Different sands

There’s a lot we don’t understand about the sediment on Mars. Water, wind, ice, changing temperatures, and volcanic eruptions can all break rocks into grains small enough for the wind to transport. The smallest grains are lofted by the wind, contributing to dust storms, and settling out as fine layers of dust. Slightly larger grains hop (saltate) along the ground and self-organize into beautiful ripples and dunes. Even larger grains (more than a few millimeters in diameter) might be moved by the wind on occasion – these grains are too big to saltate (unless the wind is really strong), but repeated impacts from smaller saltating grains can slowly make them move (Aeolian scientists call this sort of movement creep. Yep.). Those bigger grains can also self-organize, along with the smaller grains, into a different type of ripple. That could be what’s happening in the image below.

Two kinds of ripples, view is 500×625 m (0.31×0.39 mi). Be sure to click to get the full resolution view. (HiRISE ESP_055605_1695 NASA/JPL/Univ. of Arizona)

Here, a rocky surface is partly covered by a field of brown criss-crossing ripples that are ~3 m (10 ft) across. At the edge of the brown ripple field, there are larger, dark gray ripples.

Why are these two sets of ripples so different?

The only way to know for sure would be to go there in person and have a look. For now, all we can do is make some educated guesses. The two different ripple sets have different spacing, color, and orientation. It’s as if someone dumped handfuls of totally different sand in different places, except nobody I know of has hands that big. This suggests to me that we’re probably looking at different grain sizes, probably with the bigger dark gray ripples being made (at least in part) of larger grains. It takes a long time for the wind to significantly move larger grains, because they’re only likely to move a lot when the wind blows really hard. In contrast, the sand making up the smaller brown ripples might be easily moved by a more moderate (but still strong!) wind. In this case the brown ripples would “see”, and thus be affected by, more of the wind than the dark gray ripples, which might only “see” the very strongest winds. It’s as if the two sets of ripples exist in very different wind environments, which could lead them to form in different locations, with different spacing, and different orientations.

The difference in color could also be related to grain size. The wind will more easily move smaller, less dense grains. So if a source of sediment has larger grains made of one (say, a dark gray) mineral and smaller grains made of another (say, a brown) mineral, the wind could work to separate those two minerals. It’s as if the wind is the ultimate bean sorter.