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Moon of Jupiter
Callisto
Jupiter's quiet, crater-covered moon with a hidden salty sea
Key facts
- Diameter
- About 3,000 miles / 4,821 km (mean radius 2,410 km)
- Distance from Jupiter
- About 1,170,000 miles / 1,882,700 km
- One orbit
- About 16.7 Earth days
- One day (rotation)
- About 16.7 Earth days (tidally locked)
- Average temperature
- About −139 °C
- Discovered
- 1610, by Galileo Galilei (Simon Marius observed it independently)
- Named after
- Callisto, a nymph in Greek mythology
- Surface
- Ancient ice and rock, the most heavily cratered in the solar system
- Density
- About 1.83 g/cm³
- Visited by
- Pioneer 10 and 11, Voyager 1 and 2, Galileo, Cassini, New Horizons; JUICE and Europa Clipper to come
About Callisto
Callisto is a big moon of Jupiter. It is old and very cold. Its face is covered in holes. The holes are called craters. Space rocks made them long ago. Nothing much has changed since then. Callisto is dark and dusty. Under the ice there may be a sea. Four big moons go round Jupiter. Callisto is the one farthest out. It takes about seventeen days to go round. You would need a very warm coat there.
Callisto is the second largest of Jupiter's moons and the third largest moon in the solar system. It is about 4,821 km across, which is nearly the size of the planet Mercury. It orbits Jupiter at a distance of about 1.88 million km and takes about 16.7 days to go round once. Like our own Moon, Callisto rotates once every orbit, so the same side always faces Jupiter.
Callisto is the most heavily cratered object in the solar system. Its icy surface is about 4.5 billion years old and has barely changed since. Its largest feature is Valhalla, a huge ring-shaped basin whose rings stretch about 1,800 km from the centre. Callisto is far from Jupiter, so the planet's gravity does not squeeze and heat it as it does Io and Europa.
In the 1990s NASA's Galileo spacecraft found signs of a salty ocean hidden deep beneath the ice. Callisto has only a very thin atmosphere of carbon dioxide and oxygen. Galileo Galilei discovered Callisto in 1610, using one of the first telescopes. It is named after a character from Greek mythology.
Callisto is the outermost of Jupiter's four Galilean moons and, with a mean diameter of about 4,821 km, the third largest moon in the solar system after Ganymede and Titan. It orbits at about 1,882,700 km from Jupiter with a period of 16.69 days, and it is tidally locked, so one hemisphere permanently faces the planet. Galileo Galilei recorded it on 7 January 1610; Simon Marius, observing independently, proposed the name, taken from a nymph in Greek mythology who was loved by Zeus (Jupiter). The Galilean names were not widely adopted until the twentieth century.
Callisto has a density of about 1.83 g/cm³, roughly half rock and half ice, and unlike Ganymede it appears only partly differentiated. Because it lies far from Jupiter and is not locked in orbital resonance with Io, Europa and Ganymede, it has escaped strong tidal heating. The result is a surface about 4.5 billion years old and the most heavily cratered in the solar system. The largest feature, Valhalla, has a bright centre about 600 km across with concentric rings extending about 1,800 km from the centre; the Asgard basin is about 1,600 km across.
Surface temperatures average about 134 K (about −139 °C). Despite this apparent dormancy, magnetometer data from NASA's Galileo orbiter in the 1990s showed an induced magnetic field best explained by a salty subsurface ocean, thought to lie beneath an ice shell 80 to 150 km thick. A very thin atmosphere of carbon dioxide and molecular oxygen has also been detected. Pioneer 10 and 11 passed in the early 1970s, Voyager 1 and 2 imaged more than half the surface in 1979, Galileo made repeated flybys between 1995 and 2003, coming within 138 km, and Cassini (2000) and New Horizons (2007) observed it on their way elsewhere.
Radiation at Callisto is far lower than at Europa or Io, which is why ESA's JUICE mission, launched on 14 April 2023 and arriving at Jupiter in July 2031, plans 21 close flybys, and NASA's Europa Clipper is planned to make nine. A common misconception is that Callisto is geologically dead; its surface is inactive, but its interior may still hold liquid water.
Did you know?
- Callisto's surface is about 4.5 billion years old, almost as old as the solar system itself, and has hardly changed since.
- Beneath its dead-looking crust, Callisto may hide a salty ocean under an ice shell 80 to 150 km thick.
- Callisto is nearly as large as the planet Mercury, but it has only about a third of Mercury's mass because it is half ice.
Neighbours around Jupiter
Questions children ask
Why does Callisto have so many craters?
Callisto's icy surface is extremely old and has never been reshaped by volcanoes or moving ice. Every space rock that hit it over billions of years left a crater that is still there today, so it is the most heavily cratered object we know of.
Could there be life on Callisto?
Possibly, but nobody knows. NASA's Galileo spacecraft found signs of a salty ocean deep beneath the ice. That put Callisto on the list of worlds that might hold life, although its ocean is much deeper and harder to reach than Europa's.
How big is Callisto compared with our Moon?
Callisto is about 4,821 km across, while our Moon is about 3,475 km across, so Callisto is roughly 40 per cent wider. It is nearly the size of the planet Mercury, though much lighter because it is about half ice.
Which spacecraft are going to Callisto next?
ESA's JUICE mission launched on 14 April 2023 and should arrive at Jupiter in July 2031. It plans 21 close flybys of Callisto. NASA's Europa Clipper, launched in October 2024, also plans nine Callisto flybys.
Quick quiz
Three questions for space cadets. Nothing is recorded.
- Who discovered Callisto in 1610?
- What is Callisto famous for?
- About how long does Callisto take to orbit Jupiter?
Ganymede
Europa
The Moon