The smallest dunes
Remnants of erosion
January 25, 2016It’s a rock-eat-rock world
February 20, 2016
A Piece of Mars: There are two small dome-shaped dunes in this frame (0.96×0.54 or 0.6×0.33 mi). If they got any larger, they’d form slip faces. Any smaller and they’d just be random drifts of windblown sand. Dunes form at a particular size (~125 m in this case) related to the distance it takes for sand grains to accelerate to the background wind speed. This distance is bigger on Mars than on Earth, where the smallest dunes are ~20 m across. (HiRISE ESP_044198_1480 NASA/JPL/Univ. of Arizona)
3 Comments
Dr Fenton, my comments are getting published without moderation, and I don’t want to take impolite advantage, but I’m dying to ask a basic question about aeolian processes on Mars. It seems to me that there is a glaring contradiction between what the three rovers have witnessed in now close to 20 rover years of observation, and the meter-scale dune movements that other researchers are seeing from orbit. How are these dune movements possible in an environment where almost no change to smaller ripples has taken place? Correct me if I’m wrong, but I think that the only ground-truthed sand ripple movement that has been recorded took place at Gusev Crater in 2007. During the onset of a big dust storm the smallest ripples, up to roughly 1 cm height, moved a little, maybe one wavelength on average, and larger ripples didn’t move perceptibly at all. To my knowledge there has been no sand ripple movement recorded from either Meridiani or Gale. How can dunes be moving decimetres to metres per year near the rovers without the rovers frequently seeing changes to smaller features?
I’m not suggesting that the changes to dunes seen from orbit are not real. Namib Dune has clearly been recently disturbed by colluvial activity on the big slip face, and I’ve managed to find some colluvial changes to the slip faces on the “rippunes” that happened during Curi’s visit. When did the aeolian loading part happen? Has Gale been affected by sand lifting winds since Curi landed? Is there any evidence that these winds altered small sand features?
Wow, those smallest dunes are covered with a very beautiful orderly pattern of ripples, remarkably uniform in size. Namib dune, the only one we’ve seen from the ground, has a similar covering of ripples seen from orbit. Many of those ripples turned out not to be ripples after all when ground-truthed. They have slip faces much like the big slip-face on the dune. I’ve been calling these “rippunes” where I normally blog. (If anyone starts using a less whimsical name in public I’ll adopt it.) Its pretty clear that they aren’t just smaller dunes as there are no intermediates in evidence between the rippunes and the larger dunes they cover. I wonder if ALL active dunes on Mars will turn out to have rippunes on their backs? So far we’ve only had a good look at one dune and it had plenty. It doesn’t look like the rippunes grow up to be dunes, but a field of rippunes might grow higher and become a dune.
Beautiful image. Thanks for the explanation, too.