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Jupiter's Galilean Moons: Ganymede

by Dani Johnson

June 14, 2013

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Image Above: Natural color view of Ganymede from the Galileo spacecraft during its first encounter with the satellite.

Image Credit: NASA/JPL

Ganymede [gan-uh-meed] is the seventh satellite from Jupiter, the biggest planet in the Solar System. Similarly, Ganymede is the biggest satellite in the Solar System. This icy moon stands out as the only satellite with an internally generated magnetosphere and is one of three Jovian moons that may have a liquid water ocean under their thick, icy surfaces.

Name and Discovery:

Ganymede was discovered in 1610 by Galileo Galilei along with Io, Europa and Callisto. He called them "The Medicean Stars" after the wealthy Medici family. Fellow astronomers Simon Marius and Johannes Kepler discussed the idea of naming the moons after the mythical loves of Jupiter, but the idea didn't catch on for a couple hundred years. Most astronomers called them I, II, III and IV in the order of distance from Jupiter. In the 17th century we were discovering so many moons of Jupiter and Saturn that astronomers finally started calling them the familiar mythological names we know now.

Quick Facts:

  • Jupiter's four largest satellites are Io, Ganymede, Callisto, and Europa.

  • Ganymede is thought to be about the same age as Jupiter, 4.5 billion years old.

  • Ganymede is about 1.07 million kilometers from Jupiter. That's almost three times farther than the distance from the Earth to the Moon!

  • Ganymede is the seventh satellite from Jupiter.

  • It takes Ganymede a little more than seven Earth days to complete a full orbit around Jupiter, it takes our own Moon about 27 Earth days to fully orbit around the Earth.

  • Ganymede is tidally locked, meaning that one side of the satellite faces Jupiter at all times. It also means that, like our own Moon, Ganymede's day is the same length as its year.

  • Ganymede's diameter is 5,262 kilometers, making it 1 times bigger than Earth's Moon and only 2 times smaller than Earth.

  • Daytime temperatures on the surface of Ganymede are in the range of -113C (-171F) to -183C (-297F). That's only 24C to 94C warmer than the coldest recorded temperature on Earth, in Antarctica!.

  • Jupiter's moon Ganymede is the largest satellite in the solar system.

  • Ganymede would easily be considered a planet if it were orbiting the Sun instead of Jupiter.


Pioneer 10 was the first mission to return pictures of the Jovian moon. Voyager 1 and Voyager 2 were the first to deliver spectacular, high-res pictures of the satellite, but Galileo has gathered the most data in its flybys past Ganymede. On its way to Pluto New Horizons even snapped some images and gathered a little bit of data about Ganymede. Right now, Juno is en route to Jupiter and will arrive in 2016. Much like Cassini, the orbiter's primary objective is to learn about the parent planet but the close proximity will surely provide countless opportunities to observe and gather data from the Jovian satellites. The European Space Agency is also planning to launch an orbiter, JUICE, in 2022 that will spend at least three years making detailed observations and gathering data about Jupiter and the four Galilean Moons.

Above Image: Close-up of Arbela Sulcus, a long, flat feature on Ganymede. Liquid water may have flowed from within Ganymede, filling a depression, then cooling to create a smooth surface.

Image Credit: NASA/JPL

Physical Characteristics:

Ganymede is thought to have a molten iron core and a rocky mantle with a surface layer that consists mainly of silicate rock and water ice. The iron core would likely be the cause of the magnetosphere detected around the Jovian moon. Ganymede is the only satellite in the solar system with an internally generated magnetosphere, which is snugly embedded inside of Jupiter's massive magnetosphere. Galileo's Near-Infrared Mapping Spectrometer has found evidence of hydrated salt minerals on the surface of Ganymede. These minerals are thought to be mostly frozen magnesium sulfate brines that come from a layer of fluid below the surface, much like what we think is on Europa. This, along with other magnetic and surface evidence, suggests that Ganymede, as well as Europa and Callisto, might have a liquid water ocean under the icy surface.

