Jupiter's Moon Callisto is Very Likely an Ocean World
by Sarah Stanley, Eos
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.
Overview of Current Research
Recent studies have revisited the Galileo data in more detail. Unlike previous research, 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.
Methodology
The researchers utilized advanced statistical techniques alongside computational models of Callisto's ionosphere and geophysical properties to examine whether a subsurface ocean is compatible with all existing data:
| Measurement Type | Data Source | Description |
|---|---|---|
| Magnetic Measurements | NASA's Galileo Spacecraft | Magnetic data collected during eight close flybys of Callisto. |
| Statistical Analysis | Current Study | Advanced techniques used for interpreting the complex data set. |
| Modeling | Geophysical Computational Models | Models used to simulate the conditions of Callisto’s underground ocean. |
Findings
The team concluded that Callisto's ionosphere solely cannot explain all existing observations. However, a combination of a subsurface ocean and the ionosphere aligns very well with the data available. The predictions suggest that the ocean is likely at least tens of kilometers thick and is encased beneath a solid ice shell that itself could range from tens to hundreds of kilometers thick.
Significance of the Findings
This research sets the stage for upcoming spacecraft measurements that will be captured in the near future and could confirm whether Callisto is indeed an ocean world. Close-up measurements are scheduled for NASA's Europa Clipper and the European Space Agency's JUICE (Jupiter Icy Moons Explorer), both of which are already launched. China's planned Tianwen-4 mission may also observe Callisto.
Implications for Astrobiology
Should Callisto's status as an ocean world be confirmed, it would likely invoke further investigations into its potential to support life, drawing parallel interests akin to research inspired by the confirmation of Europa's ocean.
More information: Corey J. Cochrane et al, Stronger Evidence of a Subsurface Ocean Within Callisto From a Multifrequency Investigation of Its Induced Magnetic Field, AGU Advances (2025). DOI: 10.1029/2024AV001237
Journal information: AGU Advances Open Journal
Conclusion
The findings presented potentially alter our understanding of this icy moon and the broader implications for life beyond Earth. With ongoing and future missions on the horizon, the scientific community anticipates a wealth of data that could reveal much about Callisto's underground ocean and its possible habitability.
Literature Cited
1. Cochrane, Corey J. et al. (2025). Stronger Evidence of a Subsurface Ocean Within Callisto From a Multifrequency Investigation of Its Induced Magnetic Field. AGU Advances.
2. NASA (1996). Galileo Orbiter USGS Press Release, "Galileo to Study Jupiter's Moons and Magnetosphere".
3. Principal Investigator Report on Europa Clipper.
4. European Space Agency’s JUICE Mission Overview.
5. American Geophysical Union. (2020). Callisto's Salty Ocean: Understanding the Evidence.
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