Astronomy Picture of the Day
September 1, 2012

Features of Ganymede (Part I)
Features of Ganymede (Part I)

Credits: NASA - Voyager 1 Project; Credits fo the additional process. and color.: Dr Paolo C. Fienga/LXTT/IPF

This frame was acquired by the NASA - Voyager 1 Spacecraft during its approach to Ganymede, from a distance of about 272.000 Km; the center of the picture lies at approx. 13° North Latitude and and 359° East Longitude. Ganymede (also known as Jupiter III) is the largest moon in the Solar System and the seventh moon (and third Galilean Satellite) outward from the Gas-Giant Planet Jupiter. Completing an orbit in roughly 7 (seven) Earth days, Ganymede participates in a 1:2:4 Orbital Resonance with the other Jovian moons Europa and Io, respectively. Ganymede has a diameter of about 5268 km (such as approx. 3273 miles), and, even though Ganymede is something like 8% larger than the planet Mercury, it only has about 45% of Mercury's mass; furthermore, its diameter is about 2% larger than the one of the Saturnian moon Titan, which is the second largest moon in the Solar System. Ganymede also has the highest mass of all Planetary Satellites, with approx. 2,02 times the mass of the Earth's Moon.

The image that we present you in today's APOD shows detail on the Surface of Ganymede with a resolution of approx. 4,5 Km per pixel. What we see here, is a portion of a Region of Ganymede located in its Northern Hemisphere, near the Terminator. It shows a variety of Impact Structures, including both Rayed and Unrayed Impact Craters, as well as some odd-looking, Groove-like Surface Features (already discovered by the NASA - Voyager 1 Spacecraft in the lighter Regions of this Jovian moon). The most striking Surface Features, however, are the Bright Rayed Impact Craters which show a distinct light blueish color that, to some Observer, may also also appear white, with pink nuances, against the darker background. Ganymede's Surface is known to contain large amounts of Surface Water Ice and it appears that these (relatively) young Craters might have spread bright and (always relatively speaking) fresh Ice Materials all over the place. Likewise, the lighter color and Albedo (---> reflectivity) of the Grooved Areas suggests that over there, too, some cleaner and fresh Water Ice Material can be found. In fact, and as far as we know at the present day, Ganymede is composed of approximately equal amounts of Silicate Rock and Water Ice. It is a fully differentiated Celestial Body with an Iron-rich and still liquid core. Very recent studies suggest that, just like it has been speculated about the other Jovian moon Europa, even Ganymede may host a Subterranean Ocean, nearly 200 Km below its frozen Surface, somehow "sandwiched" between several different Layers of Rock and Ice.

In addition to the above, the Surface of Ganymede is composed of two main types of Terrain: the Dark Terrain, saturated with Impact Craters and dated up to 4 (four) Billion Years ago (a Terrain that covers about a third of the moon), and the Lighter Terrain, which is crosscut by extensive Grooves and Ridges and that could be, likely, much less ancient. The cause of the Light Terrain's so-called "Disrupted Geology" is not yet fully known, but it could reasonably be the result of some powerful Tectonic Activity brought about by Tidal Heating (and let us not forget that Tidal Heating - due to Tidal Friction - may also be the primary reason why a Liquid Ocean can exist, deep down and inside the otherwise Frozen Crust of Ganymede).

The NASA - Voyager 1 Spacecraft Original b/w frame has been additionally processed and then colorized, according to an informed speculation 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 1 Spacecraft and then looked outside, towards the Jovian moon Ganymede), 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 Ganymede, each having a different Albedo (---> Reflectivity) and Chemical Composition.

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