Water Vapor on the Largest Moon in the Solar System

Jupiter has a ton of moons. 79 moons in fact. 53 of these moons are named and some of these moons are heavily studied by NASA and other space agencies, including Io, Europa, Callisto, and the moon I will be talking about today: Ganymede. Recently, the Hubble Space Telescope observed that there was water vapor in Ganymede's atmosphere.

Ganymede

Ganymede was first observed by Galileo Galilei in 1610 and is named after a figure in Greek mythology named Ganymede who was a lover of Zeus and his cup-bearer in the Olympics (If you think about it, the name makes sense because Jupiter represents Zeus and Ganymede is orbiting Jupiter. Most of the other names of Jupiter’s moons have some relation to Zeus as well). Ganymede orbits Jupiter at a distance of 1,070,400 km and completes one revolution every seven days and three hours. Additionally, Ganymede is tidally locked, meaning that one side is always facing toward the planet. Ganymede also participates in orbital renaissance with two of Jupiter's moons: Europa and Io. This means that for every orbit of Ganymede, Europa orbits twice, and Io orbits for times. 

Ganymede is the largest moon in the solar system and is about 1.5 times larger than our Moon! The average density of Ganymede is 1.936 g/cm, which suggests that it is mostly made up of rocky material and watery ices. To further support water on Ganymede, the Galileo spacecraft discovered high concentrations of different sulfates, which may prove that there is an ocean under the surface of Ganymede. Additionally, previous studies have shown that Ganymede may contain more water than all of Earth’s oceans combined!

Aurorae on Ganymede

The discovery of water vapor on Ganymede is amazing, especially since prior work has suggested that ice on Ganymede’s turned from a solid to a gas directly. In this new study, researchers looked at data both old and new captured by the Hubble Space Telescope that dates back all the way to 1998. The researchers looked at pictures of auroras on Ganymede, which are made up of electrified gas, and evidence that shows that Ganymede has a weak magnetic field. Additionally, using ultraviolet signals, researchers were able to detect the presence of oxygen atoms, which were produced when charged particles erode Ganymede’s icy surface. However, the ultraviolet emissions also showed that the oxygen molecules weren’t actually pure oxygen. Even more surprising is that the Juno mission to Jupiter detected barely any pure oxygen molecules, which must mean there is a different explanation for the previous ultraviolet signals.

Scientists then focussed on Ganymede’s surface temperature, which varies greatly throughout the day. The weather has a high of minus 190 degrees Fahrenheit and a low of minus 315 degrees Fahrenheit. On the hottest spots on Ganymede, Ganymede’s surface gets hot enough to melt ice and convert it into water vapor.

They noted that this hypothesis could be the reason that they observed water vapor. The data that they collected also matched the hypothesis.

The discovery of water vapor in space is amazing and Ganymede is an amazing moon in our solar system. In fact, scientists are so curious about Ganymede that the ESA started construction on a satellite called the Jupiter Icy Moons Explorer, or JUICE for short. JUICE will study Ganymede, Callisto, and Europa, which are all thought to have liquid water beneath their surfaces. Maybe, JUICE might be able to find forms of liquid water on those moons, which in turn may show that there is life outside of Earth.

Render of JUICE Going By Ganymede

Sources:

Choi, Charles Q. “Water Vapor Detected on Huge Jupiter Moon Ganymede for 1st Time.” Space.com, Space, 27 July 2021, www.space.com/jupiter-moon-ganymede-water-vapor-discovery. 

“Ganymede (Moon).” Wikipedia, Wikimedia Foundation, 18 July 2021, en.wikipedia.org/wiki/Ganymede_(moon). 

“Jupiter Icy Moons Explorer.” Wikipedia, Wikimedia Foundation, 28 July 2021, en.wikipedia.org/wiki/Jupiter_Icy_Moons_Explorer. 

“Jupiter Moons.” NASA, NASA, 10 Sept. 2018, solarsystem.nasa.gov/moons/jupiter-moons/overview/?page=0&per_page=40&order=name%2Basc&search=&placeholder=Enter%2Bmoon%2Bname&condition_1=9%3Aparent_id&condition_2=moon%3Abody_type%3Ailike.