Exploring Ganymede

Thanks to the Hubble Space Telescope, scientists say they now have evidence of water vapor drifting through the thin atmosphere of one of Jupiter’s moons. A group of researchers led by scientists from the KTH Royal Institute of Sweden say old data from Hubble demonstrate that at least some of the surface ice of Ganymede, the largest of Jupiter’s 79 moons, has evaporated and now exists in the moon’s atmosphere as water vapor.

The unexpected discovery gives scientists hope of finding thin, wispy clouds in the atmosphere of other icy planets or moons. “These findings suggest that water vapor actually exists in the atmospheres of icy bodies in the outer solar system,” lead author Lorenz Roth told Space.com. “Now we might see it more places.”

If it didn’t orbit Jupiter, Ganymede could almost be considered a planet in its own right. Larger than both Mercury and Pluto, the icy moon has a radius of more than 1,600 miles, 77 percent that of Mars. Ganymede also boasts a magnetic field, something no other satellite in the solar system has. In 2015, scientists studying data derived from the Hubble telescope discovered evidence of a massive, 60-mile thick saltwater ocean 95 miles beneath Ganymede’s surface. And in June, a flyby from NASA’s Juno probe revealed images that led scientists to believe the surface might contain tectonic faults like Earth.

The latest research on Ganymede, published July 26 in the journal Nature Astronomy, makes the moon seem even more planet-like. After studying the first ultraviolet images of Ganymede that Hubble captured in 1998, scientists saw aurora borealis–like effects on the moon. The ultraviolet light Hubble captured suggested the moon’s atmosphere contained oxygen.

However, the light sparked by Ganymede’s auroras wasn’t a perfect match for dioxygen, the pairing of two oxygen atoms that is common in Earth’s atmosphere. Instead, analysis of the Hubble images revealed some amount of other types of oxygen-containing molecules. Initially, researchers believed they were seeing evidence of atomic oxygen—unbonded individual oxygen atoms.

But tests during Juno’s flyby revealed very little atomic oxygen in the moon’s atmosphere. Roth and his team found another explanation. They began checking temperature data from the moon and noted that it might be possible for some surface ice to skip the liquid phase and evaporate into a gas at certain places on the moon’s surface. The team identified spots where the cold moon warms up to minus 190 degrees Fahrenheit, enough heat to slowly convert solid ice into water vapor.

The relative hot spots on Ganymede matched the unexplained ultraviolet lights. That led Roth and his team to conclude the strange oxygen-related ultraviolet light was produced not by atomic oxygen, but by a single oxygen atom paired with two hydrogen atoms—water.