An interesting picture from Enceladus, currently everyone’s favourite geologically active, extra-terrestrial body:
Whilst most of us were content to simply admire the images of icy plumes emanating from Enceladus’ south pole that Cassini has sent from Saturn, Joseph Spitale and Carolyn Porco of the Lunar and Planetary Laboratory at the University of Arizona were examining them in much more detail. By using images of the same jet taken at different angles, they were able to pinpoint exactly where on the surface they were emanating from. The eruption points, shown by the coloured dots, appear to cluster in eight distinct regions (shown by the white circles), all of which are located along one of four distinctive grooves or sulci, popularly referred to as “tiger-stripes”, running across the southern polar region, .
This correlation ties in well with the notion that the jets are caused by water, formed by tidal friction-induced melting
of Enceladus’ interior (see update below), escaping through these fractures. Infra-red observations have already discovered hotspots associated with the sulci; these hotspots are represented by red polygons on the image above, and 5 of the 8 plume sources identified by Spitale and Porco are bang on top of, or at least reasonably close to, these heat sources. I suspect (supported by quotes from Porco in this PhysOrg article) that the lack of associated hotspots for the remaining sources is because they are in regions which have yet to be covered by the infra-red spectrometer.
One feature that is currently puzzling me a little is that the location of the eruption points are mainly confined to a particular section of each sulcus, marking out a linear trend which cross-cuts that of the “tiger-stripes”. It could just be that the sections closest to the south pole, in the centre of the image, are the most active. Perhaps the Nature paper which accompanies this image* holds the answer…
Update: having read the paper, it seems that the distribution of plumes and hotspots agrees reasonably well with the distribution predicted by modelling of tidal forces caused by the varying gravitational influence of Saturn and its other moons. This model suggests that the source of the plume material is quite shallow, with melting localised to areas of tidally-driven strike-slip motion on the sulci; when I wrote the above I was assuming a situation more analogous to Io, with somewhat larger scale planetary squeezing and deeper melting. Also note Andy’s comment below about the possible involvement of clathrate hydrates, where ice forms into a cage-like lattice and traps gas molecules within its structure.
*Which, rather ironically given that my latest Naturejobs column is in this weeks issue, I can’t access.