Callisto is Jupiter’s second-biggest moon, and the third-biggest moon in the solar system, after another one of Jupiter’s moons, Ganymede, and then Titan, the largest moon of the planet Saturn. As with Jupiter’s main moons Io, Ganymede, and Europa, Callisto was discovered by Galileo on January 7th of the year 1610. Below are some more interesting facts about Callisto.
Callisto is named after a mistress of Zeus
The moon is named after Callisto, a nymph, and one of the many paramours of Zeus, the King of the Gods of Greek mythology. Zeus’ wife, Hera, was less than pleased when the affair was discovered and in revenge, she changed Callisto into a bear and placed her in the sky as the constellation Ursa Major.
Callisto is 99% as big as Mercury
With a diameter of 4,820.60 km, Callisto is only 58.4 km smaller than the planet Mercury. However, it weighs only a third as much as the planet, with Callisto weighing in at 107,593,737,963,819 billion kilograms, while Mercury’s mass is 330,104,000,000,000 billion kilograms.
Callisto orbits Jupiter in 16.7 days
On average, Callisto covers a distance of 11,829 191 kilometers during each orbit, at a velocity of 29,530 km/hour. By way of comparison, the Moon orbits the Earth at a relatively sedate 3,683.4 km/h.
Callisto should be a planet, but isn’t
Based purely on its diameter, Callisto should have been a planet given the fact that it is bigger than Pluto, but since it orbits a planet and not the Sun, it is classified as a moon. Orbiting the Sun is one of the primary factors that determine whether a solar system body is a moon or a planet.
Callisto has the most craters in the solar system
Studies have revealed no geologic, tectonic, or volcanic activity on Callisto, which means that the craters on the surface cannot weather, or wear away like they do on say, Mars, where the atmosphere contributes to the weathering of craters. Because of this, the craters on Callisto are in pristine condition, and the surface of the moon is largely unchanged from when it formed about 4.5 billion years ago.
Callisto consists of rock and ice
Callisto consists of about 60% rock, with the remaining 40% being various types of ice. Studies have also found traces of water ice, various organic compounds, carbon dioxide, and different types of silicates on the surface, as well as a possible ocean of liquid water about 100 km or so beneath the surface.
Callisto is brighter than the Moon
Even though at a distance of 628,300,000 km Callisto is 180 times further away from us than our Moon is from us, it is many times brighter than our Moon when viewed through a telescope. This is because an almost uniform layer of ice on Callisto’s surface reflects roughly 20% of the light that falls onto it, whereas our Moon reflects only 0.12% of light.
Callisto is not tidally heated
Unlike the other Galilean moons, Callisto orbits too far away from Jupiter to be effected by the giant planets’ magnetosphere, which means that Callisto is not subject to the tidal friction, orbital, and rotational energy that causes the surfaces of the other moons, such as Europa, to be heated to the point where surface ice melts.
Callisto is tidally locked to Jupiter
Callisto is tidally locked to Jupiter, as our moon is tidally locked to Earth, but the small changes in the eccentricity and inclination of the moon’s orbit around Jupiter means that the percentage of the face of the moon turned toward Jupiter changes slightly over a timescale of several centuries. In practice, this means that an observer on the planet Jupiter would see more than 50% of Callisto over long timescales, much like lunar librations cause an observer on Earth to see about 59% of the Moon.
Callisto may have formed through accretion
Unlike our moon which is thought to have formed when a piece of Earth was torn out during a huge collision with a massive object, the largely undifferentiated nature of Callisto suggests that it might have formed by a process of accretion from the dust and gas that was left over after Jupiter had formed around 4.5 billion years ago.