Ganymede Fact Sheet
What Is Known About Ganymede SO FAR
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Since Ganymede has a low density of 1.94 grams/cubic centimeter (water's density = 1.00), it is estimated that the satellite is half water ice with a rocky core extending to half of the satellite's radius. The mantle is composed of ice and silicates and a crust which is probably a thick layer of water ice.
Voyager images of Ganymede have shown that the satellite has a complex geological history. Ganymede's surface is a mixture of two types of terrain. Forty percent of the surface of Ganymede is covered by highly cratered dark regions, and the remaining sixty percent is covered by a light grooved terrain which forms intricate patterns across Ganymede. The term "sulcus," meaning a groove or burrow, is often used to desribe the grooved features. This grooved terrain is probably formed by tensional faulting or the release of water from beneath the surface. Groove ridges as high as 700 meters (2000 feet) have been observed in the Voyager imagery and the grooves run for thousands of kilometers across Ganymede's surface. The grooves have relatively few craters and probably developed at the expense of the darker crust. The dark regions on Ganymede are old and rough, and the dark, cratered terrain is believed to be the original crust of the satellite. Lighter regions are young and smooth (unlike the Moon). The largest area on Ganymede is called Galileo Regio, and is one of the areas targeted by the Galileo spacecraft.
The large craters on Ganymede have almost no vertical relief and are quite flat. They lack central depressions common to craters often seen on the rocky surface of the Moon. This is probably due to slow and gradual adjustment to the soft icy surface. These large "phantom craters" are called palimpsests, a term originally applied to reused ancient writing materials on which older writing was still visible underneath newer writing. Palimpsests range from 50 to 400 km in diameter. Both bright and dark rays of ejecta exist around Ganymede's craters - rays tend to be bright from craters in the grooved terrain and dark from the dark cratered terrain.
Discovery: Jan 11, 1610 by Galileo Galilei Diameter (km): 5,262 Mass (kg): 1.48e23 kg Mass (Earth = 1) 0.0247 Surface Gravity (Earth = 1): 0.145 Mean Distance from Jupiter (km): 1,070,000 Mean Distance From Jupiter (Rj): 15.1 Mean Distance from Sun (AU): 5.203 Orbital period (days): 7.154553 Rotational period (days): 7.154553 Density (gm/cm^3) 1.94 Orbit Eccentricity: 0.002 Orbit Inclination (degrees): 0.183 Escape velocity (km/sec) 2.74 Visual Albedo: 0.43 Subsolar Temperature (K): 156 Equatorial Subsurface Temperature (K): 117 Surface Composition: Dirty Ice
Probably the most significent contribution that Galileo Galilei made to science was the discovery of the four satellites around Jupiter that are now named in his honor. Galileo first observed the moons of Jupiter on January 7, 1610 through a homemade telescope. He originally thought he saw three stars near Jupiter, strung out in a line through the planet. The next evening, these stars seemed to have moved the wrong way, which caught his attention. Galileo continued to observe the stars and Jupiter for the next week. On January 11, a fourth star (which would later turn out to be Ganymede) appeared. After a week, Galileo had observed that the four stars never left the vicinity of Jupiter and appeared to be carried along with the planet, and that they changed their position with respect to each other and Jupiter. Finally, Galileo determined that what he was observing were not stars, but planetary bodies that were in orbit around Jupiter. This discovery provided evidence in support of the Copernican system and showed that everything did not revolve around the Earth.
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Galileo published his observations in Sidereus Nuncius in March 1610:
"I therefore concluded, and decided unhesitatingly, that there are three stars in the heavens moving about Jupiter, as Venus and Mercury around the Sun; which was at length established as clear as daylight by numerous other subsequent observations. These observations also established that there are not only three, but four, erratic sidereal bodies performing their revolutions around Jupiter."
Simon Marius claimed to have observed Jupiter's moon as early as late November 1609 (about five weeks prior to Galileo) and had begun recording his observations in January 1610 at about the same time Galileo was first making his observations. However, since Marius did not publish his observations right away as Galileo had done, his claims were impossible to verify. Since Galileo's work was more reliable and extensive, he is generally given the credit for discovering the moons of Jupiter. In 1614, Marius did provide the names of the Jupiter's moons that we are familiar with today, based on a suggestion from Johannes Kepler:
"This fancy, and the particular names given, were suggested to me by Kepler, Imperial Astronomer, when we met at Ratisbon fair in October 1613. So if, as a jest, and in memory of our friendship then begun, I hail him as joint father of these four stars, again I shall not be doing wrong."
Galileo originally called the Jupiter's moons the "Medicean planets", after the Medici family and referred to the individual moons numerically as I, II, III and IV. Galileo's naming system would be used for a couple of centuries. It wouldn't be until the mid-1800's that the names of the Galilean moons, Io, Europa, Ganymede and Callisto, would be officially adopted, and only after it became very apparent that naming moons by number would be very confusing as new additional moons were being discovered.