1. Jet Propulsion Laboratory/California Institute of Technology, Pasadena, California 91109, USA
2. Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado 80303, USA
3. Planetary Sciences Group, University of Central Florida, Orlando, Florida 32816, USA
4. Science Department, Central Arizona College, Coolidge, Arizona 85228, USA
5. Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

Correspondence to: C. J. Hansen¹ Correspondence and requests for materials should be addressed to C.J.H. (Email: candice.j.hansen@jpl.nasa.gov).

Abstract

A plume of water vapour escapes from fissures crossing the south polar region of the Saturnian moon Enceladus^{1, 2, 3, 4, 5, 6}. Tidal deformation of a thin surface crust above an internal ocean could result in tensile and compressive stresses that would affect the width of the fissures⁷; therefore, the quantity of water vapour released at different locations in Enceladus' eccentric orbit is a crucial measurement of tidal control of venting. Here we report observations of an occultation of a star by the plume on 24 October 2007 that revealed four high-density gas jets superimposed on the background plume. The gas jet positions coincide with those of dust jets reported elsewhere⁸ inside the plume. The maximum water column density in the plume is about twice the density reported earlier². The density ratio does not agree with predictions⁷—we should have seen less water than was observed in 2005. The ratio of the jets' bulk vertical velocities to their thermal velocities is 1.5  0.2, which supports the hypothesis that the source of the plume is liquid water, with gas accelerated to supersonic velocity in nozzle-like channels⁹.

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