The moons of the gas giants (Jupiter, Saturn, Uranus, Neptune) are composed mainly of ice, and are geologically fascinating. For instance, Europa has features which look similar to plate tectonic features on Earth, while tiny Enceladus has a giant geyser blasting off into space. The Galileo spacecraft provided a good look at the satellites of Jupiter, while Cassini is currently doing the same for those of Saturn. I have recently completed two projects on Enceladus. One was to explain why the geysers on Enceladus are almost exactly at the south pole. It turns out that if there is warm ice in the subsurface (see left-hand figure), then Enceladus probably rolled over until this region reached the pole. This hypothesis is testable with future observations of the gravity field of the satellite, and also the distribution of impact craters. The other project was to suggest that the geysers are generated by frictional heating due to faults rubbing back-and-forth against each other (see right-hand figure).
(Left) Artist's impression of an area of warm ice within Enceladus responsible for geysering and reorientation (see the Nature article for more details).
(Right) Artistís impression of shear-heating on faults generating the observed vapour plumes above the surface (see another Nature article for details).
I am following up on various aspects of this work. During his post-doc with me, Ke (Chris) Zhang looked into the orbital evolution of Enceladus, while Guillaume Robuchon, another post-doc, is investigating the coupled thermal-orbital evolution of several icy bodies including Iapetus and Pluto. Studying Pluto is interesting because the New Horizons spacecraft will be arriving there in 2015.
I recently became a Participating Scientist on the Cassini mission, which gives me early access to the data. We are finally starting to get geophysical data for Titan, the largest Saturnian satellite and the only one with an atmosphere. Although the data are currently limited, it seems likely that Titanís ice shell is not convecting, and that it has an ocean deep beneath the surface. One of my graduate students, Erinna Chen, is now investigating the general behaviour of sub-surface oceans on icy satellites.
Publications on icy satellites
Below are some highlights; you can get a full listing here:
∑ Late-stage impacts and the orbital and thermal evolution of Tethys, K. Zhang, F. Nimmo, Icarus 218, 348-355, 2012 Reprint
∑† Thermal evolution of Pluto and implications for surface tectonics and a subsurface ocean, G. Robuchon, F. Nimmo, Icarus 216 426-439, 2011 Reprint
∑† Geophysical implications of the long-wavelength topography of the Saturnian satellites, Nimmo, F., B.G. Bills, P.C. Thomas J. Geophys. Res. 116 E11001, 2011Reprint
∑† Impact basin relaxation at Iapetus G. Robuchon, F. Nimmo, J. Roberts, M. Kirchoff, Icarus 214 82-90, 2011 Reprint
∑ Shell thickness variations and the long wavelength topography of Titan F. Nimmo, B.G. Bills Icarus 208, 896-904, 2010 Reprint
∑ The role of episodic overturn in generating the surface geology and heat flow on Enceladus C. O'Neill, F. Nimmo, Nature Geosci. 3, 88-91, 2010 Reprint
∑ Tidal heating and the long-term stability of a subsurface ocean on Enceladus J.H. Roberts and F. Nimmo, Icarus 194 675-689, 2008, Reprint(PDF)
∑ Reorientation of icy satellites by impact basins F. Nimmo and I. Matsuyama Geophys. Res. Lett. , 34, L19203, 2007. Preprint (PDF)
∑ Rotational stability of tidally deformed planetary bodies I. Matsuyama and F. Nimmo J. Geophys. Res. , 112, E11003, 2007. Reprint (PDF)
Last Modified:4th Sept 2012.