Juno Spots Salts and Organic Molecules on Ganymede’s Surface

NASA’s Juno mission continues to orbit Jupiter, gathering data on its atmosphere, composition, gravitational field, magnetic field, and radiation environment. This data is helping scientists to learn more about the planet’s formation, internal structure, mass distribution, and what is driving its powerful winds. Periodically, the spacecraft also performs flybys of Jupiter’s largest satellites (the Galilean Moons), acquiring stunning images and vital data on their surfaces. These include optical and thermal images of Io’s many active volcanoes, Europa’s icy terrain, and infrared images of Ganymede.

During its last flyby of Ganymede (June 7th, 2021), Juno collected infrared images and spectra on the moon’s surface using its Jovian InfraRed Auroral Mapper (JIRAM) instrument. According to a recent study by an international team of researchers, this data revealed the presence of salt minerals and organic molecules on the icy moon’s surface. The findings could help scientists better understand the origin of Ganymede, the composition of its interior ocean, and the way material is exchanged between the surface and interior. In short, it could help scientists determine if life exists deep inside Ganymede’s ocean.

The team was led by Federico Tosi, a Juno co-investigator and staff research scientist with the National Institute of Astrophysics – Institute of Space Astrophysics and Planetology (IAPS-INAF). He was joined by multiple colleagues from the IAPS-INAF, the Italian Space Agency (ASI), the Southwest Research Institute (SwRI), the Planetary Science Institute (PSI), NASA’s Jet Propulsion Laboratory (JPL), the Sapienza University of Rome, and Cornell University. The paper that details their findings was published on October 30th in the journal Nature Astronomy.

Since the Voyager probes buzzed through the system in 1979, the Galilean Moons (Io, Europa, Ganymede, and Callisto) have been of great interest to scientists. Ganymede, the largest of the Galileans (and even larger than the planet Mercury), has attracted much attention because of its interior ocean, which is believed to house more water than all of Earth’s oceans combined. In addition, the moon is the only satellite in the Solar System to have a magnetic field, which is believed to be due to a molten outer core counter-rotating relative to the moon itself (similar to Earth).

Due to its composition and structure – a rocky and metallic core surrounded by vast ice sheets and a liquid water ocean – scientists also speculate that there could be hydrothermal activity at the core-mantle boundary. This may provide the necessary chemical ingredients and energy to support life, similar to how hydrothermal vents are thought to have given rise to Earth’s earliest life forms. Because Ganymede’s surface crust shows signs of resurfacing events, scientists have proposed that possible indications of life (“biosignatures”) might exist on the surface.

Previous spectroscopic observations by NASA’s Galileo spacecraft, the Hubble Space Telescope, and the European Southern Observatory’s Very Large Telescope (VLT) in Chile have all hinted at the presence of salts – magnesium sulfate (MgSO₄) and possibly sodium sulfate (Na₂SO₄) – and organic molecules on its surface. However, the spatial resolution of those observations was insufficient to say for sure. On June 7th, 2021, Juno flew over Ganymede at a minimum altitude of 1,046 km (650 mi) and collected data with its JIRAM instrument (see video below) to determine if there are any potential biosignatures to be found on the surface.

Built by the Italian Space Agency (Agenzia Spaziale Italiana), the JIRAM was designed to capture the infrared light from deep inside Jupiter and probe the layered atmosphere down depths of 50 to 70 km (30 to 45 mi) below the cloud tops. The instrument has also been used to offer insights into the terrain of the Galilean moons, providing data on the surface terrain and the composition of the icy crust – which consists mainly of frozen water and volatiles like carbon dioxide, sulfur dioxide, hydrogen sulfate, and organic molecules.

During the July 2021 flyby, the JIRAM instrument obtained infrared spectra of unprecedented spatial resolution – more than 1 km (0.62 mi) per pixel. As a result, the research team was able to detect and analyze the unique spectral features of mineral salts, including hydrated sodium chloride (NaCl), ammonium chloride (NH₂Cl), and sodium bicarbonate (NaHCO₃). They also noted the possibility of aliphatic aldehydes, the type of structural units typically found in biologically active natural products. Each of these compounds has different implications.

“The presence of ammoniated salts suggests that Ganymede may have accumulated materials cold enough to condense ammonia during its formation,” said Tosi in a NASA press release. “The carbonate salts could be remnants of carbon dioxide-rich ices.”

Just as significant is where these salts were observed. According to previous modeling of Ganymede’s magnetic field, the moon’s equatorial region (up to ~40 °latitude) is shielded from energetic electrons and heavy ion bombardment from Jupiter’s powerful magnetic field. These particle fluxes are well known to impact salts and organics negatively. During its June 2021 flyby, the JIRAM instrument covered a narrow range of latitudes (10 °N to 30 °N) and a broader range of longitudes (-35 °E to 40 °E) on the moon’s Jupiter-facing side.

“We found the greatest abundance of salts and organics in the dark and bright terrains at latitudes protected by the magnetic field,” added Scott Bolton, Juno’s principal investigator from the SwRI. “This suggests we are seeing the remnants of a deep ocean brine that reached the surface of this frozen world.”

The main objective of NASA’s upcoming Europa Clipper mission and the ESA’s Jupiter Icy Moon Explorer (JUICE) is to investigate the surface of Europa and Ganymede for indications of life in their interiors. This will be evident from chemical markers that were brought to the surface through resurfacing events and plume activity, both of which are common on these Galilean Moons. It is hoped that these and other investigations will finally answer the unresolved question of whether life could exist beneath their icy crusts – something scientists have been wondering since the late 1970s.

If these missions are successful, NASA hopes to send another mission – the Europa Lander – to the surface to examine plume activity and look for biosignatures more closely. This research also previews what other missions might find in the near future, which will be sent to explore some of Saturn’s largest moons. These include Titan, Enceladus, Dione, and Mimas, all of which are believed to be “Ocean Worlds” that might also harbor life. If any or all of these missions uncover evidence of biosignatures on these moons, it will confirm that life exists well beyond Earth and will forever alter our concept of our place in it.

Further Reading: NASA, Nature Astronomy