Astronomers have uncovered the first evidence of water vapour in the atmosphere of Jupiter's moon Ganymede by using new and archival datasets from NASA's Hubble Space Telescope. According to the study published in the journal Nature Astronomy, the water vapour forms when ice from the moon's surface turns from solid to gas.


  1. Previous studies have offered circumstantial evidence that Ganymede, the largest moon in the solar system, contains more water than all of Earth's oceans. However, temperatures there are so cold that water on the surface is frozen solid.
  2. Ganymede's ocean would reside roughly 160 kilometres below the crust, therefore, the water vapour would not represent the evaporation of this ocean.
  3. Astronomers re-examined Hubble observations from the last two decades to find this evidence of water vapour.
  4. In 1998, Hubble's Space Telescope Imaging Spectrograph took the first ultraviolet (UV) images of Ganymede, which revealed colourful ribbons of electrified gas called auroral bands, and provided further evidence that Ganymede has a weak magnetic field.
  5. The similarities in these UV observations were explained by the presence of molecular oxygen (O2). However, some observed features did not match the expected emissions from a pure O2 atmosphere.
  6. At the same time, scientists concluded this discrepancy was likely related to higher concentrations of atomic oxygen (O).
  7. Ganymede's surface temperature varies strongly throughout the day, and around noon near the equator it may become sufficiently warm that the ice surface releases some small amounts of water molecules.
  8. The perceived differences in the UV images are directly correlated with where water would be expected in the moon's atmosphere. So far only the molecular oxygen had been observed.


  1. The finding adds anticipation to European Space Agency (ESA)'s upcoming mission JUpiter ICy moons Explorer (JUICE).
  2. Planned for launch in 2022 and arrival at Jupiter in 2029, JUICE 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.