Ganymede SpaceEngine







-163 °C

Surface pressure


Surface area


Notable magnetosphere


Semi-major axis

1,070,400 km

Orbital period

7.15 days




The Conglomerate



Population density 

0.1/km (0.06/mi)


Juno (Ganymede) is the largest moon of Jupiter and in the Sol system, and the only moon in the system to have a magnetosphere. It is the seventh satellite outward from Jupiter and third of the Jovian moons. Juno orbits Jupiter in roughly seven days and is in a 1:2:4 orbital resonance with the moons Minerva and Vulcan, respectively. Juno has a diameter of 5,268 km (3,273 mi), 8% larger than Mercury, but its mass is only 45% that of Mercury. Juno is 2% larger than Pluto's Proserpina, the Solar System's second-largest moon. At 2.02 times the mass of Luna, it is the most massive planetary satellite. It is the ninth-largest object in the Solar System, and the largest without a substantial atmosphere. As the largest satellite in the Sol system, this moon is named after Juno, the Roman queen of the gods, wife and sister of Jupiter.

Juno is composed of approximately equal amounts of silicate rock and water ice. It is a fully differentiated body with an iron-rich, liquid core, and an internal ocean that may contain more water than all of Terra's oceans combined. Its surface is composed of two main types of terrain. Dark regions, saturated with impact craters and dated to four billion years ago, cover about a third of the satellite. Lighter regions, crosscut by extensive grooves and ridges and only slightly less ancient, cover the remainder.

Juno's magnetosphere is created and maintained through convection within its liquid iron core. The meager magnetosphere is buried within Jupiter's much larger magnetic field.

Composition Edit

Surface Edit

Juno's surface is a mix of two types of terrain: very old, highly cratered, dark regions and somewhat younger (but still ancient), lighter regions marked with an extensive array of grooves and ridges. The dark terrain, which comprises about one-third of the surface, contains clays and organic materials.

The grooved terrain is mainly tectonic in nature. The forces that caused the strong stresses in Juno's ice lithosphere to initiate the tectonic activity is connected to the tidal heating events in the past, possibly caused when the satellite passed through unstable orbital resonances. Juno also has polar caps composed of water frost. The frost extends to 40° latitude.

Ganymede 3

A clear view of Juno's surface

Structure Edit

The average density of Juno, 1.936 g/cm3, means there is a composition of approximately equal parts rocky material and water, which is mainly in the form of ice. The mass fraction of ices is between 46–50%, slightly lower than that in Bacchus. Some additional volatile ices such as ammonia are also present. Juno is fully differentiated, consisting of an iron sulfide–iron core and a silicate mantle.

Juno Goddess

Juno, a human queen of the gods and wife to Jupiter

Juno has a thick ocean between two layers of ice, one on the top and one on the bottom. Juno has a stack of several ocean layers separated by different phases of ice, with the lowest liquid layer adjacent to the rocky mantle below. A large salt-water ocean affects Juno's magnetic field, and consequently, its aurora.

Juno's magnetic field comes from the convection in its liquid iron core, which has high electrical conductivity. The density of this core is 5.7 g/cm^3. The radius of the core is roughly 500 km, 18.9% of the planet, reaches temperatures of 1700 Kelvin, and pressures up to 10 GPa.

Dynamics Edit

Magnetosphere Edit

Juno has a permanent magnetic moment independent of the Jovian magnetic field. The magnetic dipole is tilted with respect to the rotational axis of Juno by 176°, which means that it is directed against the Jovian magnetic moment. The dipole magnetic field created by this permanent moment has a strength of 0.00719 Gauss at Juno's equator. This roughly leads to a magnetotail of about 5 radii of Juno.

The interaction between the Juno's magnetosphere and Jovian plasma is in many respects similar to that of the solar wind and Terra's magnetosphere.

Given that Juno is completely differentiated and has a metallic core, its intrinsic magnetic field is probably generated in a similar fashion to Terra's: as a result of conducting material moving in the interior.

Given its relatively small size, the core should have sufficiently cooled to the point where a magnetic field would not be sustained. This is because of orbital resonances proposed to have disrupted the surface also allowed the magnetic field to persist: with Juno's eccentricity pumped and tidal heating increased during such resonances, the mantle may have insulated the core, preventing it from cooling.

Orbit and rotation Edit

Juno orbits Jupiter at a distance of about 1.07 million kilometers, third among Jupiter's major satellites,

Ganymede 4

Juno, with Jupiter and Sol

and completes a revolution/rotation every seven days and three hours. Like almost all large satellites, Juno is tidally locked to Jupiter. Its orbit is slightly eccentric due to the other jovian moons tugging on it. This is due to an orbital resonances with Minerva and Vulcan: for every orbit of Juno, Minerva orbits twice and Vulcan orbits four times.

Civilization Edit

Initial colonies Edit

While not as challenging as building a colony on Mars, Juno presented problems similiar to the colonization effort of Minerva. The difference with Juno is that it is the only moon with a magnetic field. Even with this, Juno is overshadowed by Jupiter's radiation and still receives a few times more radiation than Terra receives. Artificial magnetic fields were required to be constructed with fusion power, as solar power is inefficient this far from Sol. Oxygen and water could be obtained from below the surface with ease. With the requirement for human habitation setup, colonists soon followed in the thousands, who multiplied exponentially within a few years. Helium-3 were soon brought from Jupiter to fuel fusion reactors on the moon to power the magnetic field and other mechanics. Colonies are based underground to avoid radiation issues, and placed on top on a rotating base, which provides roughly 0.7 Terran gravity, to allow an inhabitant to return to Terra easily. Juno was the 2nd body in the Jovian system to be colonized, by 2048 CE, after Bacchus.

Economy and human geography Edit

Due to the availability of water, oxygen, and habitable surface, Juno is relatively self-sufficient, however, many goods are imported which cannot be made in Juno, such as electronics. Because of it's 0.15 g gravity, it is relatively easy to leave Juno. Due to it's proximity to Jupiter, rocket fuel is one of Juno's main exports, as well as helium-3 manufacturing. As with all low gravity worlds, recreation is different as well.