Hubble Space Telescope Detects Evidence of Water Vapor on Jupiter's Moon Ganymede

For the first time, astronomers have discovered evidence of water vapor in the atmosphere of Jupiter's moon Ganymede. This water vapor forms when ice from the moon's surface sublimates—transitioning from solid to gas. Scientists used new data and archives from NASA's Hubble Space Telescope to make this discovery, published in the journal Nature Astronomy.

Previous studies had indicated that Ganymede, the largest moon in the solar system, contains more water than all of Earth's oceans combined. However, the extreme cold temperatures there freeze surface water. Ganymede's oceans lie beneath a crust about 100 miles thick, so the observed water vapor is not believed to represent the evaporation of these oceans.

Astronomers re-examined Hubble's observations from the past two decades to find evidence of this water vapor. In 1998, the Hubble's Space Telescope Imaging Spectrograph (STIS) captured the first ultraviolet (UV) images of Ganymede, revealing two colorful bands of charged particles known as auroral bands and confirming Ganymede's weak magnetic field.

Similarities in these UV observations were initially explained by the presence of oxygen molecules. However, some observed features did not match the expected amount of waste gas from the pure O2 atmosphere. Scientists concluded that this difference might be related to higher concentrations of atomic oxygen (O).

As part of a large observational program supporting NASA's Juno mission in 2018, Lorenz Roth of the KTH Royal Institute of Technology in Stockholm, Sweden, led a team to measure atomic oxygen levels with Hubble. Their analysis combined data from Hubble's Cosmic Origins Spectrograph (COS) in 2018 and archived images from the Space Telescope Imaging Spectrograph (STIS) from 1998 to 2010.

Contrary to the initial explanations from the 1998 data, they found almost no atomic oxygen in Ganymede's atmosphere. This means there must be another explanation for the apparent differences in these UV auroral images. Roth and his team then examined the relative distribution of auroras in UV images more closely. Ganymede's surface temperature varies dramatically throughout the day, and around midday near the equator, it can become warm enough for surface ice to sublimate a small amount of water molecules. In fact, the differences observed in the UV images directly correlate with where water vapor is expected in the moon's atmosphere.

Until now, only molecular oxygen could be observed. "This substance is produced when charged particles erode the surface ice. The water vapor we measure today originates from the sublimation of ice caused by the heating of water vapor from warm icy regions."

This discovery supplements predictions for the upcoming European Space Agency (ESA) mission, JUICE. Planned to launch in 2022 and arrive at Jupiter in 2029, it will spend at least three years conducting detailed observations of Jupiter and its three largest moons, with a particular focus on Ganymede as a planet and potential habitat.

"Ganymede is identified for detailed investigation because it provides a natural laboratory to analyze the nature, evolution, and potential habitability of icy worlds in general, its role in the Galilean moon system, and its unique magnetic and plasma interactions with Jupiter and its environment," says Roth.

"Our results could provide valuable information for the JUICE instrument team to refine their observation plans to optimize spacecraft usage."

Currently, NASA's Juno mission is closely observing Ganymede, and recently released new images of the icy moon. Juno has been studying Jupiter and its environment, known as the Jovian system, since 2016.

Understanding the Jovian system and shedding light on its history, from its origins to the possible emergence of habitable environments, will help us better understand how giant gas planets and their moons form and evolve. Additionally, it is hoped that new insights will be gained into the habitability of exoplanetary systems similar to Jupiter's moons.

The Hubble Space Telescope is an international collaboration between NASA and the European Space Agency (ESA). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the Hubble Space Telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts scientific operations on Hubble. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.

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[GregHenson-iups]

Jupiter Icy Moons Explorer (JUICE) will orbit around Ganymede.