Scientists using new and archival data from NASA’s Hubble Space Telescope have uncovered for the first time evidence of water vapor in the atmosphere of the largest moon in our solar system, Jupiter’s icy moon Ganymede, which contains more water than all of Earth’s oceans. Water vapor forms when ice from the moon’s surface sublimates—that is, undergoes a phase change from a solid to gas. Ganymede is one of three icy Galilean moons, including Europa and Callisto, that are thought to contain liquid water oceans and potential life-bearing habitats beneath their surface. Jupiter’s gravity stretches and squeezes these moons as they orbit the gas giant, heating their interiors through friction.
A 2018 study argues that the four largest moons of Jupiter, the largest planet in the solar system –three of them harboring oceans believed to 100 kilometers deep or more–may have a bigger influence on each other’s tides than the gas giant itself does generating more heat from friction and could be more suitable to hosting life than previously thought.
Tidally resonant moons — suitable to hosting life
A 2020 paper by researchers with the Lunar and Planetary Laboratory in Tucson, Ariz., argued that Galilean moons’ gravitational pulls on each other, though smaller, could be producing larger tides than Jupiter does. That’s because they are more tidally resonant with each other. The findings suggest that oceans on these moons could then generate more heat from friction and could be more suitable to hosting life than previously thought.
Temperatures on Ganymede are so cold that water on the surface is frozen solid. Ganymede’s ocean would reside roughly 100 miles below the crust; therefore, the water vapor would not represent the evaporation of this ocean.
In 1998, Hubble’s Space Telescope Imaging Spectrograph (STIS) took these first ultraviolet Discovery (UV) images of Ganymede (above), 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 (O2, composed of two oxygen atoms). 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. (NASA, ESA, Lorenz Roth KTH)
In 1998, Hubble’s Space Telescope Imaging Spectrograph (STIS) took the first ultraviolet (UV) images of Ganymede, above, which revealed in two images colorful ribbons of electrified gas called auroral bands, and provided further evidence that Ganymede has a weak magnetic field.
The similarities in these UV observations were explained by the presence of molecular oxygen (O2, composed of two oxygen atoms). But some observed features did not match the expected emissions from a pure O2 atmosphere. At the same time, scientists concluded this discrepancy was likely related to higher concentrations of atomic oxygen (O; which are single oxygen atoms).
As part of a large observing program to support NASA’s Juno mission in 2018, Lorenz Roth of the KTH Royal Institute of Technology in Stockholm, Sweden led the team that set out to measure the amount of atomic oxygen with Hubble. The team’s analysis combined the data from two instruments: Hubble’s Cosmic Origins Spectrograph (COS) in 2018 and archival images from the Space Telescope Imaging Spectrograph (STIS) from 1998 to 2010.
Located 1/2-billion miles (over 7600 million kilometers) away, Hubble can follow changes on Ganymede and reveal other characteristics at ultraviolet and near-infrared wavelengths.
A Surprise Discovery
To their surprise, and contrary to the original interpretations of the data from 1998, they discovered there was hardly any atomic oxygen in Ganymede’s atmosphere. This means there must be another explanation for the apparent differences in these UV aurora images.
Roth and his team then took a closer look at the relative distribution of the aurora in the UV images. Ganymede’s surface temperature varies strongly throughout the day, and around noon near the equator it may become sufficiently warm that the ice surface releases (or sublimates) some small amounts of water molecules. In fact, the perceived differences in the UV images are directly correlated with where water would be expected in the moon’s atmosphere.
“So far only the molecular oxygen (O2) had been observed,” explained Roth. “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.”
Phase Change from Ice to Water Finally Detected
“It is great that we could finally detected a *sublimated* atmosphere –an atmosphere transitioned from solid ice to a gas state–on an icy moon. Simulations and models have suggested such an atmosphere to be present since the 1980’s,” Lorenz Roth wrote in an email to The Daily Galaxy. “It is difficult to say exactly where and how the water atmosphere is sourced from the surface. It seems like there are some regions that both contain sufficient ice at the surface and get warm enough around noon for a phase transition to be efficient.”
This finding adds anticipation to ESA (European Space Agency)’s upcoming mission, JUICE, which stands for JUpiter ICy moons Explorer. JUICE is the first large-class mission in ESA’s Cosmic Vision 2015-2025 program. Planned for launch in 2022 and arrival at Jupiter in 2029, it will spend at least three years making detailed observations of Jupiter and three of its largest moons, with particular emphasis on Ganymede as a planetary body and potential habitat.
Ganymede was identified for detailed investigation because it provides a natural laboratory for analysis of the nature, evolution and potential habitability of icy worlds in general, the role it plays within the system of Galilean satellites, and its unique magnetic and plasma interactions with Jupiter and its environment.
“Our results can provide the JUICE instrument teams with valuable information that may be used to refine their observation plans to optimize the use of the spacecraft,” added Roth.
Right now, NASA’s Juno mission is taking a close look at Ganymede and recently released new imagery of the icy moon. Juno has been studying Jupiter and its environment, also known as the Jovian system, since 2016.
Understanding the Jovian system and unraveling its history, from its origin to the possible emergence of habitable environments, will provide us with a better understanding of how gas giant planets and their satellites form and evolve. In addition, new insights will hopefully be found on the habitability of Jupiter-like exoplanetary systems.
Source: Lorenz Roth et al, A sublimated water atmosphere on Ganymede detected from Hubble Space Telescope observations, Nature Astronomy (2021). DOI: 10.1038/s41550-021-01426-9
Image at top of page: Ganymede as seen by the Galileo spacecraft via NASA.