In July 2005 the Cassini spacecraft flew within 170 km of Saturn’s moon Enceladus and discovered an icy landscape strewn with house-sized boulders and tectonic features. Among those features are so-called “tiger stripes,” four fractures, each roughly 130 km long and flanked by a band of warm ice, that continuously spew plumes of ice, water vapor, and other gases into space. (See the article by John Spencer, Physics Today, November 2011, page 38.) Edwin Kite (University of Chicago) and Allan Rubin (Princeton University) have produced a simple model that explains the local warmth and constant eruptions. Enceladus’s surface is thought to be an ice shell roughly 35 km thick that floats on a salty liquid-water ocean. The researchers posit that the tiger stripes are open fissures a meter wide that run through the shell. As shown in the diagram, the ocean fills a fissure with water up to within a few kilometers of the moon’s surface. Diurnal tides alternatingly narrow and widen each tiger stripe and force the water table to rise and fall. Tidal forces have long been known to drive the eruptions, but earlier researchers had expected that as the vapor ascends, evaporative cooling of the liquid would cause the fissures to freeze shut in less than a year. Kite and Rubin found that although the vertical water speed in the fissure is only about 1 m/s, the heat generated by the turbulent flow is enough to keep the slot unclogged. Reassuringly, their model accounts for the 5 GW IR signal Cassini measured around the tiger stripes and explains a previously puzzling time lag between the eruptions’ peak flux and the tensile force that produces it. (E. S. Kite, A. Rubin, Proc. Natl. Acad. Sci. USA, in press.)
