Astronomy Picture of the Day
February 13, 2013

Closing-up on Valhalla
Closing-up on Valhalla

Credits: NASA/JPL - Voyager 1 Project - Credits for the additional process. and color.: Dr Paolo C. Fienga/Lunar Explorer Italia/IPF

Callisto was revealed by the NASA - Voyager 1 cameras to be a heavily cratered and - most likely (as it was thought at that time) - geologically inactive Celestial Body. This mosaic of Voyager 1 images, obtained at a distance of about 400,000 Km (about 248.000 miles) from Callisto, shows Surface details as small as 10 Km (such as 6,2 miles) across. The largest Impact Features on the Callistoan Surface are the so-called "Multi-Ring Impact Basins"; two of them are enormous and Valhalla - the prominent Impact Feature visible here, in today's APOD - is the largest one, with a Bright Central Region approx. 600 Km (a little more than 372 miles) in diameter, and Rings extending as far as about 1800 Km (approx. 1118 miles) from the center. The second largest Impact Multi-Ring Basin is named Asgard, and it measures about 1600 Km (a little more than 993 miles) in diameter.

Multi-Ring Impact Structures probably originated as a result of a Post-Impact Concentric Fracturing of the Lithosphere lying on a layer of soft or - maybe - even Liquid Material: possibly, an Ocean. Last, but not least, in our opinion, as IPF, the remarkable resemblances existing between the Callistoan Valhalla Crater and the Tyre Crater, located on the other Jovian moon Europa, (*) may tell us, in the end, VERY similar stories about these two mysterious Celestial Bodies. Stories that may talk about worlds that, certainly a long, long time ago, were completely (or almost completely) covered by water. Stories that should also tell us something about what could still be happening, even now, up there: in the depths of Callisto, Europa and, maybe, of a few other Celestial Objects orbiting around the four Gas-Giant Planets of the Solar System...

(*) note: if you want to take a look at Tyre Crater, please refer to the APOD of January, 29, 2013

The Catenae, on the other hand (for example the Gomul Catena), are long Chains of Impact Craters lined up in straight lines across the Surface of Callisto. They were probably created by objects that were tidally disrupted as they passed close to Jupiter prior to the impact on Callisto, or by very oblique Impacts.

This frame (which is an Original NASA - Voyager 1 Spacecraft false color image mosaic published on the NASA - Planetary Photojournal with the ID n. PIA 00080) has been additionally processed and then re-colorized, according to an educated guess, 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 1 Spacecraft and then looked outside, towards the Jovian moon Callisto), 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 Callisto, each having a different Albedo (---> Reflectivity) and Chemical Composition.

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