Callisto, the moon of Jupiter, is quite special. It is on par in size with the smallest planet, Mercury, and its characteristics and position in the orbit of Jupiter make it one of the best locations in the Solar System for a human base.
Callisto is the third largest moon in our system and the second biggest in orbit of Jupiter after Ganymede. It is also in the most distant orbit out of the four big moons of this enormous gas giant planet. This is far enough to be outside of the harmful effects of Jupiter’s magnetic field, which makes Europa and its fascinating subsurface ocean less accessible to us due to radiation. However, it turns out that Callisto might also have subsurface salty waters, but they are between 100 to 250 km/66 to 155 miles below the ground and might be 150 to 200 km/100 to 133 miles deep.
Salt-loving extremophile microorganisms or halophiles are the most likely type of life that could inhabit such an environment. Obviously, for now, this is just speculative; we have no evidence that life exists there. The ocean on Callisto is only heated by radioactivity and, unlike Europa’s, not by tidal effects, which would help mix water with the rock and provide nutrients for organisms. This is why it is less likely that we will find life there than on Europa or some other moons of gas giant planets.
Being a third bigger than our Moon, Callisto is the most cratered celestial object in the Solar System; its surface is ancient and shows no evidence of volcanism or geological activity. Since about half of Callisto is water and half rock, its cratered surface has, in places, glittery, white frost deposits. This is another feature necessary for the future human base; we could use the local water. Furthermore, the molecular composition of easily accessible material on the surface might make it possible to manufacture rocket fuel.
Its location near Europa makes Callisto additionally attractive for future human presence. This is why there is a great interest to know more about this intriguing moon; it will be visited by the European Space Agency’s Jupiter Icy Moon Explorer (JUICE), which will perform flybys between 2031 and 2034, NASA Clipper Mission flybys in 2030, and Chinese Tianwen-4 will enter the orbit of Callisto around 2030.
The question was: What is special about Callisto?
The size comparison between the Earth, the Moon, and Callisto.
