Jupiter and its moons

2.3 Jupiter's atmosphere (continued)

3 Discussion of Chapter 9: Rings and the satellite family

3.1 Jupiter's rings and satellites

There can be few people who have never heard of the rings of Saturn, but you may be surprised to discover that every one of the giant planets has rings, including Jupiter. Another feature common to the giant planets is that each has a numerous family of satellites. The likeness goes deeper than that, because there are strong similarities in the organization of each of these families.

There are small inner satellites ('moonlets') in near-circular prograde orbits that lie virtually in the plane of the planet's equator, some of which are intimately associated with the ring system. Farther out come the 'regular satellites', which are the only ones large enough to have taken on spherical shapes, and except at Neptune these are also in low-inclination near-circular prograde orbits. There are four of these at Jupiter, which are the ones discovered by Galileo Galilei in 1610 and collectively referred to as the galilean satellites. Beyond these are smaller 'irregular satellites' in less circular and more strongly inclined orbits, which are commonly retrograde.

Generally speaking, the outer satellites are thought to have originated as asteroids, comet nuclei or Kuiper belt objects that strayed close enough to the planet to become captured. In contrast, the large prograde satellites grew in association with their planet from a circumplanetary disc of gas and dust.. The small inner satellites are probably remnants of larger satellites that were destroyed by collision or came within the planet's Roche limit and were ripped apart by tides. The same processes could account for some of the ring material too.

Twenty-two outer satellites of Jupiter were identified in the period 1999-2001, and many of these are still denoted by provisional designations in Table 9.2. These were all telescopic discoveries, and the discovery images (i.e. the images on which the object was discovered) of one of them are shown in Figure 9.

Figure 9: The discovery images of S2000 J8 (which was awarded the name Magaclite in October 2002), made on 5 December 2000 by the University of Hawaii 2.2 m telescope. Three images are shown, covering a 71 minute period. The satellite (with a circle put round it to help you see it) clearly moves relative to the background stars.
University of Hawaii and Scott Sheppard/David Jewett

The provisional designation system for satellites is simpler than that for minor planets, as you can deduce from the examples in the table. They begin with S (to signify a presumed satellite) followed by the year of discovery. Next comes a letter to indicate the planet (J for Jupiter, S for Saturn, U for Uranus) followed by a number to indicate the order of discovery for that planet in that year. Only one discovery from the year 2000 still bears its provisional designation in the table, all the others having been awarded names in October 2002.

Too late for inclusion in the edition of Teach Yourself Planets from which Section 1 was extracted, telescopic discoveries of a further 20 retrograde and one prograde irregular outer satellites of Jupiter were announced in March-May 2003.

Question 5

What provisional designations will have been allotted to these bodies?

Answer

The first was designated S/2003 J1, the second was S/2003 J2, and so on up to S/2003 J21.

None of these exceeds about 4 km in size. The innermost of the 20 new retrograde satellites (S/2003 J3) has an orbit about a million kilometres within that of S/2001 J10, and the outermost (S/2003/J2) holds the current record as the outermost of Jupiter's entire family at 28.6 million km. S/2003 J20 became the outermost known prograde satellite, with an orbital radius of about 17 million km.

Note that these discoveries increase the total of Jupiter's known satellites to 61. You may like to reflect this by mentally correcting the total in the Jupiter planetary facts table. However, the number will go down if any of the new discoveries is discredited or, more likely, will increase as additional discoveries are announced.

Question 6

What do you think had happened that allowed the year 2000 discoveries other than S/2000 J11 to receive official names by the time Table 9.2 was prepared?

Answer

A sufficient number of observations had been made for their orbital motion about Jupiter to be adequately determined. This is the same principle as applied to the awarding of names to minor planets. Also, appropriate names (usually suggested by the discoverers) had been agreed by a committee of the IAU.

All the names of Jupiter's longer-known satellites are of characters associated with Jupiter, or his equivalent Zeus, in Greco-Roman mythology. The IAU has continued this theme for the new discoveries, with the refinement among the outer satellites that those in prograde orbits have been given mainly Latin names ending in -a whereas those in retrograde orbits have been given mainly Greek names ending in -e. Figure 10 illustrates the general disposition of the outer satellite orbits.

Figure 10: A perspective diagram of Jupiter's satellite system, showing all orbits from Callisto (the outermost galilean satellite) outwards. The orbits of 2001 discoveries are distinguished in red. Discoveries announced after the end of 2002 are not included.
University of Hawaii and Scott Sheppard/David Jewett

We probably now know more about each of the galilean satellites than we do about Mercury. Their diverse properties and large size fully justify treating them individually as we do in the following sections.