In the Jupiter system, the surface composition of the icy moons Europa and Ganymede may constrain the habitability of these satellites, known to host internal oceans of liquid water. In the recent past of these moons, underground liquid may have occasionally risen to the surface, leaving traces of its chemical composition. However, the combination of endogenous processes, that is, attributable to the genuine composition of the underground liquid, and exogenous processes, instead due to space weathering, complicates the study of the surface composition.

Larger than planet Mercury, Ganymede has a substantially thicker icy crust than Europa, and its surface composition as observed today is not necessarily representative of the internal, deep composition. Previous spectroscopic observations (NASA’s Galileo spacecraft, Hubble Space Telescope and the Very Large Telescope) hinted at salts and organics; but their spatial resolution was too low or their spectral range too narrow to make a determination.

To shed new light on the surface composition of Ganymede, in the extended mission phase the NASA Juno spacecraft flew over the satellite from a minimum altitude of 1046 km on June 7, 2021. The Italian-led Jupiter InfraRed Auroral Mapper (JIRAM) acquired infrared images and spectra at an unprecedented spatial resolution, less than 1 km per pixel, allowing the surface composition of Ganymede to be revealed at local scale. In the future, this kind of spatial resolution will typically be achieved by ESA’s JUICE spacecraft, so JIRAM measurements represented a one-shot chance to anticipate JUICE measurements by about one decade, albeit for a very small portion of the surface. In Tosi et al. (2023) we report the results obtained from the analysis of this dataset.

Besides water ice and carbon dioxide, the spectral profiles acquired by JIRAM between 2 and 5 µm suggest a mixture of mineral salts such as hydrated sodium chloride and ammonium chloride, hydrated magnesium and sodium sulfate, and possibly sodium bicarbonate or ammonium carbonate, in addition to the methyl alkane –CH3 group, potentially indicative of aliphatic aldehydes. No striking spectral signatures of exogenous compounds such as hydrated sulfuric acid, hydrogen peroxide and sulfur dioxide are observed.

JIRAM covered a narrow range of latitudes (10°N to 30°N) and a broader range of longitudes (-35°E to +40°E) in the Jupiter-facing hemisphere, flying over a variety of geological units such as grooved terrains, dark cratered terrains, and the bright ejecta of a fresh crater. The composition observed by JIRAM can vary depending on the terrain type: a larger abundance of salts and organics is not necessarily found only in dark terrains but also in some bright terrains in correspondence with the grooves. The compositional differences observed between different grooves suggest that an endogenous process, such as the extrusion of liquid from the subsurface, may determine the observed composition. Furthermore, not all dark terrains appear equally enriched in CO2, which supports a distribution controlled by geological processes.

Modelling indicates that Ganymede’s intrinsic magnetic field, a unique feature among all natural satellites explored so far, shields the surface against electrons and heavy ions up to a latitude of about 40°. Hence, the composition observed by JIRAM on Ganymede at the local scale could be the solid residue of an oceanic brine that has reached the surface, only marginally affected by space weathering in terms of radiation and deposition of exogenic materials.

The implications of these new measurements concern both the origin of Ganymede and the composition of its deep ocean, which according to geophysical models is not in direct contact with a silicate mantle today, unlike the smaller moon Europa.

The potential presence of ammoniated salts suggests that Ganymede, during its formation, accumulated materials cold enough to condense ammonia. Similarly, the presence of carbonate salts could be due to the original accumulation of carbon dioxide-rich ices. As on Earth and other icy satellites and dwarf planets such as Enceladus, Europa and Ceres, the presence of sodium at specific locations is indicative of interaction between liquid water and rocky material. This interaction may have occurred early in Ganymede’s history when accreted ice-rock mixtures experienced ice melting, and water and other primordial volatiles separated from rocks. Aldehydes, which play an important role as prebiotic precursor molecules, may have been present in an ancient hydrothermal environment.

The salt minerals and organic compounds identified spectroscopically by JIRAM on Ganymede, and their relationship with the geological characteristics of the explored area, suggest that these are the fossil of an extensive exchange between liquid water and rocky mantle that occurred up to a certain point in the history of the satellite.

Read the full paper: Salts and organics on Ganymede's surface observed by the JIRAM spectrometer onboard Juno.