Callisto Is Very Likely an Ocean World
More pocked with craters than any other object in our solar system, Jupiter’s outermost and second-biggest Galilean moon, Callisto, appears geologically unremarkable. In the 1990s, however, NASA’s Galileo spacecraft captured magnetic measurements near Callisto that suggested that its ice shell surface—much like that of Europa, another moon of Jupiter—may encase a salty, liquid water ocean.
Forward model parameter space for ocean plus ionosphere models. Left: Schematic diagram of the 4-layer interior model of Callisto, consisting of a non-conductive mantle, a conductive ocean, a non-conductive ice-shell, and a conductive ionosphere. Right: Ocean parameter space used in this study, with eight linear steps of in ocean thickness (10–150 and also including the no ocean case when ) and 10 uniform steps in log space for ocean conductivity (0.1–10.0 ). This parameter space was evaluated for six different seafloor depths (130, 170, 210, 250, 290, ), then each of these were convolved with ionospheres across 51 uniform log steps in conductance, totaling more than thousand combinations. — AGU Advances
But evidence for Callisto’s subsurface ocean remained inconclusive, as the moon has an intense ionosphere. Scientists thought this electrically conductive upper part of the moon’s atmosphere might imitate the magnetic fingerprint of a salty, conductive ocean.
Now, Cochrane et al. have revisited the Galileo data in more detail. Unlike in prior studies, this team incorporated all available magnetic measurements from Galileo’s eight close flybys of Callisto. Their expanded analysis much more strongly suggests that Callisto hosts a subsurface ocean.
In addition to reanalyzing the flyby data with advanced statistical techniques, they used computational models of Callisto’s ionosphere and geophysical properties to examine whether a subsurface ocean is compatible with all available information.
A cross section of Jupiter’s moon Callisto, showing the ionosphere surrounding the planet, the ice shell encasing it, a saltwater ocean beneath the ice, and a rocky mantle beneath the ocean. Colorful magnetic field lines move around the whole planet.
Observations acquired from the Galileo spacecraft indicate that Callisto (left) reacts inductively to Jupiter’s (right) time-varying magnetic field. New research suggests that this reaction, and its results, are indicative of the moon hosting a subsurface salty ocean. Credit: Corey J. Cochrane, NASA/JPL-Caltech
They found that Callisto’s ionosphere alone cannot explain all existing observations, but that a subsurface ocean in combination with the ionosphere can. Further exploring which scenarios best fit the data, the researchers predicted that the ocean is likely at least tens of kilometers thick, as measured from the top of the liquid ocean to its seafloor, and encased beneath a solid ice shell that itself could range from tens to hundreds of kilometers thick. Beneath the probable ocean lies a rocky interior.
These findings set the stage for spacecraft measurements that will be captured in the near future and should confirm, once and for all, whether Callisto is an ocean world. Close-up measurements are scheduled for NASA’s Europa Clipper and the European Space Agency’s JUICE (Jupiter Icy Moons Explorer) missions, both already launched. China’s planned Tianwen-4 mission may also observe Callisto.
Confirmation of Callisto’s status as an ocean world would likely prompt further investigation on its potential to support life—just as the confirmation inspired research about Europa. (AGU Advances, https://doi.org/10.1029/2024AV001237, 2025)
—Sarah Stanley, Science Writer
Citation: Stanley, S. (2025), Jupiter’s moon Callisto is very likely an ocean world, Eos, 106, https://doi.org/10.1029/2025EO250068. Published on 18 February 2025.
Stronger Evidence of a Subsurface Ocean Within Callisto From a Multifrequency Investigation of Its Induced Magnetic Field, AGU Advances (open access)
Astrobiology
