Today's Image of Mars shows some defrosting barchan dunes in the North Polar Region of Mars. Dunes like these are useful in determining the direction of the wind because the side facing the wind becomes more eroded, resulting in a sheer, less rounded face. The wind also creates horn-like features oriented in the oriented in the direction the wind predominantly blows.
This image was taken by HIRISE in the northern spring season on Mars, when the carbon dioxide ice has begun to sublimate.The dunes look speckled because the ice has sublimated in patches, revealing the dark soil underneath. The ice on the floor between the dunes has not begun sublimating because the sand underneath is not as dark as the dunes and so it takes longer to heat up.
Clicking on this image will take you to the original captioned image from HiRISE!