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.