Jupiter’s icy satellite Ganymede is the largest moon in the Solar System. Water ice on its surface is frozen solid in frigid temperatures as low as minus 185 degrees Celsius (minus 300 degrees Fahrenheit). A rain of charged particles from the Sun is enough to turn the ice into water vapor at high noon on the Ganymede. Using high-sensitivity spectra and spectral images acquired by the NASA/ESA Hubble Space Telescope, planetary researchers have now detected this water vapor in the icy moon’s tenuous atmosphere.
Roth et al. reveal the first evidence for water vapor in the atmosphere of Ganymede, the result of the thermal escape of water vapor from the moon’s icy surface. Image credit: NASA / ESA / Hubble / J. daSilva.
In 1998, Hubble’s Space Telescope Imaging Spectrograph (STIS) took the first ultraviolet (UV) pictures of Ganymede, which revealed a particular pattern in the observed emissions from the icy moon’s atmosphere.
Ganymede displays auroral bands that are somewhat similar to the auroral ovals observed on Earth and other planets with magnetic fields.
These images were therefore illustrative evidence that the moon has a permanent magnetic field.
The similarities between the two UV 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.
“Ganymede’s atmosphere is produced by charged particle sputtering and sublimation of its icy surface,” said Dr. Lorenz Roth, a researcher at the KTH Royal Institute of Technology.
“Previous far-UV observations of oxygen emissions were used to infer sputtered molecular oxygen as an atmospheric constituent, but an expected sublimated water component remained undetected.”
In the new research, Dr. Roth and colleagues carried out a combined analysis of new spectra taken in 2018 with Hubble’s Cosmic Origins Spectrograph (COS) and archival images from the STIS instrument from 1998 and 2010.
To their surprise, and in contrast to the original interpretations of the data from 1998, they discovered there was hardly any atomic oxygen in the atmosphere of Ganymede.
This means there must be another explanation for the apparent differences between the UV aurora images.
The explanation was then uncovered by the authors in the relative distribution of the aurorae in the two images.
Ganymede’s surface temperature varies strongly throughout the day, and around noon near the equator it may become sufficiently warm that the icy surface releases some small amounts of water molecules.
In fact, the perceived differences between the UV images are directly correlated with where water would be expected in the moon’s atmosphere.
“Initially only molecular oxygen had been observed. This is produced when charged particles erode the ice surface,” Dr. Roth said.
“The water vapor that we have now measured originates from ice sublimation caused by the thermal escape of water vapor from warm icy regions.”
The results were published online today in the journal Nature Astronomy.
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L. Roth et al. A sublimated water atmosphere on Ganymede detected from Hubble Space Telescope observations. Nat Astron, published online July 26, 2021; doi: 10.1038/s41550-021-01426-9
