Scientists analyzed data from the Galileo spacecraft, which explored the Jupiter system back in the 1990s. Based on these, they further confirmed suspicions that Callisto, the fourth of the planet’s largest moons, may have an ocean hidden beneath its surface.
Galileo data
Jupiter‘s most distant and second largest moon, Callisto, covered with more craters than any other object in our Solar System, seems geologically unremarkable. However, in the 1990s, NASA’s Galileo spacecraft made magnetic measurements near Callisto that suggested that the surface of its icy shell — similar to the surface of Europa, another Jupiter moon — may be covered by a salty ocean of liquid water.
But evidence for Callisto’s subsurface ocean remained inconclusive because the moon has an intense ionosphere. Scientists thought this electrically conductive upper part of the moon’s atmosphere might mimic the magnetic imprint of a salty, conductive ocean.
Now the researchers have looked at the Galileo data in more detail. Unlike previous studies, this team included all available magnetic measurements from this probe’s eight close flybys past Callisto. Their extended analysis provides much stronger evidence that Callisto contains an underground ocean.
Research on Callisto’s ionosphere
In addition to re-analyzing the flyby data using advanced statistical methods, the team used computational models of Callisto’s ionosphere and geophysical properties to investigate whether the subsurface ocean is compatible with all available information.
They found that Callisto’s ionosphere alone cannot explain all existing observations, but the subsurface ocean combined with the ionosphere can. In exploring which scenarios best fit the data, the researchers suggested that the ocean is probably at least tens of kilometers thick when measured from the top of the liquid ocean to its seafloor, and that it is enclosed beneath a solid ice shell that could be tens to hundreds of kilometers thick. Beneath the probable ocean lies a rocky interior.
Future research by spacecraft
These findings set the foundation for spacecraft measurements to be made in the near future, which should confirm once and for all whether Callisto is an oceanic world. Nearby measurements are planned for NASA’s Europa Clipper and the European Space Agency’s JUICE (Jupiter Icy Moons Explorer) missions, both of which have already launched. China’s planned Tianwen-4 mission may also observe Callisto.
Confirmation of Callisto’s status as an ocean is likely to inspire further research into its potential to support life — just as confirmation of this status has inspired research on Europa.
According to phys.org
