Jupiter’s "singing moon" Ganymede. Facts about the largest Jovian moon

Ganymede has a radius of 1,635 miles (or 2,631 kilometers).  Thus, it is considered the largest moon in the Solar System. Ganymede is larger than the Earth's moon, whose radius is only 1,079.6 miles (1,737.4 km). It’s also larger than Mercury, the smallest planet in the Solar System, which has a radius of 1,516 miles (or 2,439.7 kilometers), and Pluto, and just slightly smaller than Mars.

In fact, in a list of objects in the Solar System ordered by size, Ganymede would be in 9th place. 

This moon of Jupiter also has a mass of 1.48×10²⁰ tonnes (each tonne is equal to 2,204.6 pounds or 1,000 kilograms). This is about twice the mass of the Earth’s moon. 

Ganymede was discovered by Galileo Galilei

Ganymede is one of the four moons of Jupiter discovered in the 17th Century by Italian astronomer Galileo Galilei. Therefore, it is one of the four Galilean moons. The others are Io, Europa, and Calisto. Three of these were first spotted on the same day, January 7, 1610, as the scientists peered through his newly-improved 20-power homemade telescope. The fourth was spotted a few days later.

At first, Galileo thought that they were stars. After watching them with his telescope over several days, he realized that they described an orbit around Jupiter. Observing the "stars" over several nights, he noticed that they appeared to move in a different direction to the background stars and changed their positions relative to one another while staying close to Jupiter. He also noticed a fourth one. Within a few days, he concluded that they were not stars at all but moons orbiting around Jupiter.  He named them “Medicean Stars” after his patron, the Grand Duke of Tuscany, Cosimo II de Medici. 

To distinguish them from each other, he initially used numbers: Jupiter I, Jupiter II, Jupiter III, and Jupiter IV, in order of their distance from Jupiter. Ganymede was “Jupiter III”. 

German astronomer Simon Marius claimed to have located the satellites on January 8, 1610, but because his observations were not published until several years later, he was not credited with the findings at the time. He is credited, however, for proposing the names Io, Europa, Ganymede, and Callisto, based on Greek mythology (they were all lovers of the god Jupiter in Greek mythology), a few years later, in his book Mundus Iovialis (1614). He admits in his writings that this idea came from Johannes Kepler, though. 

Interestingly, three hundred years later, a Dutch "jury" of experts concluded that Marius had in fact independently discovered the moons of Jupiter just one day after Galileo recorded his observations.

These names, however, fell out of favor until the 20th century, when the International Astronomical Union's (IAU) Task Group for Outer Solar System Nomenclature created a formal naming process which named any moons of Jupiter after lovers, favorites, and descendants of the god Jupiter (Zeus).

Ganymede has plenty of water

Ganymede is mostly made of silicate rock and water ice. The rocky material is located on top of a metallic iron core and forms several lumps beneath the ice shell. It is believed there is liquid water located beneath the ice shell. And it's mixed with a salt called magnesium sulfate, meaning that it's salt water. 

Evidence for the existence of this underground, saline ocean was first found by NASA’s Galileo mission and was later bolstered with data from the Hubble Space Telescope. Scientists think that the ocean under this Jupiter moon’s surface is about 60 miles (100 kilometers) thick, about ten times the depth of Earth's oceans, and contains more water than all the Earth’s oceans combined.

 More evidence of water on Ganymede was collected in 2021. At that time, data from NASA's Hubble Space Telescope confirmed the presence of water vapor while studying the moon's magnetic field. The researchers found differences in the magnetic field they originally believed was due to the presence of molecular oxygen in the moon’s atmosphere. A further study suggested that these changes were instead due to the presence of water vapor produced from ice sublimation caused by the thermal escape of water vapor from the warming of icy regions.

Sublimation is a chemical process in which a solid material skips the liquid phase and turns directly into gas. In this case, the process is believed to be triggered by temperature changes near Ganymede’s equator.

Ganymede’s temperature ranges from -297ºF to -171ºF (-182ºC to -112ºC). It is always cold because it is far from the Sun and doesn’t have an atmosphere that is dense enough to conserve the heat of the little sunlight it receives. However, evidence shows that Ganymede is warmer in certain regions and at certain hours of its one-week-long day. 

Ganymede is the only moon in the solar system that has a magnetosphere

Magnetospheres have been found in several planets, including Earth, Mercury, Saturn, Mercury, and Jupiter; but not in moons. That was until 1996 when NASA’s Galileo spacecraft recorded whistling and static sounds that revealed the existence of a self-generated magnetosphere in Ganymede. 

 This suggests that Ganymede has an active interior "dynamo", most likely an iron-nickel rich liquid core as its innermost layer —located at a depth of about 250 to 800 miles (400 to 1,300 kilometers). The convection in the liquid iron, which has high electrical conductivity, would generate the magnetic field.

Just like Earth's magnetic field, Ganymede’s magnetosphere is made of charged particles that act as a shield against cosmic radiation, including radiation coming from Jupiter. And just like on Earth's, Ganymede’s magnetic field produces glowing auroras in its north and south poles.  

Electrified gases forming auroral belts were first observed in ultraviolet images of Ganymede taken by Galileo's plasma wave spectrometer, and later by the Hubble's Space Telescope Imaging Spectrograph. 

Could there be life on Ganymede?

According to NASA, a computer model of Ganymede’s interior showed that primitive life could be possible there due to the interaction of saltwater with rocky materials. 

But it could exist only under Ganymede’s thick layers of ice, where sunlight is blocked. This means that any organisms present would have to be able to survive without depending on sunlight, and would need another source of energy, much like the deep-sea creatures of Earth. They would also have to face pressures greater than those in Earth’s famous Mariana Trench, which is 36,201 feet (or 11,034 meters) deep.

In order to investigate Ganymede’s potential habitability, the European Space Agency (ESA) will send the Jupiter Icy Moon Explorer (JUICE) probe to collect data from Ganymede, as well as from two of Jupiter's other moons, Europa and Callisto. The probe is set for launch between August and September 2022 and will be inserted in orbit around Ganymede in around 2032.