By Sabine Stanley Ph.D., Johns Hopkins University
Callisto is a Galilean moon of Jupiter and is one of the biggest moons of the solar system. This mixture of rocks and ice looks similar to the largest moon, Ganymede, on the surface. However, a bit further below the surface lies a new world, which might even have one or two life elements.
Craters on Callisto
Callisto looks like Ganymede: almost as huge, as icy, and orbiting Jupiter. However, it is drastically different from the other icy moons. The first difference is that Callisto has neither plate tectonics nor strike-slip tectonics. Secondly, its surface is older than the four-billion-year old Ganymede, but has changed less and was perhaps less heated. The craters on this moon are inevitable.
Callisto is one of the most heavily cratered entities in the solar system. The largest impact crater is Valhalla, with the rings as signs of the waves propagated during the impact. The crater’s diameter is about 600 kilometers, while the rings’ diameters reach 1,800 kilometers. Unlike other planets of a similar size, there is no hill at the opposite side of this crater.
The feature at the opposite side is normally a result of shock waves after the impact. However, none of the large craters on Callisto have an opposing feature on the other side. Something must have absorbed the shock.
Subsurface Ocean in Callisto
A subsurface ocean might be the shock-absorber after each impact. The Galileo mission also detected a magnetic field, which is further evidence of a probable subsurface ocean under the icy surface. Under the ocean, there is a mixture of ice and rock. The rock dominates the mixture with depth. However, there is no evidence of an iron core. This shows a different formation process.
This is a transcript from the video series A Field Guide to the Planets. Watch it now, on Wondrium.
Most moons were hot enough to melt iron and form a dense core, but Callisto formed slowly enough not to heat up and melt. The usual rapid growth of a protoplanetary object from collisions of smaller objects did not happen in Callisto. Thus, it does not have a layered structure that leads to a core.
Considering the four spherical moons and their orbits, one can conclude that Callisto had a different story. The moons have perhaps accreted from a massive disk around Jupiter. Hence, any moon created from the disk would be absorbed by Jupiter due to frictional interaction with the disk. It took several generations of moons until the mass of the disk was low enough to create enduring moons—many of them.
Learn more about mighty Jupiter, the ruling gas giant.
Other Moons Orbiting Jupiter
Jupiter has a mini solar system of its own. In 1892, 282 years after the four Galilean moons were discovered, the fifth Jovian moon was observed: Amalthea. There was a good reason for such a long time: the other moons are so small that do not even have enough gravity to become spherical.
The most massive non-Galilean moon is Himalia, 7,000 times less massive than the smallest Galilean moon, Europa. In 2018, 12 new moons were discovered almost accidentally and joined the previously-discovered moons. Thus far, Jupiter has 79 moons, divided into two main categories.
Jovian moons are divided into two categories based on their location and orbit properties:
- The regular satellites with prograde and nearly circular orbits with low inclination, like Galilean moons.
- The irregular satellites, orbiting Jupiter on more elliptical and inclined orbits.
Other than Galilean moons, there are four other moons interior to the Galilean moons: Metis, Adrastea, Amalthea, and Thebe. Jupiter’s rings are made up of the dust created by the collisions of these moons. The particles are too small to make unique rings for Jupiter, especially compared to Saturn’s rings of large ice boulders.
Most of the moons around Jupiter orbit it in a retrograde direction, that is, are irregular. It seems that they were not even formed from the disk that created Galilean moons, and were captured by Jupiter. The farthest orbits, mainly those beyond the Hill sphere radius, are only stable in a retrograde direction. The moons are further categorized into families.
Learn more about Saturn’s moons—Titan to Enceladus.
Each family has similar characteristics, and it is likely that the members are fragments of a larger moon that underwent a large collision and broke apart in the past.
The smallest moon discovered, Valetudo, is about one kilometer in extent. This tiny moon orbits Jupiter in a prograde direction among mostly retrograde moons. Valetudo might not get too old as the possibility of colliding with other moons is high.
All of this shows how great an impact Jupiter has on its surrounding, even beyond its Hill sphere. Many of the moons are much younger than Jupiter itself, which means Jupiter keeps affecting the solar system with its enormous mass.
Common Questions About Callisto
Callisto is the Greek mythology name that Simon Marius picked for the moon after he discovered the four main moons.
Not as life-friendly as Europa, Callisto has shown some evidence of subsurface water oceans. However, no solid proof has depicted life there yet.
One day on Callisto, the second-largest moon of Jupiter and the third-largest moon of the solar system, equals approximately 16.7 Earth-days.
Jupiter has, in fact, 79 moons discovered so far. Callisto is one of the four main ones, and there are 75 other moons as well.