The water vapor forms as the icy surface of the moon turns from a solid to a gas, a process called sublimation. Astronomers uncovered this water vapor while using a combination of new and archival observations from Hubble.
Previous research has suggested that Ganymede — the ninth-largest object in our solar system — contains more water than all of Earth’s oceans put together, although the moon is 2.4 times smaller than our planet.
But Ganymede is so cold, with temperatures that can reach negative 300 degrees Fahrenheit (-184 degrees Celsius), that the surface is a frozen water ice shell. About 100 miles (161 kilometers) below this crust is likely a salty ocean — and researchers knew there was no way that ocean was evaporating through the ice shell to create water vapor.
The moon is named for a cupbearer to the ancient Greek gods. In addition to being the largest natural satellite in our solar system, Ganymede is also the only moon to have a magnetic field. This causes auroras to glow around the moon’s north and south poles.
Hubble captured its first ultraviolet images of Ganymede in 1998, revealing these auroral bands. Initially, researchers believed these auroras were due to the pure oxygen atmosphere, which was first detected using the same telescope in 1996. But some of the features couldn’t be explained and even appeared slightly different from one another.
Ganymede’s surface temperature can vary significantly over the course of a day. Around noon at the moon’s equator, it becomes warm enough for the icy surface to sublimate, or release small amounts of water molecules — which explains the differences seen in the ultraviolet images taken by Hubble. Even though Ganymede’s ice shell is as hard as rock, a stream of charged particles from the sun is enough to erode and release water vapor.
The European Space Agency’s JUICE mission, or JUpiter ICy moons Explorer, will launch in 2022. It will arrive at Jupiter in 2029 and spend three years observing the giant planet and its three largest moons. Ganymede will be included in this survey, and researchers hope to learn more about the moon as a potential habitat for life.
“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,” said lead study author Lorenz Roth, a researcher at the KTH Royal Institute of Technology in Stockholm, in a statement.
Understanding more about Ganymede can help researchers learn more about how gas giants like Jupiter and their moons form and evolve over time — and whether icy moons, which are scattered across our solar system, are habitable environments where life can find a way.