Above Image: Galileo has eyes that can see more than ours can. By looking at what we call the infrared wavelengths, the NIMS (Near Infrared Mapping Spectrometer) instrument can determine what type and size of material is on the surface of a moon. Here, 3 images of Ganymede are shown. Left: Voyager's camera. Middle: NIMS, showing water ice on the surface. Dark is less water, bright is more. Right: NIMS, showing the locations of minerals in red, and the size of ice grains in shades of blue.

Image Credit: NASA/JPL

Surface Features:

About 40% of Ganymede's icy surface is a dark brown region peppered with craters. The rest is a much lighter, grooved region with relatively few craters that probably developed at the expense of the darker region. Astrogeologists aren't sure what caused the grooves and ridges present on Ganymede's surface, but theories include tectonic fault lines and even water possibly being released under the surface. Both light and dark regions are peppered with craters that have rays surrounding them that is probably icy material from right under the surface.

Above Image: Ganymede as seen by Galileo

Image Credit: NASA/JPL

Above Image: This color picture as acquired by Voyager 1 during its approach to Ganymede

Image Credit: NASA/JPL

Ganymede also has irregular lumps (pictured below) that may be caused by rock formations either inside of ice or even completely under ice.

Above Image: The reported bulges reside in the interior, and there are no visible surface features associated with them. This tells scientists that the ice is probably strong enough, at least near the surface, to support these possible rock masses from sinking to the bottom of the ice for billions of years. But this anomaly could also be caused by piles of rock at the bottom of the ice. "The anomalies could be large concentrations of rock at or underneath the ice surface. They could also be in a layer of mixed ice and rock below the surface with variations in the amount of rock," said Dr. John Anderson, a scientist and the paper's lead author at JPL. "If there is a liquid water ocean inside Ganymede's outer ice layer there might be variations in its depth with piles of rock at the ocean bottom. There could be topographic variations in a hidden rocky surface underlying a deep outer icy shell. There are many possibilities, and we need to do more studies."

Image Credit: NASA/JPL

Above Image: This image shows some unusual features on the surface of Jupiter's moon, Ganymede. NASA's Galileo spacecraft imaged this region as it passed Ganymede during its second orbit through the Jovian system. The region is located at 31 degrees latitude, 186 degrees longitude in the north of Marius Regio, a region of ancient dark terrain, and is near the border of a large swathe of younger, heavily tectonised bright terrain known as Nippur Sulcus. Situated in the transitional region between these two terrain types, the area shown here contains many complex tectonic structures, and small fractures can be seen crisscrossing the image. North is to the top-left of the picture, and the sun illuminates the surface from the southeast. This image is centered on an unusual semicircular structure about 33 kilometers (20 miles) across. A 38 kilometer (24 miles) long, remarkably linear feature cuts across its northern extent, and a wide east-west fault system marks its southern boundary. The origin of these features is the subject of much debate among scientists analyzing the data. Was the arcuate structure part of a larger feature? Is the straight lineament the result of internal or external processes? Scientists continue to study this data in order to understand the surface processes occurring on this complex satellite.

Image Credit: NASA/JPL/Brown University

Above Image: Complex tectonism is evident in these images of Ganymede's surface. The solid state imaging camera on NASA's Galileo spacecraft imaged this region as it passed Ganymede during its second orbit through the Jovian system. The 80 kilometer (50 mile) wide lens-shaped feature in the center of the image is located at 32 degrees latitude and 188 degrees longitude along the border of a region of ancient dark terrain known as Marius Regio, and is near an area of younger bright terrain named Nippur Sulcus. The tectonism that created the structures in the bright terrain nearby has strongly affected the local dark terrain to form unusual structures such as the one shown here. The lens-like appearance of this feature is probably due to shearing of the surface, where areas have slid past each other and also rotated slightly. Note that in several places in these images, especially around the border of the lens-shaped feature, bright ridges appear to turn into dark grooves. Analysis of the geologic structures in areas like this are helping scientists to understand the complex tectonic history of Ganymede.

