In yesterday's ConTextual Image (or "CTX frame", for short) of the Red Planet (that was an image taken by the NASA - Mars Reconnaissance Orbiter on September, 15, 2014), we were able to see that many Dunes and Ripples on the Red Planet are still very active. This demonstrates that, in some areas of Mars, Sand is loose enough, and Winds are strong enough, that some significant change can occur at any given time.
Nevertheless, other Martian Dunes are clearly *inactive*. This image, which is an Extra Detail Magnification (or "EDM", for short) that represents a small portion of the Region known as Arcadia Planitia, shows us Dunes found on the Floor of an Unnamed and (relatively) small Impact Crater. Said that, if you pay attention to the aformentioned Dunes, you will notice that, unlike the Active Dunes of the Red Planet, those here are really bright, and, if you zoom in, there are several lines of evidence indicating that the Dunes themselves have become indurated (---> such as hardened into "Cohesive Sediment" or even into "Sandstone Rock").
For example, the Dunefield visible at the Southern Edge of the Crater, is cut off by a so-called "Step-Cliff", which is a data indicating the occurrence of erosion of hard material. Although fine scale Ripples on the original Dune Surface are still well preserved, we can also see large scale Fluting from the South/West to the North/East: a common Surface Texture associated with Wind-induced Sand Abrasion.
How these Dunes became indurated is still unknown, but one possibility is that this area of Mars was buried and then exhumed: a process that seems to have occurred many times in the Martian past and over various areas of the Planet. As a matter of fact, the Geologic Events of Burial, Compaction and - possibly - Ground Water Circulation, could well have indurated the Dunes, leaving them as a hard Sandstone that, when exhumed, was subsequently - and partially - eroded by a powerful Wind-action.
Mars Local Time: 15:39 (Middle Afternoon)
Coord. (centered): 39,983° North Lat. and 202,596° East Long.
Spacecraft altitude: 299,2 Km (such as about 185,803 miles)
Original image scale range: 29,9 cm/pixel (with 1 x 1 binning) so objects ~ 90 cm across are resolved
Map projected scale: 25 cm/pixel
Map projection: EQUIRECTANGULAR
Emission Angle: 3,4°
Phase Angle: 66,0°
Solar Incidence Angle: 69° (meaning that the Sun was about 21° above the Local Horizon at the time the picture was taken)
Solar Longitude: 196,7° (Northern Fall - Southern Spring)
Credits: NASA/JPL/University of Arizona
Additional process. and coloring: Lunar Explorer Italia
This picture (which is a crop taken from a NASA - Original Mars Reconnaissance Orbiter b/w and NON-Map Projected Sub-frame identified by the serial n. ESP_038143_2205) has been additionally processed, magnified to aid the visibility of the details, contrast enhanced, Gamma corrected, and then colorized in Absolute Natural Colors (such as the colors that a human eye would actually perceive if someone were onboard the NASA - Mars Reconnaissance Orbiter and then looked down, towards the Surface of Mars), by using an original technique created - and, in time, dramatically improved - by the Lunar Explorer Italia Team.