Evrim Yazgin
Cosmos science journalist
Astronomers believe they have found evidence to support a 30-year-old theory that Jupiter’s moon Callisto has a saltwater ocean.
Callisto is Jupiter’s second largest moon, with a diameter of 4,820km – the same as the solar system’s smallest planet, Mercury, and roughly a third that of Earth.
Callisto is one of the 4 Galilean satellites – so called because these 4 moons were discovered by Galileo Galilei in 1610. They were the first moons ever observed orbiting another planet.
Another of the Galilean satellites, Europa, is believed to have a salty sea of liquid water beneath a layer of ice which could be up to 35km thick.
The idea that Europa has a liquid ocean was first floated after Gerard Kuiper used a ground-based telescope to do a spectroscopic analysis in 1957 of the distant moon and found traces of water ice on its surface.
NASA’s Galileo spacecraft, which orbited Jupiter from 1995 to 2003, obtained even better evidence of a subsurface ocean.
The moon has no magnetic field of its own. However, as the moon passed through Jupiter’s powerful magnetic field, Galileo’s instruments detected a faint magnetic signature inside Europa – the most likely cause of this is a global ocean of salty water.
But the spacecraft also spotted similar signals within Callisto.
However, those results have remained inconclusive because the moon has an intense ionosphere – an upper layer of the atmosphere which is highly electrically conductive.
Scientists have been shy about ascribing the magnetic signature to a salty ocean because of this.
A new analysis of Galileo’s data published in the journal AGU Advances might confirm that the magnetic field is because of a subsurface ocean and not Callisto’s ionosphere.
It is the first time that all available magnetic measurements from Galileo’s 8 close fly-bys of Callisto have been incorporated into the same analysis.
“Our results suggest that Callisto’s response more likely arises from the combination of a thick conductive ocean and an ionosphere rather than from an ionosphere alone,” the authors write.
Modelling suggests that the ocean is thick – possibly up to 110km with a seafloor 260km beneath the surface ice layer. For comparison Earth’s oceans are on average 3.7km deep, and the deepest is the Mariana Trench at 11km.
NASA’s Europa Clipper mission which successfully launched in October 2024, is to conduct a detailed science investigation of Europa. The mission will also fly by Callisto multiple times, no doubt helping determine the presence and characteristics of any subsurface oceans of liquid water on Callisto.
