Io is one of the most marvellous bodies in the Solar System, but the intense radiation bathing its surface (Figure 2) makes it unlikely that anyone will ever be able to visit. Even robotic spacecraft cannot survive this close to Jupiter for very long, so the Galileo Jupiter orbiter made very few close fly-bys of Io. However, there is by now a remarkable collection of images from close fly-bys and farther away, represented in Chapter 9 of Teach Yourself Planets by Plate 8 (see Section 1.9), Figure 9.10, Figure 9.11, Figure 9.12 and the additional Io images below.
One of the most spectacular Galileo images of Io is reproduced in Figure 11. On this image the orange glow of the incandescent lava is genuine. It was recorded in visible light, and represents pretty much what you could have seen with your own eyes (before you were killed by the radiation!). This contrasts with most other images of active lava flows on Io (see images below) on which the hot areas were detected using infrared radiation, but were not necessarily hot enough or powerful enough to generate a detectable glow in the visible part of the spectrum. On those false-colour images, the infrared signal is displayed in red.
Most of the names on Io are taken from gods and heroes associated with fire, thunder or the Sun, or from Dante's Inferno. Eruptive centres on Io share with impact craters the distinction of being the only features whose names are not qualified by a descriptor term. Thus Prometheus (Plate 8, see Section 1.9), Zanama (Figure 9.11) and Pele (Figure 9.12) are complete names with nothing missing. However, a few eruptive centres identified by some kind of topographic attribute do take descriptor terms, one example being Pillan Patera in Plate 8 and Figure 9.12. The fissure-shaped vent in Figure 11 has not been named, but the larger feature in which it occurs is called Tvashtar Catena.
The most important general point to learn from study of Io is that it demonstrates how tidal heating can add enormously to the supply of heat leaking towards the surface of a planetary body. Tidal heating operates among the inner three galilean satellites because they are in orbital resonance (with a ratio between their periods of 1:2:4). The details of how orbital resonance leads to tidal heating are beyond the scope of this course. All you need to know is that it is to do with the regularly repeated gravitational tugs between satellites as they pass and the extent to which this prevents the tidal drag of the planet from forcing the satellite's orbits to become circular. (See Box 2 for a further clarification about orbital resonance.)
When it comes to causing tidal heating, the ratio between the orbital periods of two satellites has to be very close to a simple ratio. Two satellites of a large planet having a ratio between their orbital periods of 2.1:1 certainly will not experience tidal heating, because this ratio is too different from exactly 2:1. However, 2.01:1 or 1.99 :1 would probably be close enough.
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