One above suggests that the trails go under an overhang. I don't think there is an overhang, just some sharp ridged dunes, and oblique sunlight, so the left side of the dune is shadowed.
PHN wrote:The straight parallel black lines are likely caused by sand slides. They seem to be located on the steep surfaces of large sand dunes. My theory says that the sand starts to slide at the top and clears the reddish dust all the way down.
The straight lines are down the slip face, the side of the dune that is advancing, driven by winds from the opposite side. They must be due to the sand at the top being blown up to the lip and over the edge, to be deposited on the slip face. Longitudinal dunes form where the wind is constant and unvarying in direction, which facilitates such regular deposition. But typically, as the wind on the upwind face lifts sand into the air, over the downwind, slipface it becomes turbulent, slows and drops its sand load. In the turbulence, one would expect the sandy air to deposit a mixture of sand from a wide are of the upwind side, so the regular stripes are unexpected. At the least, the blue and pink particles (colour exaggerated, of course) must be very, very similar in size, texture and mass to be deposited so regularly. So what are they?
APOD pics used to be accompanied by some interpretation. HiRise never do.
I would be most interested to read what Areologists thought about this.
John
