Volcanic constructs on Ganymede and Enceladus : topographic evidence from stereo images and photoclinometry

Paul M. SchenkLunar and Planetary Institute, Houston, TexasJeffrey M. MooreSpace Sciences Division and the SETI Institute, NASA Ames Research Center, Moffett Field, CaliforniaAbstract. The morphology of volcanic features on Ganymede differs significantly from that onthe terrestrial planets. Few if any major volcanic landforms, such as thick flows or shieldvolcanoes, have been identified to date. Using new stereo Voyager images, we have searchedGanymede for relief-generating volcanic constructs. We observed seven major types of volcanicstructures, including several not previously recognized. The oldest are broad flat-topped domespartially filling many older craters in dark terrain. Similar domes occur on Enceladus. Togetherwith smooth dark deposits, these domes indicate that the volcanic history of the dark terrain iscomplex. Bright terrain covers vast areas, although the style of emplacement remains unclear.Smooth bright materials embay and flood older terrains, and may have been emplaced as low-viscosity fluids. Associated with smooth bright material are a number of scalloped-shaped, semi-enclosed scarps that cut into preexisting terrain. In planform these structures resemble terrestrialcalderas. The youngest volcanic materials identified are a series of small flows that may haveflooded the floor of the multiring impact structure Gilgamesh, forming a broad dome. Theidentification of volcanic constructs up to 1 km thick is the first evidence for extrusion ofmoderate-to-high viscosity material on Ganymede. Viscosity and yield strength estimates for thesematerials span several orders of magnitude, indicating that volcanic materials on Ganymede have arange of compositions and/or were extruded under a wide range of conditions and/or eruptive styles.IntroductionThe Voyager survey of the outer solar system has revealedan astonishing variety of endogenic landforms on the surfacesof icy satellites. Despite extensive endogenic resurfacing ofsomc satellites, for example, Ganymede, Enceladus, andTriton, volcanism on these surfaces has little topographicexpression. We do not see massive volcanic edifices similarto Olympus Mons or Mauna Kea. Even the best putativeexamples of volcanic landforms, seen on Ariel, Miranda,Triton, and Enceladus, exhibit rarely more than 1-2 km ofrelief [Jankowski and Squyres, 1988; Pozio and Kargel,1989; Schenk, 1991a; Croft et al., 1995]. This may berelated to the "soft" theology of ice (see Poirier [1982],Durham et aL [1983], and Kirby et al. [1987]), or torheology and volcanic styles peculiar to icy petrologicsystems [e.g., Kargel, 1991; Kargel et al., 1991]. Well over half of the surface of Ganymede (radius 2634 kin)has been covered by endogenic effusions. It is now consideredlikely that Ganymede experienced one or more episodes ofchaotic orbital behavior [Malhotra, 1991], possiblyresulting in tidal heating. Volcanic resurfacing constitutes arecord of Ganymede's compositional and thermal evolution andmay record some of these putative tidal heating events. Yetthis satellite has been the subject of morphological study for15 years without the unambiguous detection of relief-Copyright 1995 by the American Geophysical Union.Paper number 95JE01854.0148-0227/95/95JE-01854505.00generating volcanic constructions on its landscape. The styleof emplacement and composition of those volcanic depositsthat have been identified remains unclear.On any planetary surface, volcanic and other nonimpactconstructs can be difficult to recognize in spacecraft images,particularly those obtained under high Sun conditions [e.g.,Malin, 1978]. Topography is especially low on the Galileansatellites, and imaging resolution is at best -500 m/pixel.Major local albedo variations on Ganymede further complicategeologic and topographic mapping. With these problems inmind, we have undertaken the production of stereo pairs ofVoyager images (where coverage permits) to aid in the searchfor and characterization of volcanic landforms on Ganymede'ssurface. In addition to an examination of previously identifiedvolcanic materials, we report the discovery of severalpreviously unrecognized volcanic landforms and at least twotypes with substantial relief, and describe their morphologyand geologic setting. Morphology of volcanic depositsindicates that Ganymede extrusions probably had a range oftheologies and may be compositionally variable. We concludewith speculations and predictions regarding their mode ofemplacement and the implications of volcanic landforms forthe geologic evolution of Ganymede.Stereo Imaging with VoyagerThis study was motivated by the recognition that asignificant fraction of the surfaces of most of the outer planetsatellites were imaged by Voyager in stereo [e.g., Croft andSoderblom, 1991; Schenk and Moore, 1993]. We can usestereo imaging to detect subtle topographic features that were19,009