A moon orbiting Jupiter, approximately the same dimensions as Mercury, is believed to possess a vast ocean concealed beneath its frozen exterior, based on a recent investigation.

This moon of Jupiter, known as Callisto, is the farthest out of the 95 recognized moons of the planet. In the 1990s, the Galileo spacecraft from NASA observed some anomalies—Callisto seemed to react to the fluctuating magnetic field of Jupiter. Given that saltwater is a good conductor of electricity, researchers speculated that an unseen ocean could be the cause.

Nonetheless, the findings were not conclusive. Callisto also features an ionosphere, a region filled with charged particles in its upper atmosphere, which might have been influencing the magnetic phenomena rather than an ocean lying beneath the surface.

Currently, scientists are more assured than ever that Callisto is part of the expanding roster of water-abundant worlds within our solar system. And wherever there is water, there is the likelihood of life.

Callisto, ranking as the third-largest moon in the solar system, revolves around Jupiter at a distance of roughly 1.2 million miles. Discovered by Galileo Galilei in 1610, it is noted for being the most heavily cratered moon in the solar system.

The most recent support for the ocean hypothesis arises from an unexpected origin—previously collected data. The Galileo mission, launched by NASA in 1989, aimed at examining Jupiter, its moons, and its extensive magnetic field. The spacecraft had 35 encounters with the major moons of Jupiter, including eight flybys of Callisto, prior to the mission’s conclusion in 2003.

Recently, scientists revisited the data from Galileo with the help of sophisticated computer models and statistical methodologies. Unlike earlier investigations, this latest study analyzed all magnetic readings obtained during Callisto’s close encounters. The outcomes indicated that the ionosphere alone could not explain the detected magnetic interactions—however, when a concealed ocean was factored into the model, the data corresponded perfectly. The results, released in *AGU Advances*, bolster the argument for a subsurface ocean.

Researchers project that this ocean lies beneath a dense layer of ice, possibly ranging from tens to hundreds of miles in depth, likely encasing a rocky core. If validated, Callisto would share similarities with Europa, another of Jupiter’s moons celebrated for its hidden ocean.

While the presence of an ocean on Callisto remains unverified, forthcoming spacecraft missions could yield clear answers. NASA’s *Europa Clipper* and the European Space Agency’s *JUICE* (Jupiter Icy Moons Explorer) are anticipated to gather data during their respective flybys. Additionally, China’s upcoming *Tianwen-4* mission may also survey Callisto.

Following its successful launch in October 2024, *Europa Clipper* is making its way through space. Scheduled for a gravity-assist flyby of Mars on February 27, 2025, it will continue toward Europa, with an expected arrival in 2030. Meanwhile, *JUICE*, launched in 2023, will execute a flyby of Venus in August 2024 and aims to reach Jupiter by 2031.

Whether Callisto has the capacity to support life is still uncertain, but it remains on NASA’s list of captivating targets. The moon probably hosts a saline ocean and a rocky layer over 150 miles beneath its surface—both essential elements for life as we understand it. Furthermore, oxygen and hydrogen have been found in its sparse atmosphere, which continues to ignite scientific curiosity about this remote, icy realm.