Sometimes, science is about pretty images. And sometimes, it’s really not. When it comes to examining Hubble Space Telescope images of objects within our solar system, they leave a bit to be desired, especially when it comes to smaller objects, like moons. But Hubble isn’t just about pretty pictures, either.

Onboard Hubble are several spectrographs – instruments that split light into individual wavelengths. One such instrument is the Space Telescope Imaging Spectrograph, which works in wavelengths ranging from near-infrared all the way to ultraviolet. Back in 1998, this instrument took the first ultraviolet pictures of Jupiter’s largest moon Ganymede, and the resulting images revealed patterns in the atmosphere. Those patterns turned out to be auroral bands and provided scientists with evidence that Ganymede has a permanent magnetic field.

Then scientists compared the images of Ganymede’s aurorae with aurorae observed here on Earth, and while they were similar, there were also differences. Per the press release: The similarities between the two ultraviolet observations were explained by the presence of molecular oxygen, O2. The differences were explained at the time by the presence of atomic oxygen, O, which produces a signal that affects one UV color more than the other.”

Then along came NASA’s Juno mission in 2018, and many teams participated in a variety of Jupiter-based research to support the Juno mission. One team, led by Lorenz Roth, used Hubble’s Cosmic Origins Spectrograph to capture new images in an attempt to measure just how much atomic oxygen is in Ganymede’s atmosphere. And here’s where the headline would read, “The answer might surprise you!” Because it definitely surprised the researchers who discovered that there wasn’t really any atomic oxygen in the atmosphere. The original analysis was incorrect, and that meant it was time for science to do that wonderful thing where it improves with new information.

Roth and his team decided to examine the distribution of the aurorae between the newer images and the older ones. They discovered that because the surface temperature of Ganymede varies wildly throughout the day, around noon, the temps may be warm enough near the equator for the small amounts of water ice to sublimate into water vapor. And the differences between the two sets of images provided the correlation between the aurorae and where water vapor would be expected. Roth explained: Initially only the O2 had been observed. This is produced when charged particles erode the ice surface. The water vapor that we have now measured originates from ice sublimation caused by the thermal escape of H2O vapor from warm icy regions.

The results of Roth and his team’s research have been published in Nature Astronomy, and they may even prove useful to the upcoming JUICE mission from the European Space Agency. The instrument teams now have more information about where and what they might want to observe and time to make those observations plans.

Ganymede is an icy moon. It’s the ninth-largest object in our solar system. It possibly has more water than all of the oceans here on Earth, although those oceans are subsurface. Very subsurface. 160 kilometers subsurface. But all that liquid water means Ganymede’s oceans could be habitable, just like Europa’s might be. And we’re definitely rooting for Ganymede as a place for potential life. So between Juno, JUICE, and Europa Clipper, we’re pretty excited for what our future understanding of the Jovian system holds, and we’ll keep you up to date on all the Jupiter missions here on Daily Space.

More Information

ESA press release

Hubblesite press release

NASA press release

“A sublimated water atmosphere on Ganymede detected from Hubble Space Telescope observations,” Lorenz Roth et al., 2021 July 26, Nature Astronomy