For the first time: Hubble finds water on Jupiter’s moon Ganymede

Key Takeaways:

Scientists using the Hubble Telescope have detected water vapor in Ganymede’s atmosphere for the first time.
This water vapor forms as surface ice sublimates, transitioning from solid to gas.

Ganymede, Jupiter’s largest moon, likely contains more water than Earth’s oceans, but it’s frozen solid on the surface due to extreme cold.
Previous UV observations of Ganymede revealed electrified gas ribbons, hinting at a weak magnetic field and the presence of molecular oxygen.
A recent re-analysis of Hubble data indicates a scarcity of atomic oxygen, suggesting a different source

Astronomers using NASA’s Hubble Space Telescope have made a groundbreaking discovery: evidence of water vapor in the atmosphere of Ganymede, Jupiter’s largest moon. This water vapor is a result of the sublimation of ice from Ganymede’s surface, where it transitions directly from solid to gas.

While previous research had hinted at Ganymede potentially harboring more water than Earth’s oceans, the extreme cold on the moon’s surface keeps this water locked in ice form. The moon’s actual ocean lies about 100 miles beneath its crust, meaning the detected water vapor doesn’t arise from its evaporation.

Scientists re-examined Hubble’s observations spanning two decades, specifically those from 1998 when the Space Telescope Imaging Spectrograph captured the first ultraviolet (UV) images of Ganymede. These images showcased colorful ribbons of electrified gas called auroral bands, hinting at the moon’s weak magnetic field.

In 1998, Hubble’s Space Telescope Imaging Spectrograph took these first ultraviolet images of Ganymede, which revealed a particular pattern in the observed emissions from the moon’s atmosphere. The moon displays auroral bands that are somewhat similar to aurora ovals observed on Earth and other planets with magnetic fields. This was an illustrative evidence for the fact that Ganymede has a permanent magnetic field. The similarities in the ultraviolet 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. Credits: NASA, ESA, Lorenz Roth (KTH)

The presence of molecular oxygen (O2) explained some UV features, but not all. This discrepancy was attributed to higher concentrations of atomic oxygen (O). However, in 2018, as part of a program supporting NASA’s Juno mission, researchers led by Lorenz Roth analyzed data from Hubble’s Cosmic Origins Spectrograph and archived images from the Space Telescope Imaging Spectrograph. Surprisingly, they found a scarcity of atomic oxygen in Ganymede’s atmosphere, prompting a re-evaluation of the UV aurora images.

Upon closer examination, the scientists realized that the observed differences in the UV aurora images directly correlated with the areas where water would be expected in Ganymede’s atmosphere. The moon’s surface temperature fluctuates significantly, and in warm regions, particularly around noon near the equator, some water molecules sublimate from the ice surface. This revelation sheds light on the origin of the observed water vapor.

This discovery raises excitement for the European Space Agency’s (ESA) upcoming JUICE mission, which will extensively study Jupiter and its largest moons, with special emphasis on Ganymede’s potential habitability. Ganymede, with its unique magnetic interactions and plasma connections with Jupiter, serves as a natural laboratory for the analysis of icy worlds. This new understanding will not only illuminate the formation and evolution of gas giants and their satellites but also offer insights into the habitability of similar exoplanetary systems.