Astronomy Picture of the Day
February 14, 2013

Valhalla!
Valhalla!

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

This picture of the gigantic Multi-Ring Impact Basin named "Valhalla" and located on the Jovian moon Callisto, was taken by the NASA - Voyager 1 Spacecraft in the morning of March 6, of the AD 1979, at a distance of about 200.000 Km (such as approx. 124.000 miles) from the Surface of the farthermost of the 4 (four) so-called "Galilean" Natural Satellites (or moons) of the Gas-Giant Planet Jupiter. The complicated Circular Structure which characterizes Valhalla is, in a way (and according to NASA), relatively similar to other large Circular Impact Basins that can be seen on the Surface of the Earth's Moon as well as on the Planet Mercury but, as it has been already written and underlined in yesterday's APOD, we, as IPF, do believe that a true and strong resemblance between Valhalla and another Multi-Ring Impact Basin can only be found with the huge Tyre Crater; a Multi-Ring Impact Crater that is located, as you know, on the Jovian moon Europa.


Usually, the inner portions of these Multi-Ring Impact Basins are generally surrounded by Radially Lineated Ejecta and several Concentric Mountainous Ring Structures, which are thought to have formed during the Impact Event itself. However, the Callistoan Valhalla Multi-Ring Impact Basin consists of a Light-Floored Central Basin, that is approx. 300 Km (such as a little more than 186 miles) in diameter, surrounded by at least 8 (eight) to 10 (ten) discontinuous and rhythmically spaced Ridges, but no Radially Lineated Ejecta can be seen. In any case, the great number of Rings observed around Valhalla is a fact which is consistent with the hypothesis that Callisto possesses a very low Planetary Density and, also, a low internal strength. Furthermore, if you look carefully on the upper right side of the frame, at about 2 o'clock, a beautiful and extremely long and regular Catena (---> Chain of Craters, probably originated by a Cometary Strike or a Meteor that was heavily fragmented at the time of the Impact) can be seen.


This frame (which is an Original NASA - Voyager 1 Spacecraft b/w image published on the NASA - Planetary Photojournal with the ID n. PIA 02277) has been additionally processed and then 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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