The Moons of JUPITER

Jupiter has 63 moons that we know of, and they’re incredibly diverse. Most are small, rocky, inert worlds. A few are probably asteroids that were gravitationally captured when they wandered too close to Jupiter. Many of these captured moons have tilted or backward orbits, and a few of the smaller, inner moons helped form Jupiter’s faint rings.

Four of Jupiter’s largest moons – the Galilean moons – are about the size Earth’s moon or larger. With ice-covered oceans, sulfur-spewing volcanoes, magnetospheres and surfaces covered with traces of geological activity, these moons are fascinating worlds in their own right. Three of these moons may even have internal liquid-water oceans.

Other giant planets orbiting other stars may have similar kinds of moons, and because those moons may harbor life – or at least boast environments friendly to life – they’re important places to explore.

Although Juno’s mission is to explore Jupiter itself, it will also study how the moons influence the planet and its magnetosphere. For example, the gravity of the Galilean moons alters Jupiter’s shape in subtle ways. The planet’s magnetic field also sweeps up particles that are ejected from the moons’ surfaces, filling the magnetosphere.

Inner Moons
These four small moons probably provide most of the material for Jupiter’s wispy rings.    

io

Dotted with erupting volcanoes, Io is the most geologically active moon in the solar system.



IO

Io’s active surface could provide insight into Earth’s early history, when volcanoes dominated the landscape.

Io’s volcanic activity is due to the gravitational influence from Jupiter and other moons. As Io orbits, Jupiter tugs at it from the inside and the other large moons pull from the outside, slightly elongating its orbit. The fluctuating gravitational forces stretch and squeeze Io, generating internal friction and heat that powers its dynamic geology.

Every second, Io’s volcanoes spew out about a ton of particles – mostly sulfur and oxygen compounds. Io is the main supply of material that feeds into Jupiter’s magnetosphere, filling it with a gas of charged particles called plasma.    

ROTATION PERIOD
1.8 DAYS    
TEMPERATURE
-247° F    
DISTANCE FROM JUPITER
262,000 KM    


europa

Below Europa’s icy surface may be a liquid ocean – a possible environment for life.



Europa

We don’t know how thick Europa’s layer of surface ice is, but we do know that it’s smooth, with relatively few craters. There are also cracks caused by the gravitational tugs of Jupiter and the other three large moons. This squeezing and stretching probably helped to form Europa’s subsurface ocean. These gravitational forces also drive volcanic activity on the surface and on the sea floor. On Earth, these kinds of geologically active undersea environments are oases for life.

Many giant planets orbiting other stars probably have icy moons as well, and if life can exist on Europa, then maybe it can take hold elsewhere.

ROTATION PERIOD
3.6 DAYS
TEMPERATURE
274° F
DISTANCE FROM JUPITER
416,900 KM


ganymede

As the largest moon in the solar system, Ganymede is also the only one with its own magnetic field.



Ganymede

The presence Ganymede’s magnetic field suggests that it could have a molten-iron core churning at its center, like Earth. And similar to the surfaces of Europa and Callisto, Ganymede has grooved terrain.

Like Europa, Ganymede might have a subsurface ocean. Any sea that exists is probably sandwiched between two layers of ice – like Callisto – meaning the ocean wouldn’t be in contact with a rocky seafloor. As a result, the ocean wouldn’t have access to any possible sources of energy from the planet’s interior. Coupled with the fact that the ice layer is probably very thick, Ganymede isn’t as likely to be habitable.

ROTATION PERIOD
7.2 DAYS
TEMPERATURE
-256° F
DISTANCE FROM JUPITER
665,100 KM


callisto

Almost as big as Mercury, Callisto is one of the most cratered surfaces in the Jovian system.



Callisto

Covered with craters, Callisto reveals just how many asteroids and comets have whizzed by over the past couple of billion years.

Callisto may have a subsurface ocean just like Europa and Ganymede. Scientists discovered that Callisto has a weak magnetic field that fluctuates with Jupiter’s rotation. One explanation for this is that Jupiter’s rotating magnetic field induces an electrical current inside Callisto, which in turn generates a magnetic field around the moon. One way electrical currents can course through the moon is if there were a subsurface sea filled with charged particles. Callisto’s ocean, if it exists, probably sits between two layers of ice, like Ganymede.

ROTATION PERIOD
16.7 DAYS
TEMPERATURE
-247° F
DISTANCE FROM JUPITER
1,169,900 KM


Galilean Moons
Jupiter's four largest moons are called the Galilean satellites, after Italian astronomer Galileo Galilei, who observed them in 1610.

Outer Moons
More than 50 small moons have orbits farther from Jupiter than Callisto. Most of these outer moons have odd orbits, looping around at large angles relative to Jupiter’s equator or circling in the opposite direction of Jupiter’s rotation. Because moons that formed with Jupiter should be orbiting in the same direction as its rotation and roughly along its equator, these peculiar orbits suggest that these moons were asteroids captured by Jupiter’s gravity.    

Trojan Moons
Thousands of small rocks called the Trojan moons orbit the sun along with Jupiter. Unlike normal moons, the Trojan moons don’t orbit Jupiter. These objects sit in two special locations in the solar system, where the Sun’s and Jupiter’s gravity have canceled out in a way that creates gravitationally stable spots. If Jupiter’s orbit is a clock face, and the planet is at 12 o’clock with the Sun at the center, the Trojan moons are at 10 o’clock and 2 o’clock.