The study, whose findings were published in the peer-reviewed academic journal Nature Astronomy, was built on data, new and old, from NASA's Hubble Space Telescope, according to NASA.
The idea of water being present on Jupiter's moons is nothing new. In fact, one moon in particular, Europa, having water has been a staple of science-fiction for many years – most notably in the book and film adaptation of 2010: Space Odyssey Two by Arthur C. Clark – and later gained greater support after NASA in 2018 discovered water plumes on the moon.
While Europa is rather small, smaller than Earth's moon, fellow Jovian moon Ganymede is the largest moon in the solar system. In fact, it's bigger than the planet Mercury.
It was long believed based on circumstantial evidence that Ganymede contained water. Specifically, it was thought that it had more water than the entirety of Earth's oceans. But as a moon without Earth's warm atmosphere and temperature, the water in question would all be frozen. Further, the moon's ocean would not be on the surface at all, but around 100 miles underneath its thick rocky and icy crust.
The evidence dates back to 1998, when Hubble spectrograph images found evidence of molecular oxygen (O2), but the features observed didn't match up to pure O2 atmosphere.
It was first thought that this could be because of high concentration of atomic oxygen (O), but a 2018 study that was meant to support NASA's Juno mission found that there was hardly any atomic oxygen in Ganymede's atmosphere in the slightest.
The 2018 study leader, Lorenz Roth of the KTH Royal Institute of Technology in Stockholm, Sweden, found that the moon's temperature isn't always as freezing as previously believed. At some points throughout its day, the area near the moon's equator, known as the subsolar point, due to it being where sunlight shines straight down on it, can become warm enough that its icy surface lets out small amounts of water molecules - and UV imaging backs this up.
It should be noted, though, that the subsolar point changes, due to how Ganymede's orbit works. Sometimes it's on what is known as the leading hemisphere, the part of the moon facing Jupiter. But when it's on the trailing hemisphere, which faces the Sun, there is more water vapor due to being somewhat warmer. As noted by astronomer Phil Plait, this means there can be a factor of six times as much water vapor when the subsolar point is on the trailing hemisphere.
These findings are especially significant due to the anticipation of the European Space Agency's (ESA) JUpiter ICy moons Explorer (JUICE) mission, which is set to launch a probe in 2022 to Jupiter, arriving in 2029, and observe the planet and the three largest Jovian moons, including Ganymede.NASA also has plans for the Jovian moons, with a probe dubbed the Europa Clipper set to be launched to the eponymous moon in 2024 and arrive in 2030, to see if the moon can support life.