Calculations have shown that dunes of fine particles can appear on Io even without wind, driven by volcanism, which continually raises torrents of hot, semi-liquid lava to the surface.
One of Jupiter's largest satellites, Io is a remarkable place. It is the most volcanically active celestial body in the whole solar system with 400-500 volcanoes. Io's surface is continually renewed by the lava and ash streams that flow from the depths and the sulfur compounds that make up its picturesque colored patches. Individual patches are covered by winding lines that look like dunes from above.
In theory, the satellite's atmosphere is too thin and the winds too weak to move even the smallest particles of local 'sand'. Until now, therefore, these formations have been thought to be something "dune-like" rather than true moving hills made of small particles by the wind. However, new work by scientists from Rutgers University (USA) has shown that dunes can form on Io too, like on Earth or on the fantasy planet of Arrakis. The paper by George McDonald and his colleagues has been published in the journal Nature Communications.
The most detailed photos of Io have been obtained by the Galileo space probe, which operated in Jupiter's system for more than ten years, until 2003. It was these that helped to discover many of the satellite's surprising features, including areas of suspicious dunes. This work showed that Io's surface is a complex shifting combination of patches of basaltic and sulphuric lava, frozen and still flowing, clusters of small particles of volcanic ash and 'snow' consisting of frozen sulphur dioxide. If what Galileo saw on Io are indeed dunes, they must be composed of ash and sulphur snow. The only question is: by what force, if there are hardly any winds there?
To find out, McDonald and his co-authors did a mathematical modelling of the forces that act on the individual fine particles. It turns out that molten lava can come into contact with frozen sulphur dioxide as it rises closer to the surface and instantly vaporises it. Pressure of gases arising thus is enough to move quickly small solid particles and gradually to fold them in dunes.
Such a mechanism was also confirmed in the Galileo probe data: the distances between dune crests on Io, as well as their heights, matched the model predictions quite well. "Our work points to Io as a new 'dune world'," McDonald concludes. - 'We have proposed and quantitatively confirmed the way sand grains can move, forming them on the satellite. <...> It shows that the conditions in which dunes emerge are far more varied than the usual large deserts in some regions of Earth or on the fictional planet of Arrakis in 'Dune'.
