Ganymede: Exploration

A half-globe view of Jupiter's moon, Ganymede. The moon is gray with a big crater on the right side of the image. The moon also has streaks of lighter and darker materials.
This enhanced image of the Jovian moon Ganymede was obtained by the JunoCam imager aboard NASA's Juno spacecraft during the mission's June 7, 2021, flyby of the icy moon on Juno's 34th pass close to Jupiter.
NASA/JPL-Caltech/SwRI/MSSS/Kalleheikki Kannisto © CC BY

The discovery of Ganymede and Jupiter’s three other largest moons - Europa, Io, and Callisto - forever changed the way we view our solar system.

On Jan. 7, 1610, Italian astronomer Galileo Galilei was looking at Jupiter through his homemade telescope when he spotted what he thought were three small, bright stars near Jupiter. Soon, Galileo determined there were four “stars.” Within days, he had figured out that these points of light were actually moons orbiting Jupiter. These four moons were the first objects found to orbit something other than Earth or the Sun, providing strong evidence for the idea, known as the Copernican theory, that most celestial objects did not revolve around Earth.

The German astronomer Simon Marius may have observed the moons around the same time as Galileo, but Galileo is credited with the discovery. Galileo published his findings in March 1610 in a book titled “Sidereus Nuncius” or “The Starry Messenger.”

This montage shows Jupiter's four large and diverse "Galilean" satellites as seen by the Long Range Reconnaissance Imager (LORRI) on the New Horizons spacecraft during its flyby of Jupiter in late February 2007. The four moons are, from left to right: Io, Europa, Ganymede and Callisto. The images have been scaled to represent the true relative sizes of the four moons and are arranged in their order from Jupiter.
NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Galileo originally called Jupiter's moons the Medicean planets after his patrons, the Medici family. He also referred to the moons numerically as I, II, III, and IV. But it is the names that Simon Marius proposed for the moons – names suggested to him by fellow astronomer Johannes Kepler – that we use today: Io, Europa, Ganymede, and Callisto. The four moons collectively are referred to as the Galilean moons.

More than 360 years after they were discovered, NASA sent a spacecraft to study the Galilean moons. Pioneer 10 had its closest encounter with Jupiter on Dec. 3, 1973. It was the first spacecraft to obtain close-up images of Jupiter, and three of its largest moons: Ganymede, Callisto, and Europa. The spacecraft did not image Io.

Jupiter's moon Ganymede appears fuzzy, and yellowish in this image taken by the Pioneer 10 spacecraft.
This image of Ganymede was taken by NASA's Pioneer 10 on Dec. 3, 1973. Pioneer 10 was the first spacecraft to obtain close-up images of Jupiter, and three of its largest moons: Ganymede, Callisto, and Europa. Io wasn't imaged.

Pioneer 11, the sister spacecraft to Pioneer 10, took the first images of Jupiter’s polar region, flying three times closer to Jupiter than Pioneer 10. The spacecraft flew closest to Jupiter on December 3, 1974, soaring about 26,570 miles (42,760 kilometers) above the planet's cloud tops – three times closer than Pioneer 10. The spacecraft passed within 430,200 miles (692,200 kilometers) of Ganymede and discovered a high electron current flow at Ganymede’s orbit.

Ganymede Voyager 1
This picture of Ganymede was taken by Voyager 1 on March 5, 1979, from a range of 151,800 miles (253,000 kilometers). The surface shows numerous impact craters, many of which have extensive bright ray systems. Light bands traversing the surface contain alternating bright and dark lines which probably represent deformation of the icy central material.

Before NASA’s Voyager 1 and Voyager 2 arrived at Jupiter in 1979, astronomers thought that Saturn's moon Titan was the largest in the solar system. But measurements from the Voyager probes showed Ganymede is bigger than Titan, even bigger than Mercury and Pluto. Images from Voyager 1 also revealed that Ganymede had two distinct types of terrain – cratered and grooved – suggesting to scientists that the moon’s entire icy crust has been under tension from global tectonic processes.

Jupiter's moon Ganymede appears brownish gray with white splotches.
This natural color image of Ganymede was taken by NASA's Galileo spacecraft during its first encounter with the satellite. North is to the top of the picture and the Sun illuminates the surface from the right. The dark areas are the older, more heavily cratered regions and the light areas are younger, tectonically deformed regions. The brownish-gray color is due to mixtures of rocky materials and ice. Bright spots are geologically recent impact craters and their ejecta. The finest details that can be discerned in this picture are about 13.4 kilometers across. The images which combine for this color image were taken beginning at Universal Time 8:46:04 UT on June 26, 1996.

