NASA’s Juno spacecraft has discovered salts and organic compounds on the surface of Ganymede, Jupiter’s largest moon. The discovery was made during a June 2021 flyby, during which Juno studied Ganymede with its Jovian InfraRed Auroral Mapper (JIRAM) instrument, an instrument designed to study the chemistry and interactions in the atmospheres of Jupiter and its moons. Ganymede, one of these moons, is the largest moon in the solar system — at 5,268 kilometers (3,200 miles) wide, larger than the planet Mercury — and has a vast ocean beneath its icy crust.
During its 2021 flyby of Ganymede, Juno’s JIRAM instrument detected salts such as hydrated sodium chloride, ammonium chloride, sodium bicarbonate, and possibly even organic compounds known as aliphatic aldehydes. The discovery of these compounds and salts could help astronomers better understand how Ganymede formed and evolved, and perhaps shed light on the chemistry of its subsurface ocean. A study of Juno’s observations of salts and organics on Ganymede’s surface is published in the journal Nature Astronomy.
Jupiter and Ganymede. Visualization by NASA
Jupiter, which is nearby, has such a strong magnetic field that organic compounds and salts on the surfaces of its moons would have a hard time surviving. However, the region around Ganymede’s equator appears to be sufficiently shielded from the electrons and heavy ions emitted by Jupiter’s magnetic field to support the existence of these compounds.
“We found the greatest abundance of salts and organics in dark and light regions at latitudes protected by the magnetic field,” said Scott Bolton, Juno’s principal investigator at the Southwest Research Institute in San Antonio. “This suggests that we are seeing the remnants of deep ocean brine that have reached the surface of this frozen world.”
The presence of these salts and organic compounds may indicate hydrothermal activity deep beneath Ganymede’s icy surface, or interactions between its subsurface ocean and rocks deep within the planet.
Jupiter and Ganymede. Visualization by NASA
“Extensive water-rock interactions could provide such a balance and would be consistent with the presence of sodium salts as an independent indicator of aqueous alterations within Ganymede,” the authors write in a paper published in the journal Nature Astronomy on October 30.
However, the authors add, these salts could have formed through processes other than a salty internal ocean. “Because Ganymede has a much thicker crust than Europa, the exchange of matter between its deep interior and the surface may not determine its surface composition and may therefore reflect exchange of matter between the shallow crust and the surface or exogenous deposits,” they write in their study.
The Juno spacecraft was launched from Cape Canaveral, Florida, on August 5, 2011, becoming only the second mission to orbit Jupiter after NASA’s Galileo probe. Juno was designed to study the gas giant’s weather, magnetic environment, and history.
Ganymede. NASA visualization
Astronomers have found traces of water vapor for the first time in the thin atmosphere of Ganymede, a moon of Jupiter, the largest moon in the solar system, a new study has found.
The researchers say the discovery could shed light on similar watery atmospheres that may envelop other icy bodies in the solar system and beyond.
Previous studies have suggested that Ganymede, which is larger than Mercury and Pluto and only slightly smaller than Mars, may contain more water than all of Earth’s oceans combined. However, the Jupiter moon is so cold that the water on its surface is frozen. Any liquid water present on Ganymede would be trapped about 100 miles (160 kilometers) beneath its crust. The scientists detailed their findings July 26, 2021, in the journal Nature Astronomy.
Ganymede with Jupiter in the background. Visualization by NASA
Previous studies have suggested that ice on Ganymede’s surface can go from solid to gas without going through liquid, allowing water vapor to be part of the giant moon’s tenuous atmosphere. However, evidence for water has remained elusive.
In the new study, the scientists analyzed old and new data on Ganymede from NASA’s Hubble Space Telescope. In 1998, Hubble took the first ultraviolet images of Ganymede, including images of its auroras, giant lunar analogues of Earth’s northern and southern lights. Multicolored bands of electrified gas within these auroras helped prove that Ganymede has a weak magnetic field.
The ultraviolet signals detected in these auroral bands indicate the presence of oxygen molecules made up of two atoms, which are formed when charged particles break down Ganymede’s icy surface. However, some of these ultraviolet emissions were inconsistent with what would be expected from an atmosphere made of pure molecular oxygen. Previous studies had suggested that these discrepancies were due to signals from atomic oxygen, or individual oxygen atoms.
Moons of Jupiter. Visualization by NASA
As part of a major observational program in support of NASA’s Juno mission to Jupiter, researchers used the Hubble Space Telescope to measure the amount of atomic oxygen in Ganymede’s atmosphere. Surprisingly, they found that there was virtually no atomic oxygen, suggesting another explanation for the early ultraviolet signals.
The scientists focused on how much Ganymede’s surface temperature varies over the course of the day, with maximum temperatures at the equator reaching about minus 190 degrees Fahrenheit (minus 123 degrees Celsius) at midday and minimum temperatures of about minus 315 degrees Fahrenheit (minus 193 degrees Celsius) at night. At the hottest points on Ganymede, ice can become so hot that it can turn directly to steam. They noted that the differences seen in a number of ultraviolet images of Ganymede closely match the distribution of water in the moon’s atmosphere, based on its climate.
The main reason previous studies failed to detect water in Ganymede’s atmosphere is that the ultraviolet signal of molecular oxygen is very strong. This stronger oxygen signal makes it difficult to detect other signals. These results suggest that water vapor is indeed present in the atmospheres of icy bodies in the outer solar system.
