Last time, we looked at the many moons of Jupiter – well, 63 of them at any rate. If you remember, Jupiter has 67 all together, by far the largest retinue of any planet in the Solar System.
Today, I thought we’d dip into 10 facts you might not have known about the ones we had to leave out last time, otherwise known as the Galilean moons, because they were discovered in 1610 by Galileo Galilei…and were the first objects found to orbit a body that was neither Earth nor the Sun.
1. How many Galilean moons are there?
As you might have already worked out, there are 4 moons classed as Galilean moons.
This is a little montage of Jupiter, with the 4 moons mentioned above next to her. They are in the same order as mentioned, starting with Io at the top. Io, Europa, Ganymede and Callisto. They are some of the largest objects in the Solar System outside of the Sun and the eight planets in terms of mass, and are larger than any known dwarf planet. Respectively these moons are the fourth, sixth, first and third largest natural satellites in the Solar System.
Ganymede – a moon – exceeds the planet Mercury in diameter.
2. So, tell us more about Io?
Io is Jupiter’s third biggest moon with a diameter of 3642 km, (2,251 miles) making her slightly bigger than Earth’s moon. While she might be similar in size to our Moon, the fact that she has about 400 active volcanoes makes her very different. (See below)
3. Tell us more about the volcanoes
Io is the most volcanically active body in the Solar System. Her volcanoes spew out massive volumes of silicate lava, sulphur and sulphur dioxide, and because of this, Io’s appearance constantly changes.
4. And what about Europa?
Europa is the sixth-closest moon of the planet Jupiter, and the smallest of the four Galilean satellites. Even so, at just over 3,100 km (1,900 miles) in diameter, it is the sixth-largest moon and fifteenth largest object in the Solar System.
However, it is nonetheless more massive than all the known moons in the Solar System smaller than itself combined. Its bulk density suggests that it is similar in composition to the terrestrial planets, being primarily composed of silicate rock with an iron core.
It has a tenuous atmosphere composed primarily of oxygen. Although its surface is covered by cracks and streaks, craters are relatively rare. Being composed of frozen water, it gives the moon the smoothest surface of any solid object in the Solar System. The apparent youth and smoothness of the surface have led to the belief that a water ocean exists beneath it, (which could conceivably serve as an abode for extraterrestrial life).
This hypothesis proposes that heat from tidal flexing causes the ocean to remain liquid and drives geological activity similar to plate tectonics. On 8 September 2014, NASA reported finding evidence confirming earlier reports of plate tectonics in Europa’s thick ice shell – the first sign of such geological activity on a world other than Earth.
5. Anything else??
Most certainly. In December 2013, NASA reported the detection of “clay-like minerals” (specifically, phyllosilicates) – which are often associated with “organic material” – on the icy crust of Europa. In addition, NASA announced that water vapor plumes were detected on Europa and were similar to water vapor plumes detected on Enceladus, one of the moon’s of Saturn.
The Galileo mission, launched in 1989, provided the bulk of current data on Europa. No spacecraft has yet landed on Europa, but its intriguing characteristics have led to several ambitious exploration proposals. The European Space Agency’s Jupiter Icy Moon Explorer (JUICE) is a mission to Europa that is due to launch in 2022. NASA is planning a robotic mission that would be launched in the “mid-2020’s.
6. Is it true there’s a special relationship between Europa & some of the other moons?
Yes – it’s a bit complicated to explain, but, simply put, Ganymede participates in something called an orbital resonance with Europa and Io.
Every time Ganymede makes one orbit of Jupiter, Europa orbits twice and Io orbits four times. (The specifics of why can be explained by looking at something called the Laplace resonance.)
There are two hypotheses for the origin of the Laplace resonance among Io, Europa, and Ganymede. The first is that it is primordial and has existed from the beginning of the Solar System. The second, is that it developed after the formation of the Solar System.
7. We’ve just mentioned Ganymede. Tell us about her?
Ganymede is the largest moon in the Solar System, and orbits Jupiter at a distance of 1,070,400 km, (665, 116 miles). She completes a revolution every seven days and three hours. Like most known moons, Ganymede is tidally locked, with one side always facing toward the planet. It has a diameter of 5,268 km (3,273 mi), which is 8% larger than that of the planet Mercury, but has only 45% of the latter’s mass. Its diameter is 2% larger than that of Saturn’s Titan, the second largest moon. It also has the highest mass of all planetary satellites, with more than twice times the mass of the Earth’s moon.
8. And what else about Ganymede?
Ganymede is composed of approximately equal amounts of silicate rock and water ice. She is the only moon in the Solar System known to possess a magnetosphere, likely created through convection within the liquid iron core. The meager magnetosphere is buried within Jupiter’s much larger magnetic field and would show only as a local perturbation of the field lines. The satellite has a thin oxygen atmosphere that includes O, O2, and possibly O3 (ozone). Atomic hydrogen is a minor atmospheric constituent. Whether the satellite has an ionosphere associated with its atmosphere is unresolved.
9. Finally, we come to Callisto.
Callisto is the outermost of the four Galilean moons of Jupiter. It orbits at a distance of approximately 1 880 000 km (1,168,1777 miles) (26.3 times the 71 492 km radius of Jupiter itself). This is significantly larger than the orbital radius – 1 070 000 km – of the next-closest Galilean satellite, Ganymede. As a result of this relatively distant orbit, Callisto does not participate in the Laplace Resonance of the other three moons, and probably never has.
It possesses the oldest and most heavily crated surface in the entire solar System and is surrounded by an extremely thin atmosphere of carbon dioxide and molecular oxygen The likely presence of an ocean within Callisto leaves open the possibility that it could harbor life. However, conditions are thought to be less favorable than on nearby Europa. Various space probes from Pioneers 10 and 11 to Galileo and Cassini have studied Callisto. Because of its low radiation levels, Callisto has long been considered the most suitable place for a human base for future exploration of the Jovian system.
10. Are there any interesting facts we might like to know about these moons?
Of course. These 4 moons make up over 99% of the entire mass of all the moons of Jupiter.
So there you go. Only 4 moons, but a great deal of information.
And with Jupiter finished – you know where we’re headed next? – that’s right – Saturn. Tune in next time to discover what makes some of those moons so spectacular.
See you then…
An astronomy and law graduate, he is the creator of the international number one bestseller, The IX, and also has the privilege of being a member of the Science Fiction and Fantasy Writers of America and the British Fantasy Society.
When not writing, Andrew devotes some of his spare time to assisting NASA with one of their remote research projects, and writes educational articles for Astronaut.com and Amazing Stories.
He also enjoys Greek dancing and language lessons, being told what to do by his wife, and drinking Earl Grey Tea.
If you would like to find out more, visit his blog or website at:
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