If there were a contest for the most intriguing moon in our solar system, Callisto would undoubtedly be a top contender. As Jupiter’s second-largest moon, it boasts more impact craters than any other planetary body.
Scientists have speculated that beneath its scarred surface lies a vast liquid saltwater ocean, and new evidence from reanalyzed data brings us closer to confirming this long-held theory.
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Subscribe to the NewsletterA team led by NASA’s Corey J. Cochrane has found stronger evidence supporting this theory by revisiting 30-year-old data from the Galileo mission.
Originally, the team wasn’t studying Callisto but Neptune’s moon Triton, which also has a strong ionosphere. Callisto’s similar ionosphere made it an ideal test case for their methods.
Galileo’s 1995-2003 data, initially dismissed due to “noise” from Callisto’s plasma environment, was reanalyzed using advanced plasma simulations.
“We were able to leverage previously developed plasma simulations to remove this obscuring plasma noise source from the measurement so that the signal from the ocean could be analyzed independently,” Cochrane explained.
The cleaned-up data revealed an induced magnetic field, suggesting a conductive layer—likely a subsurface ocean.
Faraday’s Law of Magnetic Induction explains this phenomenon: moving a magnet near a conductive material, like a saltwater ocean, generates an electrical current and a secondary magnetic field. Callisto’s strong ionosphere had previously masked this signal, making it hard to distinguish from random noise.
NASA’s Europa Clipper and the European Space Agency’s JUICE missions, set to arrive at Jupiter in 2030 and 2031, respectively, will provide more data to confirm Callisto’s ocean.
“Proving the existence of Callisto’s ocean from new measurements simply comes down to the fact that there are more measurements available to analyze,” Cochrane said. These missions will also help estimate the ocean’s depth and the overlying ice shell’s thickness.
The search for life is another driving force. Europa, another Jupiter moon, is considered the top candidate for extraterrestrial life due to its subsurface ocean. Callisto, with its potential ocean and unexplained oxygen levels, is also a compelling target.
“It is possible that Europa’s ocean can support life because we know that it hosts the key ingredients to support it,” Cochrane noted. As Cochrane put it, the new missions will “fill in the holes” left by Galileo, bringing us closer to understanding this cosmic body.
