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This image features an artist’s impression of the moon of Jupiter Ganymede. Astronomers used archival data sets from the NASA / ESA Hubble Space Telescope to reveal the first evidence of water vapor in the atmosphere of Jupiter Ganymede’s moon, the result of thermal escape from the water vapor from the moon’s icy surface. Credit: ESA / Hubble, J. daSilva
Oxygen found on the frozen world is linked to the sublimation of surface ice
Although larger than the bloated planet Mercury, the Jovian moon Ganymede is no place for sunbathing. Located half a billion kilometers from the Sun, the water ice on its surface is frozen to freezing temperatures as low as minus 300 degrees Fahrenheit. It makes ice as hard as rock. However, a rain of charged particles from the Sun is enough to transform the ice into water vapor at midday on Ganymede. This is the first time that such evidence has been found, thanks to spectroscopic observations of aurora over Ganymede by the Hubble Space Telescope spanning two decades. Auroras are used to trace the presence of oxygen, which is then related to the presence of water molecules spraying on the surface. Ganymede has a deep ocean located about 100 miles below the surface. It’s too deep for water vapor to escape.
Astronomers have used new archival data sets from NASA’s Hubble Space Telescope to uncover evidence of water vapor in the atmosphere of Jupiter Ganymede’s moon. Steam is present due to the thermal excitation of water molecules on the moon’s icy surface. Previous research has offered circumstantial evidence of the moon containing more water than all of Earth’s oceans. However, the temperatures there are so cold that the water on the surface freezes and the ocean is about 100 miles below the crust. Credit: NASA Goddard Space Flight Center
For the first time, astronomers have discovered evidence of water vapor in the atmosphere of Jupiter Ganymede’s moon. This water vapor is formed when the ice on the moon’s surface sublimates, that is, it changes from solid to gas.
Scientists used new and archived data sets from NASA’s Hubble Space Telescope to make the discovery, published in the journal Nature astronomy.
Previous research has provided indirect evidence that Ganymede, the largest moon in the solar system, contains more water than all of Earth’s oceans. However, the temperatures there are so cold that the water on the surface is frozen. The ocean of Ganymede would reside about 100 miles below the crust; therefore, water vapor would not represent evaporation from that ocean.
Astronomers have re-examined Hubble’s observations of the past two decades to find this evidence of water vapor.
In 1998, the Hubble Space Telescope’s imaging spectrograph took the first ultraviolet (UV) images of Ganymede, which revealed colored ribbons of electrified gas called auroral bands, and provided further evidence that Ganymede has a weak magnetic field.
In 1998, the Hubble Space Telescope’s Imaging Spectrograph took these first Ganymede ultraviolet images, which revealed a particular pattern in observed emissions from the moon’s atmosphere. The moon displays auroral bands that are somewhat similar to the aurora ovals seen on Earth and other planets with magnetic fields. It was illustrative proof that Ganymede has a permanent magnetic field. The similarities in ultraviolet observations were explained by the presence of molecular oxygen. The differences were explained at the time by the presence of atomic oxygen, which produces a signal that affects one UV color more than the other. Credit: NASA, ESA, Lorenz Roth (KTH)
The similarities in these UV observations were explained by the presence of molecular oxygen (O2). However, certain characteristics observed did not correspond to the emissions expected from an atmosphere of pure O2. At the same time, the scientists concluded that this deviation was probably related to higher concentrations of atomic oxygen (O).
As part of a large observing program to support NASA’s Juno mission in 2018, Lorenz Roth of the KTH Royal Institute of Technology in Stockholm, Sweden, led the team that set out to measure the amount of atomic oxygen with Hubble. The team’s analysis combined data from two instruments: the Hubble Cosmic Origins Spectrograph in 2018 and archive images from the Space Telescope Imaging Spectrograph (STIS) from 1998 to 2010.
To their surprise, and contrary to original interpretations of the 1998 data, they discovered that there was virtually no atomic oxygen in Ganymede’s atmosphere. This means that there must be some other explanation for the apparent differences in these UV aurora images.
This image shows Jupiter’s moon Ganymede as seen by NASA’s Hubble Space Telescope in 1996. Ganymede is located half a billion miles away (over 600 million km) and Hubble can track changes on the moon and reveal d ‘other characteristics in the ultraviolet and near infrared wavelengths. Astronomers have now used new archival data sets from Hubble to reveal for the first time evidence of water vapor in the atmosphere of Jupiter Ganymede’s moon, which is present due to thermal escape from water vapor from the moon’s icy surface. Credit: NASA, ESA, John Spencer (SwRI Boulder)
Roth and his team then took a closer look at the relative distribution of auroras in UV images. Ganymede’s surface temperature varies greatly throughout the day, and around noon near the equator, it can get hot enough for the ice surface to release (or sublime) small amounts of water molecules. In fact, the differences seen in UV images directly correlate with where water would be expected in the moon’s atmosphere.
“So far, only molecular oxygen has been observed,” Roth explained. “This is produced when charged particles erode the surface of the ice. The water vapor we are now measuring comes from the sublimation of ice caused by the thermal escape of water vapor from hot icy regions.
This discovery adds anticipation to the next ESA (European Space Agency) mission, JUICE, which stands for JUpiter ICy moons Explorer. JUICE is the first large-scale mission of ESA’s Cosmic Vision 2015-2025 program. Scheduled for launch in 2022 and arrive at Jupiter in 2029, it will spend at least three years making detailed observations of Jupiter and three of its largest moons, with particular emphasis on Ganymede as a planetary body and potential habitat. .
Ganymede has been identified for detailed investigation as it provides a natural laboratory for the analysis of the nature, evolution and potential habitability of frozen worlds in general, the role it plays in the Galilean satellite system. and its unique magnetic and plasma interactions with Jupiter and its surroundings.
“Our results can provide JUICE instrument teams with valuable information that can be used to refine their observation plans to optimize spacecraft utilization,” Roth added.
Right now, NASA’s Juno mission is taking a close look at Ganymede and recently released new images of the icy moon. Juno has been studying Jupiter and its environment, also known as the Jovian system, since 2016.
Understanding the Jovian system and unraveling its history, from its origin to the possible emergence of habitable environments, will allow us to better understand how the giant gas planets and their satellites form and evolve. In addition, new information will hopefully be found on the habitability of Jupiter-like exoplanetary systems.
Reference: “A sublimated aquatic atmosphere on Ganymede detected from observations of the Hubble Space Telescope” by Lorenz Roth, Nickolay Ivchenko, G. Randall Gladstone, Joachim Saur, Denis Grodent, Bertrand Bonfond, Philippa M. Molyneux and Kurt D. Retherford, July 26, 2021, Nature astronomy.
DOI: 10.1038 / s41550-021-01426-9
The Hubble Space Telescope is an international cooperation project between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Astronomical Research in Washington, DC
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