the “prominent internal foreset structure” described by R. P. Sharp? Actually we’ve never looked at a proper cross-section of any big ripple on Mars, but the little we do know from probing them, doesn’t fit very well with the granule ripple theory IMO. They have a very uniform, distinct layer of larger grains at the surface and underneath a sharp transition to finer material, much unresolvable (< 50 microns), with no visible layering, and too few granules. Rocknest (Gale sols 58, 66) and Eldorado (Gusev sol 707) are similar, with sand size armour grains, but again with a sharp boundary near the surface to mostly very fine soil underneath. Its not until sol 802 at Gale, and then again at Namib, that we finally see a big ripple interior that’s made up mostly of sand similar to the surface.
These big ripples could turn out to be an undulating desert pavement, something like rocky desert pavements on Earth, which some now think form and persist because wind-delivered particles end up underneath the rocks on the surface, slowly lifting them up. Maybe on Mars dust ends up under the granule armour, slowly growing the “bedform” from within. There’s plenty of evidence that dust is deposited unequally on different parts of big ripples. But hey, this is just an example of other possibilities. Waves (ripples) are everywhere in nature but are caused in many different ways. I’m guessing that in the end there won’t be any true granule ripples recognized on Mars.