Did you know that Jupiter’s moon Ganymede contains more water than all of the oceans combined on the Earth? The downer though is that Ganymede’s ocean is deep buried more than 160 kilometres below its icy crust. So, water on Jupiter’s largest moon’s surface is just in solid form, or at least that is what scientists thought before they found the evidence of water vapour in Ganymede’s thin atmosphere.
The journey of the latest find dates back to 1998 when Hubble took the first ultraviolet image of Jupiter’s moon using its Imaging Spectrograph. The image featured colour aural bands — colourful ribbons of electrified gas — similar to auroras or polar lights on Earth. The band revealed that Ganymede has a weak magnetic field. However, observation by Hubble showed that the auroral bands changed over time. The reason behind this, scientists thought, was the presence of molecular oxygen and single oxygen atoms in Ganymede’s atmosphere.
As oxygen atoms affect the wavelength of ultraviolet light that scientists were observing, they were thought to be at the source of this. In 2018, when scientists tried to measure the amount of oxygen in Ganymede’s atmosphere, they were surprised as there were hardly any single oxygen atoms that could have caused the difference in the ultraviolet light’s wavelength. Looking for another explanation, scientists combined all the data and analysed spectral images and high-sensitivity spectra captured by Hubble, and they found evidence of water in the atmosphere. In fact, they also found that at a point where the sunlight directly hits the Jupiter moon, water vapour is more abundant than molecular oxygen.
“The water vapour that we measured now originates from ice sublimation caused by the thermal escape of water vapour from warm icy regions,” said Lorentz Roth, one of the authors of the study, in a statement.
According to scientists, the temperature on Ganymede varies widely throughout the day, making it really warm when the sun is overhead. This causes the ice to directly sublimate into water vapour instead of melting into liquid water. The study was published on July 26 in Nature Astronomy.
The study will help European Space Agency’s upcoming mission JUICE — JUpiter ICy moons Explorer, which will observe Jupiter’s moons for three years, especially Ganymede as potential habitat.