Ganymede vs the Moon: Size, Mass, Gravity and Ice

The short answer in numbers

The cleanest way to compare two moons is to use one reference for both. NASA JPL's Solar System Dynamics group publishes a single table of planetary satellite physical parameters, and it lists Earth's Moon with a mean radius of 1,737.4 km and Ganymede with 2,631.2 km. Double those and you get diameters of about 3,475 km and 5,262 km. NASA's Ganymede fact page rounds the same figure to about 3,270 miles (5,260 km).

Radius, GM and density come from the JPL table. The Moon's gravity, escape velocity and orbit come from NASA Goddard's Moon Fact Sheet; Ganymede's gravity and escape velocity are the values derived from its mass in standard references, and its orbit and distance are from NASA's Ganymede facts page. The ratio column is our own arithmetic.

Bigger, but lighter on its feet

The surprising row is gravity. Ganymede is about twice as massive as the Moon, yet its surface pull is about 1.43 m/s² against the Moon's 1.62. The reason is the density column. Surface gravity depends on mass divided by the square of the radius, and Ganymede's extra size outruns its extra mass. Its material is much less dense, and standard references describe it as composed of silicate rock and water in approximately equal proportions. The Moon, at 3.34 g/cm³, is almost entirely rock.

So if you weigh 70 kg on Earth, you would feel like roughly 11.6 kg on the Moon and about 10.2 kg on Ganymede, using the surface gravities above against Earth's 9.80 m/s². Escape velocity goes the other way: because Ganymede is so much bigger, a rocket needs about 2.74 km/s to leave it, a little more than the 2.38 km/s needed to leave the Moon.

One more scale check helps. Ganymede is wider than the planet Mercury, which is 4,880 km across, but it has only about 45 percent of Mercury's mass. By diameter it is the ninth-largest object in the solar system, but by mass it is the tenth.

What each one is made of

NASA describes Ganymede as having three main layers: a metallic iron core at the center, a spherical shell of rock around it, and a spherical shell of mostly ice around that. The surface is the top of the ice shell. NASA's page also reports the best evidence yet, from the Hubble Space Telescope, for an underground saltwater ocean. That ocean is estimated to be about 60 miles (100 km) thick, ten times deeper than Earth's ocean, under roughly 95 miles (150 km) of mostly ice, and it is thought to hold more water than all the water on Earth's surface.

The Moon is a different kind of body. NASA's Moon facts give it a core, mantle and crust, with a solid, iron-rich inner core 149 miles (240 km) in radius wrapped in a liquid iron shell about 56 miles (90 km) thick. Its crust is about 25 miles (40 km) thick on the near side and up to about 37 miles (60 km) on the far side. The water that does exist on the Moon is ice concentrated in permanently dark craters near the poles, not a buried ocean.

The two surfaces show their histories. Ganymede's has two kinds of terrain: very old, heavily cratered dark regions and younger, lighter regions crossed by grooves and ridges. The Moon's light highlands and dark maria are a rock story instead, and its volcanoes have been dormant for millions of years.

The magnetic field the Moon lacks

Ganymede is the only moon known to have its own magnetic field, a discovery NASA credits to the Galileo spacecraft in 1996. The field produces auroras, ribbons of glowing, electrified gas in belts around the north and south poles. Because Ganymede orbits inside Jupiter's much larger magnetic field, its auroras rock back and forth when Jupiter's field changes.

That rocking is how the ocean was found. A team led by Joachim Saur of the University of Cologne used Hubble to watch the auroras. In March 2015, scientists reported that measurements of how the aurorae moved confirmed a subsurface ocean, since the amount of rocking pointed to a saltwater ocean beneath the ice. Earth's Moon offers nothing comparable. NASA says it may once have had an internal dynamo, but today its magnetic field is very weak, and Earth's is many thousands of times stronger.

How they move

Both moons are tidally locked, so one side always faces the planet. For Earth's Moon that means a sidereal orbit of about 27.3 days, while the cycle of phases we see from Earth is about 29.5 days. Ganymede's day is the same as its orbit, about seven days and three hours.

Ganymede also belongs to a three-moon rhythm the Moon does not have. For every orbit Ganymede makes, Europa orbits twice and Io four times. NASA explains that this resonance keeps all three orbits slightly elliptical, because the moons keep lining up at the same points and tugging each other. Callisto, Jupiter's other large moon, is not part of it.

Seeing them, and visiting them

From Earth the contest is lopsided. NASA Goddard lists the full Moon's apparent magnitude at about -12.74, while Ganymede shines at about magnitude 4.6, in principle bright enough for the naked eye but lost in Jupiter's glare. Chinese records say the astronomer Gan De saw a small reddish star beside Jupiter in 365 BC, which might have been Ganymede. The accepted discovery came on 7 January 1610, when Galileo Galilei found it along with three other large moons of Jupiter. NASA notes this was the first time anyone had found a moon orbiting a planet other than Earth.

No spacecraft has orbited Ganymede yet. NASA's Juno flew past it on 7 June 2021 at about 1,038 km and photographed its craters and grooved terrain. The European Space Agency's Juice mission launched on 14 April 2023, and ESA lists its arrival at Jupiter for July 2031. Juice is planned to be the first spacecraft to enter orbit around Ganymede itself.

For another giant of the outer solar system, see our Uranus guide.