Astronomy Picture of the Day
February 21, 2013


Credits: NASA/JPL - Voyager 2 Project - Credits for the additional process. and color.: Dr Paolo C. Fienga/Lunar Explorer Italia/IPF

The Southern Hemisphere of the Uranian moon Umbriel displays heavy Cratering in this NASA - Voyager 2 image, taken on January 24, 1986, at a distance of approx. 557.000 Km (such as a little less than 346.000 miles) from the Surface of the moon. This frame, taken through the Clear-Filter of Voyager's Narrow-Angle Camera, is the most detailed image of Umbriel that we have, with a resolution of about 10 Km (such as 6,21 miles) per picture element.

Umbriel is the darkest of Uranus' larger moons and the one that appears to have experienced the lowest level of Geological Activity. It has a diameter of about 1200 Km (a little more than 745 miles) and its Surface reflects only 16% of the light that receives; in the latter respect, Umbriel is similar to the Lunar Highland Areas of our own Moon. Umbriel, as we wrote herebefore, is heavily cratered, but it lacks the numerous Bright-Ray Craters that are commonly seen on the other large Uranian Natural Satellites; this fact, among other things, results in a relatively uniform Surface Albedo (---> Reflectivity).

The prominent Impact Crater visible on the Terminator (at about 11 o'clock of Umbriel's disk) is approx. 110 Km (such as 68,3 miles) across and it shows a really bright Central Peak. On the other hand, the strangest Surface Feature visible in this image (on the Limb of Umbriel, right at 12 o'clock of the disk) is a curious-looking Bright Ring which represents the most reflective area seen on Umbriel itself. The Bright Ring is about 140 Km (approx. 87 miles) in diameter and it lies near the moon's Equator. The nature of the Bright Ring is unknown, although it might be a very large Frost Deposit, perhaps associated with (located on top and near) the Rim of a huge Impact Cater. The few white spots that can be barely seen against the black background are not stars, but just just image artifacts, due to 'noise' in the data received on Earth.

This frame (which is the Original NASA - Voyager 2 b/w image published on the NASA - Planetary Photojournal with the ID n. PIA 00040) has been additionally processed and then colorized, according to an educated guess carried out by Dr Paolo C. Fienga (LXTT-IPF), in Absolute Natural Colors (such as the colors that a human eye would actually perceive if someone were onboard the NASA - Voyager 2 Spacecraft and then looked outside, towards the Neptunian moon Triton), 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 others, the existence of different Elements present on the Surface of Triton, each having a different Albedo (---> Reflectivity) and Chemical Composition.

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