Ganymede with Jupiter in the background. Visualization by NASA
An asteroid impact could have ‘completely removed the original surface’ of Ganymede. A new study suggests that a giant asteroid slammed into Ganymede, Jupiter’s largest moon, with such force that it caused a dramatic and permanent change in the moon’s orientation about 4 billion years ago. The scientists say the decisive impact also significantly affected the geological and internal evolution of the moon, which is larger than Mercury.
Computer modeling by planetary scientist Naoyuki Hirata of Japan’s Kobe University predicts the asteroid would have been about 186 miles (300 kilometers) wide, or 20 times the size of the fateful one that wiped out the dinosaurs some 66 million years ago.
The location of the asteroid’s impact on Jupiter’s moon Ganymede coincides almost exactly with the meridian farthest from Jupiter. This meant that Ganymede had undergone a reorientation of its axis of rotation, and allowed planetary scientist Naoyuki Hirata of Japan’s Kobe University to calculate what type of impact could have caused it. Naoyuki Hirata
Only a collision of this magnitude would be strong enough to destabilize Ganymede and cause the tidally locked moon to begin spinning on its axis, according to a study published by Hirata in the journal Scientific Reports.
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“This is a neat attempt to turn back the clock using computer modelling to explain the distribution of scars on Ganymede,” Lee Fletcher, a planetary scientist at the University of Leicester in the UK who was not involved in the new study, told The Guardian.
Hirata estimates that the subsequent process unfolded about 1,000 years after the asteroid hit. According to the new study, the space object likely slammed into Ganymede at an angle of 60 to 90 degrees, leaving a crater 1,400 to 1,600 km wide.
Jupiter’s moon Ganymede is covered over much of its surface with furrows that form concentric circles around one particular spot (red cross), leading researchers in the 1980s to conclude that they were the result of a major impact event. Naoyuki Hirata
Evidence for Ganymede’s reorientation comes from its surface, which is pockmarked with vast striations — concentric rings of depressions thought to be the fragmented remains of bowl-shaped depressions left by asteroid impacts. A new analysis of Ganymede’s largest striation system, which scars the moon just below its equator, suggests that an asteroid impact caused the moon to rotate so that its crater faces away from Jupiter.
In a recent statement, the researcher said the asteroid impact would have “completely obliterated the original surface” of Ganymede, with the crater reaching as much as 25 percent of the size of the Jupiter moon. The impact could have significantly affected the moon’s geology and interior, which scientists believe contains a hidden saltwater ocean containing more water than all of Earth’s oceans combined.
This enhanced image of Jupiter’s moon Ganymede was taken by the JunoCam camera aboard NASA’s Juno spacecraft as it flew past the icy moon on June 7, 2021, during Juno’s 34th pass of Jupiter. NASA/JPL-Caltech/SwRI/MSSS/Kalleheikki Kannisto
Scientists are using infrared light to better study Jupiter’s moon Ganymede, the largest moon in our solar system.
Using the Jovian Infrared Auroral Mapper (JIRAM) instrument aboard Juno, which detects infrared light invisible to the human eye, the Juno science team has created a new infrared map of Ganymede that they hope will help them better understand the Jupiter moon’s icy crust and the ocean that lies beneath, according to NASA.
Ganymede. NASA visualization
“Ganymede is larger than Mercury, but virtually everything we’re exploring on this mission to Jupiter is on a massive scale,” Scott Bolton, Juno’s principal investigator at the Southwest Research Institute in San Antonio, said in a NASA statement. “The infrared and other data Juno collected during the flyby hold fundamental clues for understanding the evolution of Jupiter’s 79 moons from their formation to the present day.”
During Juno’s most recent close approach to Ganymede, which followed a much closer flyby on June 7, 2021, the spacecraft passed within 31,136 miles (50,109 kilometers) of Ganymede’s surface.
The close flyby allowed Juno’s JIRAM instrument to see Ganymede’s north polar region for the first time. The instrument also collected data on the composition of matter at both low and high altitudes on the moon, according to the statement.
New infrared image of Ganymede from NASA’s Juno spacecraft. NASA/JPL-Caltech/SwRI/ASI/INAF/JIRAM
The data Juno collected during this flyby complements those from previous close encounters, as well as observations by previous probes such as NASA’s Voyager mission and the agency’s Galileo, New Horizons, and Cassini spacecraft. By observing the moon in infrared light, the team was able to learn more about Ganymede’s composition to better understand not only the moon itself, but planets like it as well.
“We found that Ganymede’s high latitudes are dominated by water ice with small grains, which is the result of intense bombardment by charged particles,” said Alessandro Mura, a Juno co-investigator from the National Institute for Astrophysics in Rome, in the same statement.
“In contrast, low latitudes are shielded by Ganymede’s magnetic field and contain more of the original chemistry, mostly non-water-ice components such as salts and organics,” Mura added. “It is critical to characterize the unique properties of these icy regions to better understand the space weathering processes that the surface undergoes.”
In this image taken by Juno, dark plumes of ejecta surround Kittu Crater on Ganymede. ASA/JPL-Caltech/SwRI/MSSS. Image processing: Thomas Thomopoulos © CC BY
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