Astronomy Picture of the Day
May 27, 2015

Rhea's uneven Horizon
Rhea's uneven Horizon

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

Gazing off toward the Horizon of a Celestial Body is always thought-provoking, no matter what Celestial Body's Horizon it is. Rhea's Horizon, however (and it can be seen here quite clearly) is slightly (actually, in our opinion , as IPF, highly) irregular and deeply battered by countless Impact Craters, so thoughts inevitably turn towards the Forces that shaped this icy world in the way that we can see today.

The surface of Rhea (approx. 949 miles - or about 1527,26 Km - across) has been sculpted largely by Cratering, and each and every Impact Crater is a reminder of a collision which occurred sometime in the moon's History. As you know, on more geologically active worlds - like Earth or Titan - the Impact Craters would be erased (---> destroyed, eliminated) by the occurrence of phenomena like Erosion, Volcanic Activity or Tectonics; but, on quieter (---> actually, totally still) worlds like Rhea, the Impact Craters remain until they are disrupted - or even covered up - by new Impact Craters or by the Ejecta of a subsequent impact.

The illuminated Terrain seen here is on the Trailing Hemisphere of Rhea; North is up and rotated 12° to the right (Dx). In this view, the NASA - Cassini Spacecraft was at a so-called "Sub-spacecraft Latitude" of North. The image was taken in Visible Light, with the NASA - Cassini Spacecraft's Narrow-Angle Camera on February, 10, 2015.

This view was obtained at a distance of approximately 35.000 miles (such as about 56.326,9 Km) from Rhea and at a Sun-Rhea-Cassini Spacecraft, or Phase, Angle of 76°. Image scale is roughly 1100 feet (such as about 335,28 meters) per pixel.

This frame (which is an Original NASA - CASSINI Spacecraft's b/w and NON Map-Projected image published on the NASA - Planetary Photojournal with the ID n. PIA 18316) has been additionally processed, contrast enhanced, magnified to help the visibility of the Surface details, Gamma corrected and then re-colorized - according to an educated guess (or, if you wish, an informed speculation) carried out by Dr Paolo C. Fienga - 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 Northern Latitudes of the Saturnian moon "Rhea"), 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 Rhea, 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 Rhea - 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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