Jupiter’s Moons and Rings

The lord of the planets has an entourage of 67 moons at the latest count. No doubt there are more yet to be discovered. The four largest are known as the Galilean moons after their discovery by pioneering Italian astronomer Galileo Galilei in January 1610. For many years they were referred to simply by the Roman numerals I, II, III and IV but since the discovery of the fifth moon, Amalthea, in the late 19th century they are now called Io, Europa, Ganymede and Callisto after characters from mythology who consorted with Jupiter.

All of the Galilean Moons are of sufficient size that their own gravity has pulled them into a spherical shape with heavier elements at their cores, lighter ones at or near the surface and even atmospheres. If you could take the planet Mercury along with our Moon and set them in descending order of size among the Galilean Moons the sequence would be Ganymede, Mercury, Callisto, Io, the Moon and Europa.

In stark contrast, the rest of the Jovian moons are no more than a twentieth of the size of the Galileans and many are much smaller. These diminutive moons lack sufficient mass to form as spheres and are merely odd-shaped conglomerates of rock. Many, if not all, are probably captured asteroids drawn in by Jupiter’s gravity. Images taken by the Galileo space probe revealed potato-shaped rocks ranging in size from dozens of miles down to only one or two miles across. Some of these tiny worlds are small enough to fit inside Hyde Park in London.


Io – Jupiter's innermost large moon
Io – Jupiter’s innermost large moon

The innermost of the Galilean Moons and with a diameter of 3640 km (2262 miles), Io is slightly larger than our Moon. When the Voyager 1 probe first imaged Io back in 1979, astronomers were shocked to see the most geologically active world in the Solar System rather than the boring, ice-covered ball of rock they’d been expecting.

Whirling around its orbit in less than two days, Io is the focus of a stupendous tug-of-war between Jupiter’s gravity on one side and that of its companion moons on the other. As a result Io’s surface is continuously squeezed in and out like a gigantic sponge ball, a process which generates massive internal heating. Volcanoes are constantly erupting, spewing lava across the surface and firing immense sulphur plumes hundreds of miles into space.

In contrast to the grey, cratered terrain normally associated with moons, Io’s surface is multi-coloured due to a predominance of sulphur and sulphurous compounds, varying from bright yellow to white, orange, red, black and green. Constant resurfacing means that geological features don’t last very long on Io, any impact craters it might have had would have soon been filled in with lava.


Next out from Io is Europa, slightly smaller than our Moon at a diameter of 3130 km

Europa – second closest large moon
Europa – second closest large moon

(1945 miles), but as intriguing as Io is dynamic. Europa’s surface consists of an icy crust as hard as rock in temperatures that plunge to 200 degrees centigrade below zero. This might sound as though Europa is indeed nothing more than a deep frozen ball of rock but prepare for a surprise.

Images from the Voyager and Galileo probes revealed that Europa’s surface looks similar to the Earth’s arctic regions. The surface is criss-crossed with a myriad of scar-like lines, some running for hundreds of miles. Astronomers realised that Europa also experiences a gravitational tug-of-war like Io but to a lesser degree. Even so, there is sufficient internal heat for warm ice to well-up beneath the icy surface and break through it. This process produces the long scar features. The intriguing part is that there could well be a warm liquid ocean deep below the ice crust and, as everyone knows, heat and water are conducive to life.

Europa looks the most likely candidate for extant extraterrestrial life in the Solar System. Already there are serious proposals afoot to mount robotic drilling missions in a quest to find out what, if anything, lurks beneath the ice. Just maybe, there is some form of aquatic life waiting to be discovered in the Europan depths. Time will tell, but astronomy does have a habit of turning up surprises.


Ganymede – the largest moon in the Solar System
Ganymede – the largest moon in the Solar System

Ganymede is the largest moon in the Solar System as befits the largest planet. At a diameter of nearly 5262 km (3270 miles) Ganymede is over a third of the diameter of Earth, retains a very thin atmosphere and would qualify as a planet if it were in orbit around the Sun instead of Jupiter. Ganymede will present a special challenge for telescopes during NAW, as its size is sufficient for major surface features to be observed or imaged with large aperture instruments above 10 inches.

As we move outwards through the Galilean Moons, it becomes noticeable that the effects of Jupiter’s gravity are steadily reduced. Ganymede’s surface is much older, more cratered and far less geologically active than both Io and Europa. The trio orbit in resonance in the ratio of 1 to 2 to 4, so Io races around its orbit 4 times in the 8 days it takes Ganymede to orbit once.

Ganymede’s surface is ancient and relatively little has happened there within about the last 4 billion years. The oldest parts are heavily cratered like our Moon and probably date from a similar time, when asteroid and comet impacts were prolific throughout the Solar System. There are lighter, grooved regions covering about two-thirds of the surface, the origins of which remain unclear but are thought to have been formed by internal heating processes similar to those on Europa.



Callisto completes the Galilean quartet and has the oldest and darkest surface of the four. The surface is a jumbled mix of rock and ice and heavily cratered like our Moon. This is clear evidence that there is little or no geological activity on Callisto and the surface has been little changed since Callisto formed.

Some of the craters are surrounded by strange concentric ring structures, the origins of which are uncertain. Callisto is not in orbital resonance with Io, Europa and Ganymede but will be in the distant future. The other three moons will gradually slow in their orbits and drift outwards to a point where Callisto joins the resonance. Callisto’s quiescent nature, combined with its safe orbital distance from Jupiter, would make it the preferred destination for any future manned mission to the Jovian system.

Ring System

Ever since the first telescopic observations were made during the early 17th century, Jupiter’s companion gas giant Saturn has been known to boast a spectacular ring system. By the way, Saturn is also visible during NAW although not so well-placed as Jupiter.

In recent times when probes flew by the gas giants in turn, it was revealed that all of them have their own ring systems. Jupiter has four tenuous rings but unfortunately they are too faint to be seen by Earth-bound telescopes.