With Europa and Enceladus getting most of the attention for their subsurface oceans and potential to host life, other frozen worlds have been left in the shadows—but the mysterious Jovian moon Ganymede is now making headlines.

While Ganymede hasn’t yet been observed spewing plumes of water vapor like Saturn’s moon Enceladus, Jupiter’s largest moon is most likely hiding an enormous saltwater ocean. Hubble observations suggest that the ocean—thought to sit under 150 km (95 miles) of ice—could be up to 100 km (60 miles) deep. That’s 10 times deeper than the ocean on Earth.

Ganymede is having a moment because NASA’s Juno mission observed salts and organic compounds on its surface, possibly from an ocean that lies beneath its crust of ice. While Juno’s observations can't provide decisive evidence that this moon has an ocean that makes Earth look like a kiddie pool, the Juno findings are the strongest evidence yet of salts and other chemicals making it to the exterior of Ganymede.

Fathoms below

The surface of Ganymede is already known to be made of water ice. Juno’s JIRAM (Jovian Infrared Auroral Mapper) instrument has now used its infrared vision to identify substances that included hydrated and ammoniated salts, sodium bicarbonate, hydrated silica, and what might be aliphatic aldehydes, which can potentially build more complex organic compounds. Hydrated salt (hydrated sodium chloride) may hint at a briny ocean below the surface ice. Juno mission scientists think that ammoniated salt (ammonium chloride) found on the surface could possibly mean that as Ganymede formed, it somehow accumulated substances cold enough to make ammonia condense. Carbonate salts might be leftovers from ices rich in carbon dioxide.

“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 scientists wrote in a study recently published in Nature.

Anything endogenic originates from the inside of a moon, planet, or other body, while exogenic substances originate on the surface. If the salts and organics found really are endogenic, it means they somehow rose from the depths of Ganymede. They may have traveled in water that oozed through cracks in the surface instead of being ejected in the form of vaporous plumes, such as those on Enceladus.

JIRAM did not find exogenous compounds such as hydrogen peroxide or hydrated sulfuric acid, both of which are found on the surface of Europa, another frozen Jovian world, though they had been detected near the poles of Ganymede in previous studies. The lack of exogenous compounds (at least based on what JIRAM was able to see) in these salty deposits may be evidence that the discovered compounds came from a briny ocean.

Reaching the surface

Whether the compounds found by JIRAM actually originated deep in the interior or closer to the surface is still unknown. Without definitive evidence of that ocean, the Juno scientists also acknowledge the possibility that organics and salts could have somehow originated in the shallower layers of the crust. Ganymede’s crust is much thicker than Europa’s, which means it would be more difficult for any substance from a subsurface ocean to make it all the way through that crust.

Connections have been made between aliphatic aldehydes and liquid water activity, at least on Earth. Other researchers have also seen what might be signs of them in the water vapor plumes issuing from Enceladus. If so, that would strengthen the case for a subsurface ocean origin, as Enceladus’ vapor also contains some of the same salts found on the surface of Ganymede, and these salts are considered endogenous. They are thought to come from interactions between liquid water and rocks, especially silicate rocks.

The discovery of organics and aliphatic aldehydes on Ganymede inevitably raises another question: Does Ganymede have what it takes to support life? It might. Aliphatic aldehydes, which have been found in some types of carbonaceous meteorites that have fallen to Earth, are precursors of carboxylic acids and amino acids. Alien seekers shouldn’t get too excited about this. Organics are everywhere in space, so their presence on Ganymede shouldn’t be too surprising. Still, that could continue to spark the imaginations of those who want to believe.

Nature Astronomy, 2023.  DOI: 10.1038/s41550-023-02107-5