Thanks to data collected by NASA’s Hubble Space Telescope, astronomers were able to find the first ever evidence of water vapor in the atmosphere of Ganymede. The water vapor found on Jupiter’s moon was a result of sublimating ice on the surface (this is when it turns from solid to gas).
Past research has claimed that Ganymede – the largest moon in our Solar System – has more water than all of our planet’s oceans combined, but since the temperatures on the moon are so cold, the water is completely frozen on the surface and its ocean would have to be located approximately 100 miles underneath the crust. The daytime temperature on Ganymede is between -297 and -171 degrees Fahrenheit (-183 to -113 degrees Celsius). With that being said, the water vapor observed by Hubble over the past two decades could not have been part of the underground ocean.
To understand this better, we have to look back at the initial observations of Ganymede. Back in 1998, the first ever ultraviolet photos of the moon were taken by Hubble’s Space Telescope Imaging Spectrograph. They showed colorful auroral bands that experts believed were caused by molecular oxygen (O2); however, further analysis indicated that the moon did not have pure molecular oxygen in its atmosphere and suggested that there might have been a higher amount of atomic oxygen.
Now skipping forward, a team that was led by Lorenz Roth from the KTH Royal Institute of Technology in Stockholm, Sweden, decided to measure how much atomic oxygen was in Ganymede’s atmosphere by using data collected by Hubble's Cosmic Origins Spectrograph in 2018, as well as old photos from 1998 to 2010 that were taken by the Space Telescope Imaging Spectrograph (STIS).
Incredibly, they found that there wasn’t very much atomic oxygen in the atmosphere and that there had to be another explanation for the differences noticed in the ultraviolet aurora images. The moon’s surface close to the equator could possibly warm up enough that ice vapor is released as Roth explained, “So far only the molecular oxygen had been observed,” adding, “This is produced when charged particles erode the ice surface. The water vapor that we measured now originates from ice sublimation caused by the thermal escape of water vapor from warm icy regions.” (The ultraviolet images can be seen here.)
This is an exciting new discovery and we will hopefully learn more when the European Space Agency launches its JUICE (JUpiter ICy moons Explorer) mission in 2022. It will arrive at the Gas Giant in 2029 and will study Jupiter in addition to its three largest moons including Ganymede.
For the time being, NASA’s Juno mission is currently studying Ganymede and several recent photos have been released of the moon. (The pictures can be seen here.)
The study was published in the journal Nature Astronomy.