Using data from the Jovian InfraRed Auroral Mapper (JIRAM) aboard NASA’s Juno probe, planetary scientists detected hydrated sodium chloride, ammonium chloride, and sodium/ammonium carbonate, as well as organic compounds on the surface of Jupiter’s moon Ganymede.
This image of Ganymede was captured by NASA’s Juno mission during a close flyby in June 2021. A color-enhanced version of the image was created by citizen scientist Thomas Thomopoulos. Image credit: NASA / JPL-Caltech / SwRI / MSSS / Thomas Thomopoulos.
The surface composition of icy satellites, apart from the ubiquitous presence of water ice, is an exceptional question with important implications.
Composition can provide clues to the origin and evolution of the body and thus pave the way for habitability.
Subsurface oceans of liquid water, if present, can interact with the ice surfaces above, directly affecting the habitability of the oceans and the detection of possible trace elements of extraterrestrial life.
Ganymede, the largest of Jupiter’s moons, has long been of great interest to planetary scientists because of its vast subsurface ocean.
Previous spectroscopic observations by NASA’s Galileo spacecraft and NASA/ESA’s Hubble Space Telescope, as well as ESO’s Very Large Telescope, suggested the presence of salts and organics, but the spatial resolution of these observations was too low to make a determination.
On June 2021, Juno flew over Ganymede at a minimum altitude of 1,046 km (650 mi).
Shortly after the time of closest approach, the JIRAM instrument acquired infrared images and infrared spectra of the lunar surface.
JIRAM was designed to capture infrared light (invisible to the naked eye) that emerges deep inside Jupiter, probing the weather layer up to 50 to 70 km (30-45 miles) below the gas giant’s cloud tops.
But the instrument has also been used to peer into the terrain of the moons Io, Europa, Ganymede and Callisto.
JIRAM data from Ganymede acquired during the flyby achieved unprecedented spatial resolution for infrared spectroscopy – better than 1 km (0.62 mi) per pixel.
As a result, Juno scientists were able to detect and analyze the unique spectral properties of materials without water ice, including hydrated sodium chloride, ammonium chloride, sodium bicarbonate, and possibly aliphatic aldehydes.
“The presence of ammonium salts suggests that Ganymede may have accumulated materials cold enough to condense ammonia during its formation,” said Dr. Federico Tosi, Juno co-investigator from Italy’s National Institute for Astrophysics.
“The carbonate salts could be remnants of carbon dioxide-rich ice.”
Previous modeling of Ganymede’s magnetic field determined that the moon’s equatorial region up to about 40 degrees latitude is shielded from the energetic electrons and heavy ions produced by Jupiter’s hellish magnetic field.
It is well known that the presence of such particulate streams negatively affects salts and organic matter.
During the June 2021 flyby, JIRAM covered a narrow range of latitudes (10 degrees North to 30 degrees North) and a wider range of longitudes (minus 35 degrees East to 40 degrees East) in the Jupiter-facing hemisphere.
“We found the largest amount of salts and organic matter in dark and light terrains in latitudes protected by the magnetic field,” said Dr. Scott Bolton, Juno principal investigator at the Southwest Research Institute.
“This suggests that we are seeing remnants of deep ocean brine that reached the surface of this frozen world.”
The findings were published in a journal Astronomy of nature.
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F. True et al. Salts and organics on Ganymede’s surface as observed by the JIRAM spectrometer on board Juno. Nat Astron, published online October 30, 2023; doi: 10.1038/s41550-023-02107-5