The Galileo spacecraft, the first to orbit Jupiter, made the major discovery that Ganymede has its own magnetosphere – a region of charged particles that surrounds some planets but had never before been found around a moon. Galileo made several flybys of Ganymede with multiple instruments collecting data on the moon.

Galileo even captured sounds of whistling and static caused by Ganymede's magnetosphere. The probe flew within 519 miles (835 kilometers) of Ganymede, sending back stunning close-up images showing ancient cratered ice fields adjacent to or overlain by younger ice volcanic plains, ridged ice mountains, deep furrows, and smooth broad basins that are products of tectonic forces.

Giant Jupiter with its reddish brown and white swirls looms large over the moon, Ganymede - which looks like a small gray and white marble.
Jupiter casts a baleful eye toward the moon Ganymede in this enhanced-contrast image from NASA's Cassini spacecraft.
NASA/JPL/University of Arizona

During Galileo’s extended mission, NASA’s Cassini spacecraft flew by Jupiter on its way to Saturn. For a few weeks, both spacecraft observed Jupiter together. Cassini sent back amazing images of Ganymede dwarfed by Jupiter as it made its closest approach to the planet in December 2000.

Grayish moon Ganymede with bright spots.
This is New Horizons' best image of Ganymede, taken with the spacecraft's Long Range Reconnaissance Imager (LORRI) camera at 10:01 Universal Time on Feb. 27, 2007, from a range of 2.2 million miles (3.5 million kilometers). Dark patches of ancient terrain are broken up by swaths of brighter, younger material, and the entire icy surface is peppered by more recent impact craters that have splashed fresh, bright ice across the surface.
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

NASA’s New Horizons spacecraft came within 1.4 million miles (2.3 million kilometers) of Jupiter in 2007 as it headed to Pluto. It was the eighth spacecraft to visit Jupiter. As it approached, New Horizons took images of Ganymede, Callisto, Io, and Europa. It captured nearly 700 observations, including images of Ganymede on Feb. 27, 2007.

Blue and white bands circle grayish moon Ganymede
Hubble images of Ganymede's auroral belts (colored blue for illustration) are overlaid on a Galileo orbiter image of the moon. The amount of rocking of the moon's magnetic field suggests that the moon has a subsurface saltwater ocean.

NASA’s Hubble Space Telescope has provided important contributions to what we know about Ganymede – including the best evidence yet that Ganymede may have an underground saltwater ocean, opening up the possibility that Jupiter’s largest moon is another candidate for life beyond Earth. Because Ganymede has a magnetic field, it has auroras. Since Ganymede is close to Jupiter, its magnetic field lies within, or is embedded in, Jupiter’s magnetic field - a field within a field. (Ganymede is the primary scientific target of ESA’s JUICE mission which will also study Jupiter’s moons Europa, and Callisto.)

When Jupiter’s magnetic field changes, the auroras on Ganymede also change, “rocking” back and forth. By watching the rocking motion of the two auroras, scientists using Hubble were able to determine that a large amount of saltwater exists beneath Ganymede’s crust affecting its magnetic field.

On June 7, 2021, NASA’s Juno spacecraft flew closer to Jupiter’s ice-encrusted moon Ganymede than any spacecraft in more than two decades. Less than a day later, Juno made its 34th flyby of Jupiter. This animation provides a “starship captain” point of view of each flyby. For both worlds, JunoCam images were orthographically projected onto a digital sphere and used to create the flyby animation. Synthetic frames were added to provide views of approach and departure for both Ganymede and Jupiter.

During its June 7, 2021, flyby Juno captured its best images yet of Ganymede, flying closer to Jupiter’s largest moon than any spacecraft in more than two decades.

The photos show the surface of Ganymede in remarkable detail, including craters, clearly distinct dark and bright terrain, and long structural features possibly linked to tectonic faults.

A grayish moon with bright spots and a "belly button" crater at the bottom.
This image of Ganymede was obtained by the JunoCam imager aboard NASA’s Juno spacecraft during its June 7, 2021, flyby of the icy moon. At the time of closest approach, Juno was within 645 miles (1,038 kilometers) of its surface – closer to Jupiter’s largest moon than any other spacecraft has come in more than two decades.

“This is the closest any spacecraft has come to this mammoth moon in a generation,” said Juno Principal Investigator Scott Bolton of the Southwest Research Institute in San Antonio. “We are going to take our time before we draw any scientific conclusions, but until then we can simply marvel at this celestial wonder.”​