Icarus 199 (2009) 189–196 Contents lists available at ScienceDirect Icarus www.elsevier.com/locate/icarus Endogenic heat from Enceladus’ south polar fractures: New observations, and models of conductive surface heating Oleg Abramov ∗ , John R. Spencer Department of Space Studies, Southwest Research Institute, 1050 Walnut St., Suite 300, Boulder, CO 80302, USA article info abstract Article history: Received 24 February 2008 Revised 18 July 2008 Accepted 29 July 2008 Available online 4 September 2008 Keywords: Enceladus Geological processes Ices Thermal histories Volcanism Linear features dubbed “tiger stripes” in the south polar region of Enceladus have anomalously high heat fluxes and are the apparent source of the observed plume. Several explanations for the observed activity have been proposed, including venting from a subsurface reservoir of liquid water, sublimation of surface ice, dissociation of clathrates, and shear heating. Thermal modeling presented in this work, coupled with observations from the Cassini Composite Infrared Spectrometer (CIRS) instrument, seeks to elucidate the underlying physical mechanism by constraining vent temperatures and thermal emission sources, using a model in which the observed thermal signature results primarily from conductive heating of the surface by warm subsurface fractures. The fractures feed surface vents, which may themselves contribute to the observed thermal emission. Model variables include vent temperature, presence of a surface insulating layer, vent width, time-variable heat input, and heat sources other than the central vent. Results indicate that CIRS spectra are best fitted with a model in which the surface is heated by narrow vents at temperatures as high as 223 K. Although equally good fits can be obtained for vent temperatures in the range of 130 to 155 K if the vents are wider (180 m and 22 m respectively) and dominate the emission spectrum, these models are probably less realistic because vents with these temperatures and widths cannot supply the observed H 2 O vapor flux. The lack of emission angle dependence of the thermal emission when July 2005 and November 2006 CIRS observations are compared also argues against thermal emission being dominated by the vents themselves. Thus, results favor high-temperature models, possibly venting from a subsurface liquid water reservoir. However, a fracture filled with liquid water near the surface would produce significantly higher radiances than were detected unless masked by a thermally insulating surface layer. Models that best match the CIRS data are characterized by small fractions of the surface at high temperatures, which strengthens the case for the vents and/or their conductively-heated margins being the primary heat source. Models where the thermal emission is dominated by conductive heating of the surface from below by a laterally-extensive buried heat source cannot reproduce the observed spectrum. Models with a 10 cm thick upper insulating layer produce a poor match to the CIRS spectra, suggesting high thermal inertias near the tiger stripes. Finally, tiger stripe thermal emission measured by CIRS varied by less than 15% over the 16 month period from July 2005 to November 2006.  2008 Elsevier Inc. All rights reserved. 1. Introduction The south polar region of Enceladus, a small icy satellite of Sat- urn, consists of young, tectonically deformed terrain dominated by roughly parallel, 2-km wide linear depressions dubbed “tiger stripes” (e.g., Porco et al., 2006). Recent observations by multi- ple instruments on the Cassini spacecraft describe an anomalously high heat flux associated with these tiger stripes, along with ac- tive plumes of water vapor and ice particles that originate from them (Porco et al., 2006; Spencer et al., 2006; Hansen et al., 2006; Waite et al., 2006; Spahn et al., 2006; Spitale and Porco, 2007). * Corresponding author. Fax: +1 303 492 2606. E-mail address: oleg.abramov@Colorado.edu (O. Abramov). Several explanations for the observed elevated temperatures and the resulting plume have been proposed, including venting from a subsurface reservoir of liquid water (Porco et al., 2006; Schmidt et al., 2008), sublimation of surface ice (Spencer et al., 2006), decompression and dissociation of clathrates (Kieffer et al., 2006), and shear heating (Nimmo et al., 2007). These mechanisms predict a range of vent temperatures: ∼140 K for clathrate de- compression (Kieffer et al., 2006), >180 K for sublimation of H 2 O (Spencer et al., 2006), and up to 273 K for the shallow reservoir of liquid water (Porco et al., 2006). The elevated vent tempera- tures would conductively heat the nearby surface, contributing to the thermal signature observed by Cassini. The thermal model- ing presented in this work, coupled with observations from the Cassini CIRS instrument, seeks to constrain the vent temperatures 0019-1035/$ – see front matter  2008 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2008.07.016