Jupiter's Attendants

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By Lesa Moore

In the last episode of this Solar System series (Sept. '99 issue), I talked about Jupiter. This time, let's take a look at its moons, and a few other interesting bits of space debris known as Trojan Asteroids.

The Galilean Moons

The four brightest moons of Jupiter, with diameters greater than 3000km, are observable in any telescope. Indeed, the telescope with which they were discovered by Galileo in 1610 was only about 2.5cm in diameter! This means you can also see these "Galilean moons" with binoculars - the trick is to hold them steady by mounting them on a tripod, or resting them against a pole or a fence.

Io is the closest of these moons to Jupiter. We all know how Earth's Moon raises tides in our oceans. In a similar way, Io has "tides" caused by the influences of Jupiter and the other large moons. Without any liquid on the surface, though, these tides actually change the shape of the rocky moon itself. The constant tidal flexing warms the mantle, producing volcanism. The Voyager spacecraft observed the first active volcanoes ever seen anywhere other than Earth when it flew by in 1979. Numerous active volcanoes were identified, and Io is the most volcanically active body in the Solar System, completely resurfacing the top 10cm of crust in only a few thousand years. There is not one single impact crater on Io. The sulphur compounds in the crust give the lava flows and volcanic peaks colours of red, yellow, black and white. Photos of Io resemble pizza!

Europa is the next large moon out from Jupiter. Its surface is extremely smooth, the product of an ice crust floating on a slush which is warmed, again, by tidal flexing. Recent images from the Galileo spacecraft show how this surface looks similar to Antarctic ice floes, with chunks of ice crust which have cracked and drifted, then refrozen into new patterns, like a jigsaw-puzzle gone wrong. This is one likely target for a search for extra-terrestrial life, since liquid water beneath the crust could possibly support living organisms.

Next out is Ganymede. It was once thought that Saturn's largest moon, Titan, was the largest moon in the Solar System. Early measurements, however, included what was later discovered to be a thick atmospheric layer. So it turned out that Ganymede, not Titan is the largest moon of all, with a diameter of 5276km. Ganymede is composed of rock and ice, but has no atmosphere, so has a cratered appearance.

Callisto, the fourth large moon, is similar to Ganymede, but slightly smaller. Its claim to fame is that it is the most heavily cratered body in the Solar System.

If you want to individually identify these moons when you view them through a telescope, there are charts to help you on pp 102-104 of "Astronomy 2000".

Other Moons

The lesser-known moons of Jupiter are the other 12. They fall into three groups of 4 each. The first four, Metis, Adrastea, Amalthea and Thebe lie inside the orbit of Io. These all have diameters of less than 300km.

Whereas the planes of the orbits of these inner moons, and the Galilean moons, are inclined at only half a degree, the members of the next group have orbits inclined by 26 to 29 degrees. Leda, Himalia, Lysithia and Elara orbit much farther from Jupiter than the moons we have looked at so far (see table).

The final group orbits almost twice as far away again - Ananke, Carme, Pasiphae and Sinope have orbits inclined by 147 to 163 degrees to the ecliptic, and all orbit in retrograde motion (whereas most objects in the Solar System spin and revolve west to east, retrograde motion means they go the opposite way).

The other interesting fact about Jupiter's moons is that they are all tidally locked, permanently keeping the same side facing toward Jupiter.

Space Debris IV: Trojan Asteroids of Jupiter

Unlike moons, which orbit around a planet, trojan asteroids share the same orbit with their parent planet in its passage around the Sun (i.e., they are co-orbital). The Trojan Asteroids of Jupiter are the best known, but Mars has one called 5261 Eureka.

The strange asteroid 3753 Cruithne, while not a true trojan, has an unusual horseshoe-shaped orbit inclined to Earth's, but at the same distance as Earth is from the Sun. The small moons of Saturn, Janus and Epimetheus, also exhibit similar behaviour to Earth and Cruithne, respectively.

Let's look at the orbits of Jupiter's curious entourage. Trojan asteroids are found around two favourite spots, defined as Lagrange points L4 and L5 (see diagram). Lagrange points are positions where bodies are able to maintain stable orbits in three-body systems.

Jupiter's retinue consists of 400 or more objects, and the name "Trojan" derives from a generalization of the names of some of its largest co-orbital asteroids. Examples are 588 Achilles, 624 Hektor and 911 Agamemnon. For some unknown reason, there are many more in the leading Lagrange point (L4) than in the trailing one.

The Trojans spread out to occupy tadpole-shaped areas around L4 and L5, when compared with the positions of Jupiter and the Sun. Cruithne's horseshoe-shaped orbit mentioned above circuits around L4, L3 and L5, relative to Earth and the Sun.


  • Various websites (no longer available)
  • Sky & Space Dec/Jan 1999/2000 p12
  • Atlas of the Solar System, Krisciunas, K. and Yenne, B. 1991

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