Astronomy Picture of the Day
February 14, 2015

Longitudinal South Polar Dune (CTX Frame)
Longitudinal South Polar Dune (CTX Frame)

Credits: NASA/JPL/University of Arizona - Credits for the additional process. and color.: Dr Paolo C. Fienga/LXTT/IPF

This beautiful Contextual Frame was taken by the NASA - Mars Reconnaissance Orbiter during the late Martian Southern Spring (such as January, 24, 2015 on Earth), when the Red Planet is near its Perihelion (---> the closest distance to the Sun). In this picture we can clearly see the effects of Dry Ice Sublimation on a so-called "Longitudinal Dune" that is located in the far Southern Hemisphere of the Red Planet. The bright Patches that are well visible on both sides of the Dune are small areas which are still covered by Frost while, on the other hand, the dark areas are Frost-free.

In general terms (and "normal" - for the place we are considering - weather conditions), Longitudinal Dunes form when the Dominant Winds blowing over the Region, switch in between two common directions. Based on the direction of this Dune’s long Crest and taken into account the orientation of the smaller Ripples, it appears that the Dominant Winds here blow from the East and from the North/West. However, it would require taking multiple HiRISE images of this location over time before we could say this definitively.

The broad base of this Dune may indicate that the Dune Sand (---> the Sand forming the Dune) has spilled out from areas once covered in Ice. During the next Martian Southern Hemisphere's Winter (about half a Mars year - or one Earth year - from when this image was taken), this Dune will be, once again, fully covered by Frost and, possibly, by Solid Carbon Dioxide Ice, which will make unable the Winds to blow the Volatiles Material away (at least until the Volatiles themselves shall begin to sublimate in the next coming Martian Spring). The whole scene here is approx. 250 meters (such as about 820 feet) across.

Mars Local Time: 15:32 (Middle Afternoon)
Coord. (centered): 67,861° South Lat. and 207,704° East Long.
Spacecraft altitude: 250,9 Km (such as about 155,808 miles)
Original image scale range: 25,1 cm/pixel (with 1 x 1 binningso objects75 cm across are resolved
Map projected scale: 25 cm/pixel
Emission Angle: 8,9°
Phase Angle: 46,3°
Solar Incidence Angle: 53° (meaning that the Sun was about 37° above the Local Horizon at the time the picture was taken)
Solar Longitude: 265,8° (Northern Fall - Southern Spring)
Credits: NASA/JPL/University of Arizona
Additional process. and coloring: Lunar Explorer Italia

This picture (which is a NASA - Original Mars Reconnaissance Orbiter b/w and NON-Map-Projected CTX frame identified by the serial n. ESP_039568_1120) has been additionally processed, magnified - in order to make the details more visible -, 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.

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