The discovery was made while examining data gathered by Juno’s Jovian InfraRed Auroral Mapper (JIRAM) spectrometer, a device used to study the chemistry of the atmospheres of Jupiter and several of its moons. A paper printed earlier this week in Nature Astronomy described the find.
Juno’s JIRAM spotted salts such as hydrated sodium chloride, ammonium chloride, sodium bicarbonate, and possibly organic compounds called aliphatic aldehydes, which the National Institutes of Health (NIH) describes as “essential building blocks for the synthesis of more complex organic compounds.”
“The composition and spatial distribution of these salts and organics suggest that their origin is endogenic, resulting from the extrusion of subsurface brines, whose chemistry reflects the water–rock interaction inside Ganymede,” the paper said.
"We found the greatest abundance of salts and organics in the dark and bright terrains at latitudes protected by the magnetic field," said Scott Bolton of the Southwest Research Institute, who is Juno's principal investigator and one of the paper’s co-authors. "This suggests we are seeing the remnants of a deep ocean brine that reached the surface of this frozen world."
Today, Ganymede’s vast ocean can only be found beneath its ice surface, as the huge moon has almost no atmosphere. However, water still spews up into the skies over Ganymede through huge geysers, which Juno also observed during the 2021 flyby.