Everything in its Right Place - Galileo's Moons Quiz

Answer: Ganymede

Ganymede is the largest of the four Galilean moons, and the largest natural satellite in the Solar System. Its orbit around Jupiter puts it as the seventh closest, and the third closest of the Galilean group, as well as it creating an orbital resonance with two of its fellows - for every orbit Ganymede makes, Europa makes two and Io four. Ganymede's surface is a mix of older, darker regions with abundant impact craters, and lighter, younger regions with an extensive system of grooves and ridges, believed to have been created by tectonic activity caused by tidal heating, a process caused by Ganymede being pulled and flexed as a result of Jupiter's gravity.

Answer: Io

Io is the innermost of the four Galilean moons, taking just 42 hours to complete an orbit of Jupiter. Because Io is in an orbital resonance with Europa, the next of the Galilean moons out, completing two orbits for Europa's one, it is subject to extreme tidal forces between Europa and Jupiter.

These cause a phenomenon called tidal heating that has resulted in an extreme level of volcanism on the surface. As a result, unlike other similarly sized bodies in the Solar System, Io is virtually devoid of impact craters as its surface is being constantly remade by volcanic eruptions.

Answer: Europa

Europa is the smallest of the four Galilean moons, and is the second in sequence out from Jupiter after Io. Like Io, Europa's surface is fairly devoid of impact craters, instead having a large number of cracks and striations across it, due to its crust being covered by water-ice.

As a result, Europa has the smoothest surface of any known body in the Solar System, as the crust is constantly remade. The 1:2:4 orbital resonance it shares with Io and Ganymede causes a process of tidal heating - the gravitational tugging of Jupiter plus the other moons causes friction within Europa, leading to an internal heat source believed to be a cause of its smooth surface.

Answer: Callisto

Callisto is the outermost of the four Galilean moons, taking 16.5 days to complete a single orbit of Jupiter. Unlike the other three, it is not in an orbital resonance, and thus not subject to the same process of tidal heating. As a result, unlike Io and its volcanism, or Europa and its ice-crust and sub-surface ocean, the surface of Callisto is not being constantly reformed, with no evidence of any subsurface activity, making it possibly the oldest and most cratered surface of any body in the Solar System.

In fact, Callisto has such a high density of craters on its surface that any new impact crater formed from a future impact would likely lead to the erasure of an existing crater.

Answer: Tuscany

Cosimo II was born in 1590, the eldest child of Ferdinando I, Grand Duke of Tuscany, a state located in the north-west of the Italian peninsula with its capital at Florence, ruled over by the Medici family. In 1605, Ferdinando appointed Galileo Galilei as Cosimo's tutor, a position he occupied for three years until 1608.

The following year, Cosimo succeeded his father as Grand Duke, and proceeded to become Galileo's patron, appointing him as the Court Mathematician. It was around this time that Galileo, in early experiments with a refracting telescope, discovered the four moons of Jupiter.

Initially naming the objects as "Cosimo's stars", after his patron, he was subsequently advised to name them the "Medici stars", with each named for Cosimo and his three brothers.

Answer: Simon Marius

Although Galileo is primarily credited with the discovery of the four large moons of Jupiter, at around the same time the German astronomer Simon Marius was also undertaking observations of Jupiter with a telescope. He too noticed the objects in orbit, and independently published his findings in 1611, after Galileo's initial publication of the discovery.

In 1614, Marius published again, which included ideas for names of the objects, including one based on a suggestion by Johannes Kepler, where the four moons would be named after lovers of Zeus from Greek mythology.

Although not widely used initially, by the mid-20th century, when more moons were discovered orbiting Jupiter, the naming system came to be the standard for satellites in the Solar System.

Answer: Callisto

Callisto's orbit extends to approximately 1.8 million km from Jupiter, considerably greater than the next of the four Galilean moons inward, Ganymede. Thanks to its distance, Callisto is not locked into the orbital resonance of the other three and, as such, does not suffer from the gravitational flux that leads to the process of tidal heating that the others undergo, which has led to its surface not being appreciably renewed. Callisto's distance from Jupiter means it is much less affected by the planet's magnetic field and radiation belt.

This, combined with its geological inactivity, makes it the most suitable location for a prospective human base for manned exploration of Jupiter.

Answer: Io

Eighteen months after its launch, Voyager 1 began its approach to Jupiter, with part of its mission being to explore Jupiter's moons. On 5 March, the spacecraft made its flypast of Io; at the time, Io was believed to be a dead world, but images revealed a lack of impact craters on the surface.

In one of the spacecraft's images, mission scientist Linda Morabito noticed a plume emerging from Io. Further analysis saw nine other such plumes in other images, indicating that Io was in fact geologically active. Images taken four months later by Voyager 2 showed several surface features had changed since Voyager 1's flyby, suggesting a high level of volcanism. Study of Io has shown that it has as many as 400 active volcanos, making it the most geologically active body known in the Solar System.

Answer: Europa

When the two Voyager spacecraft performed their flybys of Jupiter, among the data returned were photographs of the surface of Europa. The smoothness of the crust suggested to scientists that beneath its surface may be a sub-surface ocean of liquid water. Data from the Galileo orbiter, which undertook detailed observations of Europa, provided more evidence of this. Any ocean that does exist maintains its liquid state as a result of tidal heating, where the gravitational action of Jupiter causes friction in Europa's core, generating heat.

This process has been speculated to also cause hydrothermal vents that may harbour extra-terrestrial life, similar to those in the deep ocean environment on Earth.

Answer: Ganymede

When Simon Marius gave names to the four objects discovered orbiting Jupiter, it was at the suggestion of Johannes Kepler that they be named after lovers of Zeus. The largest of the four was named for Ganymede, a Trojan hero that Zeus became enamoured with and carried off to Mount Olympus to serve as the cup bearer to the gods.

However, the name Ganymede, as with the other three, was not generally used until the mid-20th century, with instead a simple designator system of Roman numerals used, giving Ganymede the name Jupiter III, as the third satellite of Jupiter. Marius's naming system was revived following the discovery of new moons of Saturn closer than those already discovered, to prevent the confusion of having to renumber them all.