ICARUS 119, 385–404 (1996) ARTICLE NO. 0026 The Volcanic and Tectonic History of Enceladus JEFFREY S. KARGEL U.S. Geological Survey, Flagstaff, Arizona 86001 E-mail: jkargel@iflag2.wr.usgs.gov AND STEFANIA POZIO Reparto di Planetologia, Viale dell’ Universita ` 11, 00185 Rome, Italy Received April 17, 1995; revised September 28, 1995 concentration of cracks or grabens and flattened, flooded, and rifted craters. Pit chains and cratered domes suggest explosive Enceladus has a protracted history of impact cratering, cryo- volcanism. Smooth plains may have formed by cryovolcanic volcanism, and extensional, compressional, and probable equivalents of flood-basalt volcanism. Pure H 2 O would be diffi- strike–slip faulting. It is unique in having some of the outer cult to extrude through an icy crust and is cosmochemically Solar System’s least and most heavily cratered surfaces. Encela- improbable as a cryovolcanic agent. Density relations rule out dus’ cratering record, tectonic features, and relief elements have eutectic brine lavas on Enceladus, but NH 3 –H 2 O volcanism been analyzed more comprehensively than done previously. is possible. Current steady-state tidal dissipation may cause Like few other icy satellites, Enceladus seems to have experi- melting of ammonia hydrate at a depth of 25 km if the crust enced major lateral lithospheric motions; it may be the only is made of ammonia hydrate or 100 km if it is made of water icy satellite with global features indicating probable lithospheric ice.  1996 Academic Press, Inc. convergence and folding. Ridged plains, 500 km across, consist of a central labyrinthine ridge complex atop a broad dome surrounded by smooth plains and peripheral sinuous ridge belts. 1. INTRODUCTION The ridged plains have few if any signs of extension, almost no craters, and an average age of just 10 7 to 10 8 years. Ridge Enceladus has a complex surface (Figs. 1–4) and a dy- belts have local relief ranging from 500 to 2000 m and tend to namic history despite its diminutive stature among icy sat- occur near the bottoms of broad regional troughs between swells. Our reanalysis of Peter Thomas’ (Dermott, S. F., and ellites (its disk, diameter 499 km, could fit within Arizona). P. C. Thomas, 1994, The determination of the mass and mean Pre-Voyager models suggested that large icy satellites density of Enceladus from its observed shape, Icarus, 109, 241– should be geologically evolved but Enceladus should not 257) limb profiles indicates that high peaks, probably ridge be (Consolmagno and Lewis 1978, Peale et al. 1980). Hints belts, also occur in unmapped areas. Sinuous ridges appear to the contrary came in 1980 from (1) Earth-based observa- foldlike and are similar to terrestrial fold belts such as the tions, which showed that Saturn’s tenuous E-ring may have Appalachians. If they are indeed folds, it may require that the been ejected from Enceladus (Baum et al. 1981), and (2) ridged plains are mechanically (perhaps volcanically) layered. low-resolution Voyager 1 images of Enceladus, which Regional topography suggests that folding may have occurred showed a high albedo and no large craters. Enceladus’ along zones of convective downwelling. The cratered plains, in extraordinary nature was recognized in Voyager 2’s high- contrast to the ridged plains, are heavily cratered and exhibit resolution (1–3 km/pixel) images of 43% of the satellite, extensional structures but no obvious signs of compression. Cratered plains contain a possible strike–slip fault (Isbanir which showed evidence of a protracted geologic history Fossa), along which two pairs of fractures seem to have 15 km with episodes of volcanism and tectonism (Smith et al. of right-lateral offset. The oldest cratered plains might date 1982) (Figs. 1–3). Many publications have contributed from shortly after the formation of the saturnian system or the knowledge of Enceladus’ shape, mean density, surface fea- impact disruption and reaccretion of Enceladus. Another area tures, and composition, photometry, geologic history, ther- of cratered plains has modified craters (e.g., Ali Baba and mal history, resurfacing mechanisms, and associations with Aladdin), which some workers have explained by anomalous Saturn’s E-ring. There has been no previous complete tec- heat flow and viscous relaxation; lateral shear and shield-build- tonic mapping of Enceladus in areas covered by Voyager ing volcanism also may have been important. A young rift-like structure (northern Samarkand Sulci) has few craters and a 2 images, no tectonic mapping at all in conventional projec- 385 0019-1035/96 $18.00 Copyright  1996 by Academic Press, Inc. All rights of reproduction in any form reserved.