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Ganymede: Facts About Jupiter's Largest Moon

Jupiter’s moon Ganymede is the largest satellite in the solar system. Larger than Mercury and Pluto, and only slightly smaller than Mars, it would easily be classified as a planet if were orbiting the sun rather than Jupiter.

Voyager Ganymede Jupiter
NASA's Voyager 2 spacecraft snapped this color image of Jupiter's moon Ganymede, the largest satellite in the solar system, on July 7, 1979 from a distance of 745,000 miles (1.2 million kilometers).
Credit: NASA

Facts about Ganymede

Age: Ganymede is about 4.5 billion years old, about the same age as Jupiter.

Distance from Jupiter: Ganymede is the seventh moon and third Galilean satellite outward from Jupiter, orbiting at about 665,000 miles (1.070 million km). It takes Ganymede about seven Earth days to orbit Jupiter.

Photos of Ganymede, Jupiter's Largest Moon

Size: Ganymede’s mean radius is 1,635 miles (2,631.2 km). Due to its size, Ganymede can be viewed with the naked eye. Early Chinese astronomical records show the discovery of a moon of Jupiter, mostly likely the first observation of Ganymede. Although Ganymede is larger than Mercury it only has half its mass, classifying it as low density.

Temperature: Daytime temperatures on the surface average -171F to -297F, and night temperatures drop to -193C. It is unlikely that any living organisms inhabit Ganymede.

This montage compares New Horizons' best views of Ganymede, Jupiter's largest moon, gathered with the spacecraft's Long Range Reconnaissance Imager (LORRI) and its infrared spectrometer, the Linear Etalon Imaging Spectral Array (LEISA).
This montage compares New Horizons' best views of Ganymede, Jupiter's largest moon, gathered with the spacecraft's Long Range Reconnaissance Imager (LORRI) and its infrared spectrometer, the Linear Etalon Imaging Spectral Array (LEISA).
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

The Discovery of Ganymede

Ganymede was discovered by Galileo Galilei on Jan. 7, 1610. The discovery, along with three other Jovian moons, was the first time a moon was discovered orbiting a planet other than Earth. Galileo’s discovery eventually led to the understanding that planets orbit the sun, instead of our solar system revolving around Earth.

Ganymede’s Name

Galileo called this moon Jupiter III. When the numerical naming system was abandoned in the mid-1800s, the moon was named after Ganymede, a Trojan prince in Greek mythology. Zeus, a counterpart of Jupiter in Roman mythology, carried Ganymede, who had taken the form of an eagle, to Olympus, where he became a cupbearer to the Olympian gods and one of Zeus’ lovers.

Exploration of Ganymede

Several spacecraft have flown by Jupiter and its moons. Pioneer 10 arrived first, in 1973, followed by Pioneer 11 in 1974. Voyager 1 and Voyager 2 returned striking photos during their flybys. The Galileo spacecraft passed as low as 162 miles (261 km) over the surfaces of the Galilean moons and produced detailed images.

Ganymede sound and signal illustration

Characteristics of Ganymede

Ganymede has a core of metallic iron, which is followed by a layer of rock that is topped off by a crust of mostly ice that is very thick. There are also a number of bumps on Ganymede’s surface, which may be rock formations.

In February 2014, NASA and the United States Geological Survey unveiled the first detailed map of Ganymede in images and a video animation created using observations from NASA's Voyager 1 and Voyager 2 spacecraft, as well as the dedicated Jupiter-orbiting Galileo spacecraft.

Ganymede’s surface is made up of primarily two types of terrain—about 40 percent is dark with numerous craters and 60 percent is lighter in color has grooves that form intricate patterns to give the satellite its distinctive appearance. The grooves, which were likely formed as a result of tectonic activity or water being released from beneath the surface, are as high as 2,000 feet and stretch for thousands of miles.

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.

It is believed that Ganymede has a saltwater ocean 124 miles below its surface, compared to Europa, which has an extensive ocean closer to the surface.

New Map Reveals Geology of Jupiter's Moon Ganymede
A global image mosaic of Jupiter's moon, Ganymede created with images from the Voyager and Galileo missions.
Credit: Wes Patterson

Ganymede’s Atmosphere

Ganymede has a thin oxygen atmosphere – too thin to support life. It is the only satellite in the solar system to have a magnetosphere. Typically found in planets, including Earth and Jupiter, a magnetosphere is a comet-shaped region in which charged particles are trapped or deflected. Ganymede’s magnetosphere is entirely embedded within the magnetosphere of Jupiter.

- Kim Ann Zimmermann

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