Eroding Mesas Forming Logitudinal and Barchan Dunes
Science Theme: Aeolian Processes   
This image shows sand dunes within the Hellespontus region of Mars.
The sand comes from the layered mesas and knobs, low in the image, that have been eroded by formative winds blowing from east to west (in this image from bottom to top, east being bottom, west being top).
The primary indicator is the orientation of the barchan dunes, a dune type that on Earth has its horns in the downwind direction. Also, the steepest faces are on the dunes’ eastern side, indicating easterly winds.
In addition to the barchans, some of the dunes are organized into a longitudinal form,  whereby their long axis is parallel to the wind. These are seen emanating from the sand next to the mesas and knobs, as well as from some barchan horns.

Acquisition date: 16 March 2008 ,  Local Mars time: 15:09
Range to target site from satellite : 256.9 km (160.6 miles)
Original image scale range:51.4 cm/pixel, so objects ~154cm across are resolved
Sun about 16° above the horizon
Spring time on Mars