Image Credit: NASA/JPL/Brown University

Above Image: View of two impact craters that are superimposed on Memphis Facula, a large bright circular feature in the otherwise generally dark terrain in Galileo Regio on Jupiter's moon, Ganymede. These are thought to be impact craters because they share many of the features of such structures on other planets, including steep walls, flat floors, and central mountain peaks. Bright icy material is exposed on the walls, rims and peaks of these features and darker material can be seen covering the floors and streaming down the inner walls of the craters. The dark material may have been concentrated on the crater floors during the impact events. A dark line near the crater rim may be exposures of layered bedrock which has been uplifted. These craters have been degraded to the degree that their ejecta and surrounding secondary crater fields are no longer visible. The crater on the left (Chrysor) is about 6 kilometers (km) in diameter and the larger one on the right (Aleyn) is about 12 km wide. Smaller craters are seen as bright circles on the crater floors and in the surrounding areas. The density of these superposed impact features allows scientists to estimate the age of the surface and the age of the craters, thought to be many hundreds of millions of years old. Memphis Facula, a large 350 km diameter bright feature on which the craters are situated, appears to have formed from the excavation of bright water ice material during an ancient, large impact event.

Image Credit: NASA/JPL/Brown University

Above Image: View of a chain of craters named Enki Catena on Jupiter's moon, Ganymede. This chain of 13 craters probably formed by a comet which was pulled into pieces by Jupiter's gravity as it passed too close to the planet. Soon after this breakup, the 13 fragments crashed onto Ganymede in rapid succession. The Enki craters formed across the sharp boundary between areas of bright terrain and dark terrain, delimited by a thin trough running diagonally across the center of this image. The ejecta deposit surrounding the craters appears very bright on the bright terrain. Even though all the craters formed nearly simultaneously, it is difficult to discern any ejecta deposit on the dark terrain. This may be because the impacts excavated and mixed dark material into the ejecta and the resulting mix is not apparent against the dark background.

Image Credit: NASA/JPL/Brown University

Above Image: This image is a computer-generated perspective view of ridges in the Uruk Sulcus region of Jupiter's moon, Ganymede. This area is part of the bright grooved terrain that covers over half of Ganymede's surface, where the icy surface has been fractured and broken into many parallel ridges and troughs. Bright icy material is exposed in the crests of the ridges, while dark material has collected in low areas. The topographic information, which was generated from imaging of the same area on two successive flybys of Ganymede by NASA's Galileo spacecraft, reveals elevation differences of a few hundreds of meters between the highest and lowest points in this area.

Image Credit: NASA/JPL/Brown University


The atmosphere on Ganymede is thin and made up almost entirely of Oxygen. The atmosphere is caused by charged particles from Jupiter's radiation belt that hit Europa's icy surface and produce water vapor. The same radiation coming off of Jupiter hits the water vapor and knocks the hydrogen away from the oxygen. Because hydrogen weighs less it escapes and leaves the oxygen behind.

Orbit and Rotation:

Ganymede, Europa and Io orbit their planet in a 1:2:4 resonance, meaning that for every time Ganymede orbits one time Europa orbits twice and Io orbits four times. They are also tidally locked, which means that each moon keeps the same hemisphere facing Jupiter during their orbit.

The orbit of Ganymede isn't very eccentric, meaning that it is almost circular rather than oval, but it's possible that some of the surface features were caused during a past occurrence of a more eccentric orbit. An eccentric orbit would cause the satellite to alternate between being closer to Jupiter and then farther away from Jupiter over and over again. The changes in the massive gravitational pull from the Planet would deform and crack the surface of the satellite.


In the myth Jupiter, the king of the gods, fell in love with Ganymede and brought him to Mount Olympus on the wings of an eagle and gave him the job as cupbearer so they could be lovers.


Extra Content:

Featured Photo:

Above Image: Why does Jupiter have rings? Jupiter's rings were discovered in 1979 by the passing Voyager 1 spacecraft, but their origin was a mystery. Data from the Galileo spacecraft that orbited Jupiter from 1995 to 2003 later confirmed that these rings were created by meteoroid impacts on small nearby moons. As a small meteoroid strikes tiny Adrastea, for example, it will bore into the moon, vaporize, and explode dirt and dust off into a Jovian orbit. Pictured above is an eclipse of the Sun by Jupiter, as viewed from Galileo. Small dust particles high in Jupiter's atmosphere, as well as the dust particles that compose the rings, can be seen by reflected sunlight.

Image Credit: M. Belton (NOAO), J. Burns (Cornell) et al., Galileo Project, JPL, NASA

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by Dani Johnson

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