Juno discovers salts and organic compounds on Ganymede’s surface

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After a flyby of Jupiter’s largest moon, Ganymede, in 2021, Juno and a team of scientists have discovered the presence of mineral salts and organic compounds on the surface of the icy moon. The new findings will allow scientists to better understand where Ganymede came from and the composition of its deep subsurface ocean.

For planetary scientists and astrobiologists, Ganymede has long been of great interest, as the planet is theorized to house a massive ocean beneath its icy surface. Furthermore, scientists have been regularly performing spectroscopic observations of Ganymede and its surface features for decades to learn more about the composition of the moon as a whole. NASA’s Galileo mission, the joint NASA/ESA Hubble Space Telescope, and the European Southern Observatory’s Very Large Telescope (VLR) have all contributed to these spectroscopic observations and have even hinted at the presence of the salts and organic compounds Juno discovered with its recent flyby.

However, the spatial resolution of the observations from Galileo, Hubble, and the VLT were far too low, and scientists weren’t able to confidently confirm the presence of the compounds without further observations by a mission that could provide spectroscopic observations of Ganymede with a high resolution. Thankfully, Juno was able to provide those high-resolution spectroscopic observations with its recent flyby using the mission’s Jovian Infrared Auroral Mapper (JIRAM) spectrometer.

The flyby, which occurred on June 7, 2021, saw Juno fly just 1,046 kilometers above the surface of Ganymede. JIRAM was activated shortly after Juno made its closest approach to Ganymede, and the instrument immediately began observing Ganymede in infrared, collecting infrared images and infrared spectra of the moon’s surface. By observing Ganymede in the infrared, scientists were able to see the chemical footprint of Ganymede’s surface by detecting the infrared light reflected off of its surface.

JIRAM is designed to observe and investigate the infrared light that reaches from the very deep atmospheric layers of Jupiter. However, as Juno began performing more flybys of Jupiter’s icy moons, scientists began using JIRAM to investigate the surface features and composition of moons like Io, Europa, Callisto, and now Ganymede.

So, what did scientists find in the new Juno observations?

JIRAM’s observations of Ganymede’s surface were collected with a spatial resolution of better than one kilometer per pixel — a spatial resolution that is unprecedented in infrared spectroscopy. From this high-resolution data, scientists were able to directly detect and analyze spectral features of non-water-ice materials on Ganymede’s surface. Some of these non-water-ice materials include hydrated sodium chloride, ammonium chloride, sodium bicarbonate, and possibly even aliphatic aldehyde.

While these compounds may seem like just another list of organic compounds and salts, to scientists, these compounds and salts tell the story of Ganymede’s formation and what Ganymede’s surface composition was like in its early days.

Data from JIRAM superimposed on an image of Ganymede’s grooved surface. (Credit: NASA/JPL-Caltech/SwRI/ASI/INAF/JIRAM/Brown University)

“The presence of ammoniated salts suggests that Ganymede may have accumulated materials cold enough to condense ammonia during its formation. The carbonate salts could be remnants of carbon dioxide-rich ices,” said lead author Federico Tosi, a Juno co-investigator from Italy’s National Institute for Astrophysics in Rome.

Interestingly, it seems as though the salts and organic compounds are localized to areas in which Ganymede’s magnetic field can shield them from surrounding radiation. Modeling of Ganymede’s magnetic field has shown that the equatorial region of the moon is shielded from Jupiter’s immense magnetic field by Ganymede’s own magnetic field. Jupiter’s magnetic field creates energetic electron and heavy ion bombardment on its surrounding moons, and particle fluxes such as those created by Jupiter’s magnetic field are known to negatively affect salts and organic compounds similar to those found on Ganymede.

To further investigate this, teams imaged a narrow range of latitudes and a broader range of longitudes in the Jupiter-facing hemisphere of Ganymede using JIRAM during Juno’s 2021 flyby. In the observations, scientists looked for areas where the salts and organic compounds were most abundant.

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

As mentioned, analyzing and understanding the salts and organic compounds present on the surface of Ganymede is vital to scientists gaining a broader understanding of not just Ganymede’s origins but also the origins of Jupiter’s system of large icy moons, which are Ganymede, Callisto, and Europa. Each of these icy moons are believed to house large subsurface oceans beneath their crusts, making each of them some of the most potentially habitable locations in the solar system.

By better understanding the makeup of each of the moons, scientists can further assess the habitability of the moons and their subsurface oceans. As Juno continues its mission around Jupiter, it’ll continue to fly past each of the icy moons every few years or so, providing scientists with more detailed observations that will allow them to better understand the moons. Since the Ganymede flyby in 2021, Juno has already performed two flybys of Europa, the first in October 2021 and the second in September 2022, and is set to flyby volcano-covered Io later this year on Dec. 30.

What’s more, upcoming missions to the icy moons, such as ESA’s JUICE, which launched in April, and NASA’s Europa Clipper mission, which is currently scheduled for launch in October 2024, will exclusively study the icy moons using instruments designed to investigate the characteristics of icy moons.

Tosi et al.’s findings were published in the Nature Astronomy journal on Oct. 30. 

(Lead image: Image of Ganymede taken by Juno during its June 2021 flyby. Credit: NASA/JPL-Caltech/SwRI/MSSS/Kalleheikki Kannisto (CC BY))

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