For the first time, scientists have found evidence of water vapor in the atmosphere of Jupiter’s moon Ganymede.

Ganymede is the largest moon, and the ninth-largest object, in the Solar System.

The moon may hold more water than all of Earth’s oceans. But the super cold temperature on Ganymede freezes the surface water. However, scientists believe an entire ocean may lie about 160 kilometers (99 miles) below the crust. And you know what they say? Where there is water there could be life as we know it.

As water plays such an essential role in life on Earth, the presence of water has been vital in the search for other habitable planets and moons.

And now, NASA announced that scientists have found evidence for water vapor in the atmosphere of Ganymede. They have achieved this by looking through the last two decades of data from the legendary Hubble Space Telescope.

Apparently, this water doesn’t come from the underground ocean. The water vapor on Ganymede forms when ice from its surface sublimates, meaning the water turns from solid to gas.

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So the story began in 1998. That’s when Hubble’s Space Telescope Imaging Spectrograph (STIS) took the first ultraviolet (UV) pictures of Ganymede.

These pictures revealed colorful ribbons of electrified gas called auroral bands. It also provided further evidence that Ganymede has a weak magnetic field.

The similarities between the two UV observations were explained by the presence of molecular oxygen, O2. At the time, the presence of atomic oxygen, O, explained the differences. The oxygen produces a signal that affects one UV color more than the other.

In 2018, a Swedish team led by Lorenz Roth of the KTH Royal Institute of Technology in Stockholm, measured the amount of atomic oxygen using Hubble Telescope data from 1998 to 2010.

Surprisingly, and contrary to the original interpretations of the data from 1998, the team discovered there was hardly any atomic oxygen in Ganymede’s atmosphere. This means there must be another explanation for the differences between the UV aurora images.

Later on, Roth and his team uncovered the explanation when they took a closer look at the relative distribution of the aurora in the UV observations. Ganymede’s surface temperature varies strongly throughout the day. And around noon near the equator, it may become sufficiently warm that the icy surface releases some small amounts of water molecules. In fact, the perceived differences between the UV images are directly correlated with where water would be expected in the moon’s atmosphere.

“So far only the molecular oxygen 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.”

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Even though the study doesn’t say much about the moon’s potential for alien life, this water vapor helps scientists better understand Ganymede’s atmosphere. Previously, they only knew that it contained oxygen.

However, the European Space Agency (ESA) is planning to launch a mission to Jupiter in 2022. They call the mission JUICE which stands for JUpiter ICy moons Explorer.

ESA expects JUICE to arrive at Jupiter in 2029. The explorer will spend at least three years making detailed observations of Jupiter and three of its largest moons, with a special focus on Ganymede as a planetary body and potential habitat.

Ganymede can act as a natural laboratory to analyze the nature, evolution, and potential habitability of icy worlds in general, its role within the Galilean moons’ system, and its unique magnetic and plasma interactions with Jupiter its environment.

Roth believes their results can provide the JUICE mission with valuable information that ESA may use to refine their observation plans to optimize spacecraft’s use.

At the moment, NASA’s Juno mission is active and taking close observations of Ganymede. Juno has been studying the Jovian system since 2016.

Scientists published their study in the journal Nature Astronomy.

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