Curiosity about sand dunes (part 2/2)
Today is December 21, 2015 (northern winter and southern summer solstice on Earth). On Mars it is Ls = 84º, Mars Year 33 (about 12 sols from northern summer and southern winter solstice on Mars). It is sol 1200 of Curiosity’s mission on Mars, and the rover is working its way around the southern side of Namib Dune. Part 1 of my previous post shows part of the windward (northeastern) side of High Dune. This time I’ll show pictures of the slip face of Namib Dune.
The dunes in this part of the Bagnold Dune Field are slowly marching towards the southwest. Wind blows from the north-northeast and the sand piles up, only to oversteepen on the lee side and form avalanches in what we call a slip face. This happens over and over as the dune moves: saltating sand flies over the dune crest and settles on the upper slip face (what we call grainfall). When enough sand piles up, it oversteepends, and eventually there’s a slope failure: stuff higher up moves down, like a little landslide (what we call grainflow). We see grainflows most commonly when the wind is blowing nearly directly across the crest of the dune.
But when is nature ever so uniform? Sometimes people ask me how strong the wind blows on Mars, as if I could just give them a single value that would apply to all of Mars (its mountains, polar caps, steep crater rims, and flat plains) at all times (its CO2-covered winters, convectively-turbulent summer days, regions prone to seasonal dust storms, and nighttime low-lying flows). Go outside on Earth for a moment and tell me if you can feel the wind moving from different directions, at different speeds, and do it again in 12 hours and again in 6 months. In most places you won’t get the same result, and it is the same on Mars.
You don’t get the same winds blowing here in the Bagnold Dune Field either, even though the dunes are telling us that the strong winds mostly blow from the north-northeast. What can the slip face tell us about that? Let’s have a look:
What an awesome view! Here’s another view of the whole slip face:
It’s totally different from the other side of the dune, which is covered in ~2 m sized ripples that are themselves covered in smaller ripples. Instead, this looks like a giant wall of sand, textured with features that all look like they’ve moved downhill. The smoother surfaces are probably the newest: these are fresh grainflows. But some of the slip face is covered in small ripples. Here’s what I mean:
What you’re seeing is the interplay of at least two different winds. There’s the main wind that blows over the crest, which forms grainflows, and then a secondary wind that blows in a different direction, forming ripples out of sand from the grainflows. Based on the orientations of other ripples, that secondary wind probably blows from the northwest, which is roughly along the slope of the slip face. That northwesterly wind is responsible for making the large ripples at the base of the slip face – when that wind blows, those large ripples would march towards the camera.
It’s pretty typical for two to three wind directions to dominate all other winds in a region, at least in terms of sand transport. On Earth this usually comes down to seasonal changes in weather patterns: winter storms vs. steady summer winds. Perhaps where you live, most of the weather arrives from one direction, but occasional storms may blow in from another direction. Those winds that blow strongest are most likely to move the most sand. This appears to be the case on Mars as well.
It looks to me like the most recent wind activity has formed grainflows, suggesting that the NNE wind has been more recently active than the ripple-forming wind. However, most of the slip face is covered in small ripples, suggesting that this NW wind was, until recently, the prevailing wind. The ripples weren’t able to fully rewrite the topography of the slip face, as you can see they cover slightly larger undulations that were probably older grain flows – this supports the idea that they are formed by a secondary wind that cannot move enough sand to rewrite the entire dune (if it did then the slipface would point towards the southeast, instead of towards the southwest as it does now). We’re probably seeing a seasonal tradeoff between the NNE and NW winds. I might even cautiously suggest that the grainflow-forming NNE wind is active in the current season (local autumn) and that the ripple-forming NW wind blew in a previous season (perhaps local spring or summer). I’d love to get a shout-out from the REMS folks, if they can pull out any new wind data from their partially-broken anemometer.
Happy solstice everybody, and I hope you have a good holiday.