Astronomy Picture of the Day
August 30, 2013

Phoebe's Skyline (Part I)
Phoebe's Skyline (Part I)

Credits: NASA/JPL/Space Science Institute - Credits for the additional process. and color.: Dr Paolo C. Fienga/LXTT/IPF

Images like this one, showing bright, wispy Streaks (which are thought to be made by Water-Ice and that were revealed - probably - by the occurrence of an event of subsidence of the Crater Walls), are leading to the view that Phoebe is an Ice-rich Celestial Body, overlain with a thin layer of Dark Material (Cosmic Dust, possibly). An obvious down-slope motion of Material along the Inner Walls of the Impact Craters visible in this image, is the cause for the bright Streaks (which, by the way, are so bright to result slightly over-exposed in the picture), and a more than significant slumping has also occurred along the Inner Wall of the Major Impact Crater - located on top left of the picture.

The slumping of Material might have occurred because a (relatively) small Cosmic Projectile punched into the steep Slope of the Inner Wall of a pre-existing (and perhaps larger) Impact Crater. Another possibility, however, is that the Material collapsed because of the shock-waves that were caused by another (and most likely major) Impact that occurred elsewhere on Phoebe. Note that the bright, exposed areas of Ice, are not very uniform along the Inner Wall of all the visible Craters.

Small Impact Craters are also exposing bright Streaks of Material on the hummocky Floor of the larger Crater. Elsewhere on this image, there are local areas of Outcropping along the larger Crater Wall where - most likely - is located some denser, and therefore more resistant Material. Whether these Outcrops are just large Rocky Blocks being exhumed by the occurrence of Landslides, or actual "Bedrock", is not currently known. The Impact Crater located on the left of the frame (the one that shows most of the bright Streamers), is about 45 Km (such a little less rthan 28 miles) in diameter, front to back as viewed. The larger Depression in which the Crater sits, is on the order of 100 Km (about 62,1 miles) across. The Slopes, going from the Rim down to the Hummocky Floor, are approximately 20 Km (such as 12,42 miles) long; many of the bright Streamers visible on the Crater's Inner Wall are about 10 Km (such as 6,21 miles) long.

This image was obtained at a Phase, or Sun-Phoebe-Cassini Spacecraft, Angle of 78°, and from a distance of approx. 11.920 Km (such as about 7.402,3 miles) from the  Surface of Phoebe. The image scale is approximately 70 meters (such as 229,6 feet) per pixel.

This frame (which is an Original NASA - CASSINI Spacecraft b/w frame published on the NASA - Planetary Photojournal with the ID n. PIA 06075) has been additionally processed, contrast enhanced, Gamma corrected and then colorized, according to an educated guess carried out by Dr Paolo C. Fienga (LXXT/IPF), in Absolute Natural Colors (such as the colors that a human eye would actually perceive if someone were onboard the NASA - CASSINI Spacecraft and then looked outside, towards the Saturnian Outer moon, Phoebe), by using an original technique created - and, in time, dramatically improved - by the Lunar Explorer Italia Team. Different colors, as well as different shades of the same color, mean, among other things, the existence of different Elements (Minerals) present on the Surface of Phoebe, each having a different Albedo (---> Reflectivity) and Chemical Composition.

Note: it is possible (but we, as IPF, have no way to be one-hundred-percent sure of such a circumstance), that the actual luminosity of Phoebe- as it is in this frame - would appear, to an average human eye, a little bit lower than it has been shown (or, better yet: interpreted) here.

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