Io - Jupiter's amazing volcanic moon

Although the Galileo spacecraft has been in orbit around Jupiter for over 4 years, and has provided some remarkable images of Jupiter and several of its moons (including Europa), high resolution images of Io have been acquired only during the past few months (since October). The results from some of the recent data are published in several papers in the May 19^th issue of Science⁽¹⁾.

Io is the innermost of the Galilean Moons (the four moons discovered by Galileo in 1610). It is slightly larger than our moon, and unlike most bodies in the outer part of the solar system, it has a rocky composition, dominated by silicate minerals. It also appears to have an iron core.

The visit of the two Pioneer spacecraft in 1979 overturned everything that we thought that we knew about the geology of Io. In view of its relatively small size Io was assumed to be geologically dead, like our moon, but the Pioneer images showed that it is virtually free from impact craters (which would indicate old rocks), and alive with evidence of recent volcanism, including active eruptions. Its surface is a bright collage of yellows, browns and reds along with black lava flows. Io is strongly affected by the gravitational pull of Jupiter and of Europa and Ganymede, and the internal heat produced by the resulting tidal forces is enough to produce hot magmas in the mantle.

The Galileo spacecraft is equipped with a variety of sensors. It can acquire images in both the visible and infrared parts of the spectrum and this data can be used to help determine the composition of both solids and gases. The image resolution depends on the height of the satellite above the surface. For the recent encounters the pixel size ranges from around 10 m to several 100s of metres, as compared with several kilometres for most of the Pioneer images. Galileo also has sensors which can be used to determine the temperature at the surface.

Some of the findings of the recent close encounters with Io can be summarized as follows:

• There are no steep-sided volcanic cones and domes on Io, and hence it is concluded (McEwen et al.) that all of the magma is strongly mafic, and that there has been no differentiation of a silica-rich crust (as on earth).
  
• Magma temperatures range up to around 2000 K (1725 C), further confirming their ultramafic composition. The flows at the Zamana volcano have crenulated margins and other features which are similar to pahoehoe sheet flows on earth. It is predicted that lava flows on Io may be sufficiently liquid to flow in a turbulent manner and there is evidence of this in the way that channels have been carved by the Zamana flows.
  
• There is undoubtedly a lot of sulphur being emitted by the Ionian volcanoes, and it is also suggested (McEwen et al.) that some of the lava flows are in large part liquid sulphur. Both S₂ and SO₂ are detected in some of the gaseous plumes. The reds and browns observed in many areas may be a product of red S₃ and S₄ formed by polymerization of the S₂ (Spencer et al.(b)).
  
• The eruption rate in some cases is very high. At the Pillan volcano, for example, ultramafic lava was observed to be covering the landscape at a rate of more than 300 square metres per second.
  
• Some of the Ionian volcanic centres are consistently active. For example, activity at the Prometheus volcano (on the Io equator) has been observed in every image acquired by the two Voyager spacecraft (in 1979) and by all of the encounters with the Galileo spacecraft (since 1995). It appears that most of the emission at Prometheus is gaseous and sulphur rich. It is proposed by Keiffer et al. that this type of eruption is similar to the so-called "rootless" eruptions seen on earth where lava flows over an area with surface water and an "eruption" takes place when the water boils. At Io the lava commonly flows over areas with a thick deposits of S and SO₂, and the heat from the lava takes these materials well beyond their vaporization points. Where there is a conduit towards surface the material boils out in a huge plume (tens to hundreds of kilometres high) and the surrounding liquid and vapour mixture, beneath the lava, migrates towards the conduit to feed the plume. At Prometheus it appears that this process has continued for over 20 years.
  
• At the Tvashtar volcano (in the far north) there is evidence of a 40 km long fissure with a curtain of lava fountain rising as much as 1.5 km above the surface. There is also evidence of lava flowing to either side of the fissure. A similar fissure is observed at the Zamana voclano.
  
• There are no volcanic mountains on Io (owing to the low magma viscosity) but there are mountains, some rising as high as 16 km above the planetary surface. Most of these appear to be tilted blocks formed by thrust faulting which is driven by crustal compression.

1. References

McKewen A. and others, Galileo at Io: Results from high-resolution imaging, Science, V 288, p. 1193-1198.

Spencer J. and others (a), Io's thermal emission from the Galileo photopolarimeter radiometer, Science, V 288, p. 1198-1201.

Lopes-Gautier, R. and others, A close-up look at Io from Galileo's near-infrared mapping spectrometer, Science, V 288, p. 1201-1204.

Keiffer, S., and others, Prometheus: Io's wandering plume, Science, V 288, p. 1204-1208.

Spencer J. and others (b), Discovery of gaseous S₂ in Io's Pele plume, Science, V 288, p. 1208-1210.

There is more information on Io, including some spectacular images, at the following websites.

Nasa: http://www.jpl.nasa.gov/galileo/images/io/ioimages.html

Students for the Exploratio and Development of Space: http://www.seds.org/nineplanets/nineplanets/io.html

Science (from McEwen et al.): http://www.sciencemag.org/feature/data/1049308.shl

Steven Earle, 2000. Return to Earth Science News