Juno Spots Salts and Organic Molecules on Ganymede’s Surface

Juno Spots Salts and Organic Molecules on Ganymede’s Surface

Thanks to the flyby of Ganymede by the Juno spacecraft in 2021, a team of scientists has created the most precise map to date of the presence of salts and organic materials on the surface of Jupiter’s largest moon. Based on what they are made of and where they are found, these salts and organic materials came from inside Ganymede.

They were pushed out of an underground, salty ocean whose chemistry reflects how water and rocks interact inside Ganymede. The authors believe that their work will lead to a better understanding of Ganymede’s origins and the composition of its internal ocean.


Discovered in 1610 by the Italian astronomer Galileo, Ganymede is the largest moon in the solar system, orbiting around Jupiter. Apart from the fact that it doesn’t orbit the Sun, Ganymede exhibits many planetary features: it is, for example, larger than the planet Mercury and has a completely differentiated structure, with the densest materials concentrated at its center. A mantle of silicate rocks surrounds a liquid iron core that makes up Ganymede.

Additionally, it has a thin atmosphere and possesses its own magnetic field. What intrigues scientists the most is the presence of a liquid-water ocean beneath its icy surface, initially theorized in the 1970s and gradually confirmed through observations by the Galileo spacecraft and the Hubble Space Telescope. Interacting with the rocky mantle, this internally rich saltwater ocean could provide favorable conditions for the emergence of life.

The Juno mission extended to uncovering the secrets of Jupiter’s moons. Launched by the U.S. space agency in 2011, the Juno probe was initially designed to study Jupiter, especially its internal layers and atmospheric composition. The mission was first extended from 2018 to 2021 and then from 2021 to 2025 (or until the end of the probe’s life), during which it was aimed at certain Jupiter’s moons, including Europa and particularly Ganymede.

Juno flew over the largest moon in the solar system on June 7, 2021, revealing magnificent images of its icy surface covered in craters. The probe subsequently flew over the moon Europa, which has also been suspected of harboring an internal ocean, in October 2021 and September 2022. Its flyby of Io, the most geologically active object in the solar system, is scheduled for December 2023.

The Presence of Salts and Organic Molecules Confirmed.

Originally built to probe Jupiter’s internal layers, the Jovian Infrared Auroral Mapper (JIRAM) spectrometer onboard the Juno probe was able to observe Ganymede’s surface and collect data at a resolution never before achieved—greater than one kilometer per pixel. An infrared spectrometer measures the infrared radiation from a surface to deduce its chemical composition.

By analyzing the data collected by the JIRAM spectrometer, a team of scientists has confirmed the presence of salts and organic molecules on Ganymede’s surface for the first time. They present their findings in the journal Nature Astronomy. Previous observations made by the Galileo probe and the Hubble Space Telescope had already suggested the presence of salts and organic molecules on the moon’s surface, but the low spatial resolution of those observations had not allowed for a definitive discovery.

Valuable Insights Into Its Internal Ocean

According to the data collected by the Juno probe, Ganymede’s surface, mostly composed of icy water, contains mineral salts such as ammonium chloride (NH2Cl), sodium bicarbonate (NaHCO3), and sodium chloride (NaCl, used as table salt). It also appears to have organic molecules.

According to scientists, the presence of each of these compounds has implications for our understanding of Ganymede and its evolution since its formation. For example, the presence of ammonia-rich salts suggests that during its formation, Ganymede aggregated materials cold enough to condense ammonia, while the presence of carbonate salts could indicate the past presence of carbon dioxide-rich ice.

The spatial distribution of these compounds is also a crucial clue. They are mainly detected in the equatorial region of the moon, in an area protected from Jupiter’s magnetic field, so their origin is endogenous. According to scientists, the salts on Ganymede’s surface would come from its internally salty liquid water ocean (referred to as brine).

Scientists believe that the presence of these compounds is an important marker of interactions between the rocky mantle materials and the liquid water in its internal ocean and could indicate the presence of hydrothermal activity in its depths. To learn more, scientists are now relying on the Juice probe, launched a few months ago towards the Jovian system, to study the subglacial oceans of Jupiter’s moons.