Astronomy Picture of the Day
January 24, 2013

The Many Faces of Dione
The Many Faces of Dione

Credits: NASA/JPL/Space Science Institute - Credits for the additional process. and color.: Elisabetta Bonora and Marco Faccin/Lunar Explorer Italia/IPF

The Icy Surface of the Saturnian moon Dione includes, among other things, some Heavily Cratered Terrain, Moderately Cratered Plains, Lightly Cratered Plains, and several Areas which show the presence of deep and wide Tectonic Fractures (notice that all these Geological Features are well visible in this beautiful picture). The Heavily Cratered Terrain shows a number of Impact Craters which are larger than 100 Km (a little more than 62 miles) in diameter. The Plain Areas, on the other hand, tend to have Impact Craters which are less than 30 Km (such as about 18,6 miles) in diameter. Some of the Plains, however, are more Heavily Cratered than others. Much of the Heavily Cratered Terrain is located on the Trailing Hemisphere of Dione, while the less Cratered Plains are found on the Leading one.

This characteristic of Dione tells us just the opposite of what a few scientists would have expected; Shoemaker and Wolfe, for instance, proposed a Cratering Model for a Tidally Locked Satellite with the highest Cratering Rates on its Leading Hemisphere and the lowest ones on the Trailing Hemisphere. This Model, therefore (if it is one-hundred-percent correct - a fact, this one, that we have no way to prove with absolute cetainty), suggests that during the period of heavy bombardment, Dione was (better yet: could have been) tidally locked to Saturn in an opposite orientation as to the present one. Just because Dione is a relatively small Celestial Body (approx. 1120 Km - such as about 695,5 miles - in diameter), we can assume that an impact causing an approx. 35-Km-diameter Impact Crater should have been strong enough to alter the Course (---> Motion and Orientation) of the moon, as to its Parent Planet. Now, since there are so many Impact Craters on Dione which are way larger than 35 Km (such as about 21,7 miles), we can logically speculate that Dione itself might have changed both its Motion and Orientation a few times during the aforementioned period of heavy bombardment. Eventually, once that the bombardment ended, the moon was finally able to set itself on a definitive and stable Course around Saturn.

As a matter of fact, the Pattern of Cratering and the bright Albedo of its Leading Side suggest that Dione has remained in its current Course for several billion years. Like the Jovian moon Callisto, Dione's Impact Craters lack the high Central Features (---> Peaks/Uplifts) that are often seen, instead, on the Moon and Mercury; this circumstance, in our opinion, is probably due to slumping (---> Mass Wasting) of the weak Icy Crust over Geologic Times, but a final answer to this kind of question, as you can imagine, will probably never given with an absolute certainty. Just out of curiosity, if you pay special attention to the visible portion of the Limb of Dione, as it appears in this highly defined picture, you may realize that its shape is NOT, in fact, perfectly spherical.

This image, which is a combination of 5 (five) Original NASA - CASSINI Spacecraft b/w frames whose ID numbers are N00199629/30/31/32 and 33, has been additionally processed and then colorized in Natural Colors (such as the colors that a perfect human eye - or an Electronic Eye - would have perceived while looking at Dione from Cassini's vantage point) 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 Dione, each having a different Albedo (---> Reflectivity) and Chemical Composition.

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