Observational and Theoretical Constraints on Plume Activity at Europa

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5422 Ices, 5480 Volcanism (6063, 8148, 8450)

Scientific paper

The recently-detected plume activity on Enceladus [1] has raised the question of whether Europa, too, might be active. The few Galileo images devoted to searches for plumes yielded no detections; comparisons between Voyager and Galileo images suggest that less than ~1mm of resurfacing has happened in the past 20 years over lengthscales of a few km [2]. Cassini observations of Europa's oxygen torus [3] suggest a column abundance and loss rate roughly consistent with modelled O sputtering rates [4,5]. However, the tenuous atmosphere does appear to be spatially non- uniform [6]. The observations suggest that plumes or other non-sputtering sources produce vapour at rates less than roughly 10~kg/s, or less than 10% of the Enceladus plume rate [1]. One possible source of vapour on Europa is shear heating [7,8]. For nominal Europa parameters the predicted rate of vapour production is roughly 1~kg/s per km of fault and the vapour exit velocity is ~450~m/s, much less than Europa's escape velocity. These results suggest that the bulk of the vapour will reimpact the surface after forming a plume approximately 70~km high. The resulting thermal anomaly due to vapour recondensation is ~2~K. To generate a total vapour production rate of 10~kg/s requires roughly 10~km of active faults. If there is a single plume, the local resurfacing rate is ~0.05~mm/yr, compatible with the observational resurfacing constraints [2]. Using a global lineament map [9] and assuming equi-spaced active faults, areas predicted to show most intense shear heating are two regions near the S pole (at ~90° and ~270° longitude) and one smaller patch near the N pole at ~270°. Shear heating, in addition to vapour production, may also cause elevated surface temperatures resulting in thermal segregation of ice [10]. These predictions may be compared with existing observations from Galileo, Cassini, and Earth-based telescopes [e.g. 6], and may assist in the planning of potential future spacecraft missions. [1] Porco C. et al., Science 311, 1393-1401, 2006. [2] Phillips, C.B. et al., JGR 105, 22579-22597, 2000. [3] Hansen, C.J. et al., Icarus 176, 305-315, 2005 [4] Smyth, W.H. and M.L. Marconi, Icarus 181, 510-526, 2006 [5] Shematovich, V.I. et al., Icarus 173, 480-498, 2005. [6] McGrath, M.A. et al., in Bagenal, F. et al., eds., CUP, 2004. [7] Nimmo, F., E. Gaidos, JGR 107, 5021, 2002. [8] Nimmo, F. et al., Nature 447, 289-291, 2007. [9] Crawford, Z.A., R.T. Pappalardo, LPSC XXXVII, 2264, 2006. [10] Spencer, J.R., Icarus 69, 297-313, 1987.

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