David A. Rothery Teach Yourself Planets, Chapter 9, pp. 107-39, Hodder Education, 2000, 2003.
Copyright © David Rothery
Of the welter of revelations provided by the Voyager tours of the outer Solar System, the discovery of active volcanoes on Io probably ranks top of the list. Prior to this, most people had assumed that bodies of Io's size, whether rocky like Io or icy like its companions, would be geologically dead like our own, similarly sized, Moon. This is because their small size makes them incapable of having retained enough primordial heat or generating adequate radiogenic heat to keep their lithospheres thin and to drive mantle convection sufficiently close to the surface for melts to escape.
However, it is now realized that the orbital resonance that exists between the three innermost galilean satellites results in tidal heating. For every one orbit completed by Ganymede, Europa completes two and Io four. This means that the satellites repeatedly pass each other at the same points in their orbits, and the consequent internal stresses experienced by the satellites provide a source of heat that keeps their interiors warmer than they would otherwise be. The effect is greatest for Io, which is closest to Jupiter and hence experiences the strongest tidal forces.
There are often more than a dozen volcanoes erupting on Io at any one time. These are identified either by seeing an 'eruption plume' powered by the explosive escape of sulfur dioxide and rising 100-400 km above the surface (Plate 8), or by infrared detection of a hot spot. The record for the highest local temperature is at least 1400°C.
[Click 'view document' to open Plate 8. Galileo image of lo recorded on 28 June 1997. There are two eruption plumes visible. One is 140 km high and is seen in profile above the limb; this emanates from a volcano named Pillan Patera. The other plume, from a volcano named Prometheus, is seen from directly above, and lies near the centre of the disc. It is shown enlarged in the inset at upper left; the bluish dark ring is the outline of the plume and this casts a reddish shadow over the surface to its right. NASA.]
Io's density is slightly greater than that of the Moon, and it is clear that Io is a dominantly silicate body, like the terrestrial planets. Gravity and magnetic observations by Galileo confirm that it has a dense, presumably iron-rich core below its rocky mantle. Spectral data show that Io's surface is covered by sulfur, sulfur dioxide frost and other sulfur compounds. However, these are no more than thin, volatile, veneers resulting from volcanic activity and the crust as a whole is some kind of silicate rock.
Io has an atmosphere of sulfur dioxide, and atomic oxygen, sodium and potassium. The surface pressure is less than a millionth of the Earth's but nearly a billion times greater than the atmospheric pressure of the Moon or Mercury. Io's atmosphere continually leaks away into space, contributing to a 'cloud' of sodium and potassium that falls inwards towards Jupiter and to a magnetically confined belt of ionized sulfur that stretches right round Jupiter, concentrated around Io's orbit. The atmosphere is replenished by a combination of volcanic activity and collisions onto Io's surface by high-speed ions channelled by Jupiter's magnetic field. When Io passes into the shadow of Jupiter its atmosphere can be seen faintly glowing in an auroral display caused by these same magnetospheric ions impinging on the atmosphere (Figure 9.10).
Io's surface is totally dominated by the results of volcanic activity (Figure 9.11). There are lava flows up to several hundred km in length and vast swathes of mostly flat terrain covered by fallout from eruption plumes. Most of the lava flows are now believed to have formed from molten silicate rock, which is often discoloured by a sulfurous surface coating, but there are probably some flows that formed from molten sulfur too. Here and there volcanoes rise above the general level of the plains, and their summits are occupied by volcanic craters (described as 'calderas') up to 200 km across formed by subsidence of the roof of the volcano after magma has been erupted from within. No impact craters are visible, because the volcanic eruptions deposit fresh materials across the globe at an average rate of something like a centimetre thickness per year.
Over 500 volcanoes have been identified on Io, and about 100 have been seen to erupt on images from Voyager or Galileo (or both), or by means of infrared telescope studies. The long duration of the Galileo mission enabled many changes on Io's surface to be documented (Figure 9.12).
Io's volcanoes appear to be randomly distributed, and Io certainly lacks the kind of well-defined global pattern displayed by the Earth (Figure 5.5 [figure not included in this course]). Thus unlike Earth, which gets rid of heat from its interior by plate tectonics, and Venus, where heat escapes by conduction probably punctuated by orgies of resurfacing every half billion years or so, Io's heat escapes by means of hordes of volcanoes. One factor that probably influences the difference between the Earth and Io is that, to maintain a steady state, Io has to lose heat at a rate of about 2.5 watts per square metre, compared to only 0.08 watts per square metre in the case of the Earth. Possibly, the tidal heating experienced by Io is sufficient to keep a large fraction of its mantle partially molten.
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