The sand is black sand, and the white lines are the remnants of frost leftover on the surface of the black sand. The dunes are free of the frost remnants, because they are slowly moving. This means we can fully see the color of the black sand, which plays with visual perspective. Here’s the same area in an earlier photo when it was completely all frosted over: https://static.uahirise.org/images/2023/details/cut/ESP_076510_2230-2.jpg