Ganymede is the largest moon of Jupiter and the largest moon in the Solar System. It is about 5,260 kilometers (3,270 miles) across. That figure is its mean diameter, or average width. Ganymede is wider than the planet Mercury, and it is the only moon known to have its own magnetic field.

Size

NASA gives Ganymede's mean radius as 2,631.2 kilometers, which makes it 5,262 kilometers across. NASA's Jet Propulsion Laboratory lists the same radius, give or take 1.7 kilometers.

Ganymede is a little less than twice as dense as water. Scientists think it has three main layers. At the center is a core of metallic iron. Around it is a shell of rock, and around that is a thick shell made mostly of ice, whose top is the surface.

Size comparisons

• Ganymede is about one and a half times as wide as the Moon, which is 3,475 kilometers (2,159 miles) across.
• Saturn's moon Titan, the second-largest moon and the next smaller object in the explorer, is 5,150 kilometers (3,200 miles) across. That is only about 2% narrower than Ganymede.
• Mars, the next larger object in the explorer, is 6,779 kilometers (4,212 miles) across. Ganymede is a little more than three-quarters as wide.
• Ganymede is about 8% wider than Mercury, which is 4,879 kilometers (3,032 miles) across, but it has less than half of Mercury's mass.
• If Jupiter were shrunk to the size of a basketball, Ganymede would be a bead about 9 millimeters (0.35 inches) across, orbiting about 1.8 meters (6 feet) away.

An ocean under the ice

Ganymede probably has an ocean of salty water beneath its icy crust. NASA estimates that the ocean is about 100 kilometers (60 miles) deep, 10 times deeper than Earth's oceans. It lies under a crust about 150 kilometers (95 miles) thick, made mostly of ice. The ocean may hold more water than all the water on Earth's surface.

The best evidence came from the Hubble Space Telescope. Ganymede has auroras, glowing belts of gas around its north and south poles. As Jupiter's magnetic field changes, the auroras rock back and forth. The amount they rock suggests that a salty ocean lies beneath the surface.

A moon with a magnetic field

Ganymede is the only moon known to make its own magnetic field. NASA's Galileo spacecraft discovered it in 1996. The field comes from Ganymede's metallic iron core, and it is what causes the auroras. Because Ganymede orbits close to Jupiter, its magnetic field lies inside Jupiter's.

Discovery and exploration

Galileo Galilei discovered Ganymede on January 7, 1610. Together with three other large moons of Jupiter that he found, it was among the first moons known to orbit a planet other than Earth. Ganymede circles Jupiter about every 7.2 days, at an average distance of about 1.07 million kilometers (665,000 miles).

NASA's Juno spacecraft photographed Ganymede up close in June 2021. The European Space Agency's Juice spacecraft launched in April 2023. As of 2026, it is due to reach Jupiter in July 2031 and to finish its mission in orbit around Ganymede. It would be the first spacecraft to orbit a moon in the outer Solar System.

Sources

• Jovian Satellite Fact Sheet. NASA, 2023.
• Planetary Satellite Physical Parameters. NASA Jet Propulsion Laboratory.
• Ganymede: Facts. NASA, 2025.
• Ganymede. NASA.
• Titan: Facts. NASA, 2025.
• Juice factsheet. European Space Agency.
• Saturnian Satellite Fact Sheet. NASA, 2025.
• Moon Fact Sheet. NASA, 2024.
• Mars Fact Sheet. NASA, 2025.
• Mercury Fact Sheet. NASA, 2024.
• Jupiter Fact Sheet. NASA, 2024.
• Official Basketball Rules 2026: Basketball Equipment. FIBA, 2026.