Planetary science

Sodium at Io

It has been known for some years that Jupiter's satellite Io has sodium as a component of its atmosphere. The source, it now seems, is sodium chloride emitted by volcanoes on Io's surface.

Io, the innermost of Jupiter's four Galilean satellites, is anomalous in various respects. One curiosity is its atmosphere, and the 'plasma torus' of ionized matter through which the planet orbits. On page 45 of this issue, Lellouch et al.1 describe observations that will help settle thinking about a puzzling constituent of the atmosphere and torus, and its origins.

The true dimension of Io's strangeness first became apparent only in 1974, when R. A. Brown identified the intense emission of yellow 'sodium D lines' in spectroscopic studies of Io. It was soon established that these emissions come from an extended region — the 'neutral torus' — that is much larger than the visual disk of Io and which shares its orbit around Jupiter2,3. Two years later came the discovery that this region is also occupied by Io's plasma torus, which consists of ionized material that has been created by the impact of electrons on neutral molecules and atoms. The satellite's orbit is embedded in the torus, which has turned out to be the source of most of the ions in Jupiter's magnetosphere.

Ground-based measurements provided further understanding of the torus. But it was spacecraft encounters with Io (Fig. 1) that provided the next surprises. Data from Voyager 1's 1979 encounter not only revealed intense emissions in the far-ultraviolet part of the spectrum, primarily due to sulphur and oxygen ions in several states of ionization, but also identified widespread volcanism and geyser activity on Io's surface. The source of the sulphur and oxygen ions was quickly shown to be sulphur dioxide; and, simultaneously, it was realized that the volcanic and geyser activity was due to intense internal heating of Io, a result of the tidal flexing by Jupiter. Studies of the Galileo Orbiter data have established that some of the volcanism occurs at unusually high temperatures.

Figure 1: Images of Io.
figure1

NASA

The main picture, in true colour, was taken by the Galileo Orbiter in 1999. The inset, showing Jupiter with Io at the lower left, is an image from the Voyager 1 mission (1979). Io is about 3,600 km in diameter.

But what was the origin of the sulphur and oxygen in the neutral atmospheric clouds and plasma torus? The ions in the torus are forced by Jupiter's magnetic field to rotate at nearly the same rate as the planet, and therefore bombard Io's atmosphere and surface at energies of hundreds of electron volts. This was identified as the source of the sulphur and oxygen — through a process called sputtering, ion bombardment of SO2 on Io's surface releases atoms and molecules or ions into the atmosphere and neutral torus; the atoms and molecules are then dissociated and ionized by energetic electrons in the plasma torus. But the nature of the compound constituting the sodium source remained a puzzle. Theoretical modelling had shown that it is probably sodium chloride, and this is now confirmed by Lellouch et al.1, who report its detection in vapour form.

This part of the story begins two years ago with Küppers and Schneider's identification of chlorine ions in the plasma torus4. Their observation in the near infrared was quickly confirmed in the far ultraviolet5. On the theoretical side, Fegley and Zolotov6 extended their earlier predictions of the gases and vapours that would be expected to be emitted by volcanic magmas on Io. To their assumed magma composition, taken to occur at high temperatures (1,000–2,000 K), they added sodium, potassium and chlorine in cosmic abundances. This chemical model produced copious NaCl vapour (along with sodium atoms, potassium chloride, SO2 and other sulphur compounds).

Using the 30-m IRAM (Institut de Radio-Astronomie Millimétrique) radio telescope, sited near Granada in Spain, Lellouch et al. have now identified two spectroscopic lines of NaCl vapour, at wavelengths of 1.3 mm and 2.1 mm, in Io's atmosphere. They also detect three lines of SO2 and provide a tentative detection of KCl. The NaCl occurs only in patches in the atmosphere, consistent with there being just a few volcanic sources. An alternative, but much less likely, explanation is that the NaCl stems from evaporation or sputtering from material condensed on SO2 snowdrifts on Io's surface. Earlier observations of SO2 with the same telescope show that SO2 is also distributed patchily, and for this more volatile molecule both sources — volcanism and sputtering — are plausible.

The amount of Na and Cl in the torus, in various forms, has been calculated to be only some 2% of the total composition. The NaCl supply rate found by Lellouch et al. is too large to be consistent with this figure, but their estimate includes some uncertain parameters and the agreement appears to be satisfactory. Thus, 28 years after Brown's discovery of sodium at Io, we may be close to understanding how it reaches the atmosphere and torus.

References

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Correspondence to Donald M. Hunten.

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Hunten, D. Sodium at Io. Nature 421, 30–31 (2003). https://doi.org/10.1038/421030a

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