Jupiter and its moons

1.7 Satellites

David A. Rothery Teach Yourself Planets, Chapter 9, pp. 107-39, Hodder Education, 2000, 2003.

Copyright © David Rothery

The satellite system of each giant planet begins closest to the planet with small moonlets associated with the ring system in near-circular orbits, then come larger 'regular' satellites also in near-circular orbits close to the planet's equatorial plane, followed by an outer family of small 'irregular' satellites in elongated, inclined and in most cases retrograde orbits.

Most satellites are in synchronous rotation, always keeping the same face towards their planet. This means that one side of a satellite, known as the 'leading hemisphere', faces permanently towards the direction of orbital travel while the 'trailing hemisphere' faces away from the direction of travel. We know comparatively little about most of the inner moonlets and small outer satellites of the giant planets in general, other than that most have densities probably less than that of ice and so are likely to be heaps of rubble and that the inner moonlets are probably the sources of the ring material.

Chapter 02 described how most large satellites of giant planets grew from a disc of gas and dust around each planet. Most inner moonlets are probably fragments of formerly larger satellites that were destroyed by collisions or tidal forces, whereas the irregular outer satellites, especially those in retrograde orbits, are likely to be captured comets or asteroids. These are very dark, and are probably covered by tholins or other carbonaceous material, whatever their internal compositions.

Some basic information about Jupiter's family of satellites is given in Table 9.2. Images of its inner moonlets (Figure 9.9) show that these have the irregular shapes expected of bodies too small for their own gravity to pull them into spheres, the size limit for which appears to be a radius of about 200 km for an icy body. Amalthea and Thebe both have craters that are quite large relative to their own sizes, especially on their leading hemispheres, which is the side most vulnerable to impact damage. Amalthea is dark red in colour, which could be a sulfurous coating derived from its volcanically active neighbour Io, or indicate a tholin-rich surface similar to Jupiter's Trojan asteroids or Centaurs.

Table 9.2: Satellites of Jupiter [not including discoveries made in 2003 and subsequently][.] Orbital distances and periods of the unnamed (provisionally designated) satellites are uncertain (R = retrograde, ? = value unknown)[.]

Name Distance from planet's centre Radius (km) Orbital period (days) Mass Denary (g/cm3)
Metis 127,960 30 × 17 0.295 1 × 1018 kg 3.0
Adrastrea 128,980 13 × 10 × 8 0.298 0.2 × 1018 kg 3.0
Amalthea 181,300 131 × 73 × 67 0.498 7.5 × 1018 kg 3.1
Thebe 221,900 55 × 45 0.675 0.8 × 1018 kg 3.0
Io 421,600 1821 1.769 8.93 × 1022 kg 3.53
Europa 670,900 1565 3.552 4.80 × 1022 kg 3.01
Ganymede 1,070,000 2,631 7.155 14.8 × 1022 kg 1.94
Callisto 1,883,000 2,403 16.69 10.8 × 1022 kg 1.83
Themisto 7,507,000 4 130.0 ? ?
Leda 11,165,000 10 240.9 ? ?
Himalia 11,461,000 85 250.6 ? ?
Lysithea 11,717,000 18 259.2 ? ?
Elara 11,741,000 43 259.6 ? ?
S/2000 J11 12,555,000 2.0 287 ? ?
S/2001 J10 [Euporie] 19,394,000 1.0 553 R ? ?
S/2001 J7 [Euanthe] 21,027,000 1.5 620 R ? ?
Harpalyke 21,105,000 2.2 623 R ? ?
S/2001 J9 [Orthosie] 21,116,000 1.0 623 R ? ?
Praxidike 21,147,000 3.4 625 R ? ?
S/2001 J3 [Hermippe] 21,252,000 2.0 632 R ? ?
locaste 21,269,000 2.6 631 R ? ?
Ananke 21,276,000 10 630 R ? ?
S/2001 J2 [Thyone] 21,312,000 2.0 632 R ? ?
S/2002 J1 22,931,000 1.5 714 R ? ?
S/2001 J6 [Pasithee] 23,029,000 1.0 716 R ? ?
S/2001 J8 [Kale] 23,124,000 1.0 721 R ? ?
Chaldene 23,179,000 1.9 724 R ? ?
Isonoe 23,217,000 1.9 726 R ? ?
S/2001 J4 [Eurydome] 23,219,000 1.5 721 R ? ?
Erinome 23,279,000 1.6 728 R ? ?
Taygete 23,360,000 2.5 732 R ? ?
Carme 23,404,000 15 734 R ? ?
S/2001 J11 [Aitne] 23,547,000 2.0 741 R ? ?
Kalyke 23,583,000 2.6 743 R ? ?
Pasiphae 23,624,000 18 744 R ? ?
Megaclite 23,806,000 2.7 753 R ? ?
S/2001 J5 [Sponde] 23,808,000 1.0 749 R ? ?
Sinope 23,939,000 14 759 R ? ?
Callirrhoe 24,102,000 4 759 R ? ?
S/2001 J1 [Autonoe] 24,122,000 2.0 765 R ? ?
Figure 9.9: Jupiter's four innermost known satellites, imaged by Galileo and shown at their correct relative sizes[.] From left to right: Metis (60 km long), Adrastea (20 km long), Amalthea (247 km long) and Thebe (116 km long)[.] Two views are shown for Amalthea and Thebe: top the leading hemisphere, bottom the trailing hemisphere[.]
NASA

Jupiter's irregular satellites are very poorly known. Many of them are recent telescopic discoveries, as can be deduced from the provisional designations that some bear instead of names in Table 9.2. There may be hundreds of others more than 1 km across that still await discovery. The satellite S/1975 J1 was first seen briefly in 1975 but was lost again until 2000, when it was designated as S/2000 J1 until it became clear that the two objects were the same. It received the name Themisto late in 2002, when ten other discoveries from 1999 and 2000 were also given names. Leda, Himalia, Lysithea, Elara and S/2000 J11 are the only other irregular satellites to have prograde orbits, and those five could be remnants of a single body that broke apart during capture by Jupiter. The outermost satellites all have retrograde orbits. These tend to fall in groups, in which several adjacent orbits have almost the same inclination. Each group is probably the remains of a body that broke apart during capture by Jupiter.

Jupiter has four large satellites, Io, Europa, Ganymede and Callisto, orbiting between its inner and outer satellite families. These were discovered in 1610 by Galileo Galilei, and are known collectively as the galilean satellites. Individually they are substantial bodies and are really worlds in their own rights. Some of their important properties are listed in Table 9.3.

Table 9.3: Planetary facts for the galilean satellites

Io Europa Ganymede Callisto
Equatorial radius (km) 1821 1565 2631 2403
Mass (relative to Earth) 0.0149 0.00803 0.0248 0.0181
Density (g/cm3) 3.53 3.01 1.94 1.83
Surface gravity (relative to Earth) 0.18 0.13 0.14 0.13
Composition of surface rocky salty ice dirty ice dirty ice
Mean surface temperature −150°C −150°C −160°C −160°C

The decrease in density outwards from Io through Europa to Ganymede and Callisto reflects the fact that Io is ice-free, Europa has a 100 km thick covering of ice, and Ganymede and Callisto each contain about 40 per cent of ice. This would be a natural consequence of a decrease in temperature with distance away from the hot, young Jupiter within the disc of gas and dust from which these bodies grew. They are described in turn in the following sections.