Frost Covered Gullies in Terra Sirenum (CTX Frame + EDM - Absolute Natural Colors; credits: Dr Paolo C. Fienga - Lunexit Team)

PSP_001552_1410 shows Gullies in a Unnamed Crater located in Terra Sirenum (Southern Hemisphere of Mars).
This image was acquired during the Winter, which explains the abundant frost (the bright material) seen throughout the image. The frost is likely water frost, as opposed to CO2, because temperatures at this latitude probably do not get cold enough for Carbon Dioxide to condense.

The formation mechanism of Gullies is much debated. Several theories support erosion by liquid water, while others favor dry debris flows or Carbon Dioxide. A major unknown is, if the Gullies are formed by liquid water, does the water originate from the Surface or Subsurface?

Also interesting about this scene is the fact that the Gullies occur at multiple elevations along the same Crater Wall. This is uncommon on Mars. Gullies, whether or not they are found in conjunction with an obvious horizontal layer, usually form at the same elevation on a given slope. It is unknown what caused these Gullies to form at multiple elevations. Their locations are suggestive of a distributed water source, which also favors a Surface, rather than a confined Subsurface origin of water, such as an Aquifer.

Acquisition Date: November, 25th, 2006
Mars Local Time (M.L.T.): 15:40 (Middle Afternoon)
Latitude (centered): 38,9° South
Longitude (East): 195,9 °
Range to Target Site (or Spacecraft Altitude as to the pictured area): 250,5 Km (such as approx. 156,6 miles)
Original Image Scale Range: 50,1 cm/pixel (with 2 x 2 binning) so objects ~150 cm across are resolved
Map Projected Scale: 50 cm/pixel and North is up
Map projection: EQUIRECTANGULAR
Emission Angle: 2,4°
Phase Angle: 76,9°
Solar Incidence Angle: 75° (with the Sun about 15° above the Local Horizon)
Solar Longitude: 140,6° (Northern Summer)