JOVIAN SYSTEM DATA ANALYSIS PROGRAM PROPOSAL SUMMARY

"Ridge and trough terrain" is a generic term for sets of subparallel ridges and troughs observed on several icy outer planet satellites, and Ganymede's grooved terrain is the best known example of ridge and trough terrain. The nature of Ganymede's ridge and trough terrain provides important constraints on the interior and surface evolution of the satellite, notably its differentiation and tidal evolution. However, the resolution of Voyager images severely limited the conclusions that could be drawn regarding its detailed morphology, deformational style, and the relationship of bright grooved terrain to dark terrain on Ganymede. Because it is fundamental to understanding the satellite, imaging of Ganymede's grooved terrain has been a keystone of the Galileo SSI plan for imaging the satellites of Jupiter. Galileo's discovery of ridge and trough terrain on Europa makes comparison to Ganymede of fundamental importance in understanding the comparative evolution of the Galilean satellites. The objective of this study is to use Galileo SSI high resolution images to examine and categorize the detailed local-scale morphology of ridge and trough terrain on Ganymede, in order to determine the morphological variety and deformational styles of the terrain, and its evolutionary sequence. This will include comparison of ridge and trough terrains within Ganymede's dark and bright terrains. This study will address the satellite's local-scale stress history, its lithospheric character, and its broader-scale geological history. Conclusions from the Ganymede phase of this study will be used to gain insight into the origin of ridge and trough sets on Europa. This research will address the origin and evolution of ridge and trough terrain on Ganymede and comparison to Europa through the following approach: First, ridge and trough terrains in high resolution Galileo images of Ganymede will be defined and classified. A classification will be developed for the morphological types observed in both bright and dark terrain. From the range of morphologies observed and categorized, type example ridge and trough terrains will be defined based on their morphological character. Detailed lineament maps will be produced for each of the type example ridge and trough terrains, and their immediate local surroundings. Second, the characteristics of each type example ridge and trough terrain type will be tested against compiled morphological predictions of possible magmatic and tectonic models for their origin. Once a likely means of origin for ridge and trough terrain types is inferred, the tectono-stratigraphic relationships among ridge and trough sets will be used to derive local and regional stress/strain relationships in the Galileo target regions. Third, the results will be synthesized through intercomparison of the type example terrains and other Galileo target sites. This will center on development of an evolutionary sequence for the variety of ridge and trough terrains on Ganymede, including analysis of the transition from dark terrain to bright grooved terrain. Fourth, this study will address implications of the style and variety of ridge and trough terrains for the local, regional, and global geological history of Ganymede. This will include analysis of the style and degree of strain represented by the terrains. Potential examples of lithospheric flexure in the Galileo images will also be analyzed, to derive properties of Ganymede's elastic lithosphere at the time of its deformation. Fifth, morphological types of ridge and trough terrain will be identified and classified from Galileo high resolution images of Europa, and compared with those on Ganymede. Similarities or differences in morphologies will be used to infer whether similar or different processes have formed the various ridges and trough terrain types on the two satellites. The origin and evolution of ridge and trough terrain is the fundamental input to models of the surface and interior evolution of Ganymede, constraining a large variety of models of surface tectonism and volcanism as well as the satellite's interior differentiation, volume change, mantle convection, and tidal evolution. Application to Europa is important in understanding that satellite's history in a comparative framework, and will serve to focus investigation of models of Europa's ridges and troughs. The results of this study will provide a context in which to understand ridge and trough terrain on other icy satellites, notably when Cassini arrives in the saturnian system. Although I have not submitted a separate Education and Public Outreach proposal, I plan to continue my extensive public outreach and education activities, including public presentations of Galileo's discoveries in the jovian system. The proposed research is separate from, but related to, activities perfomed through affiliation with the Galileo SSI team during the spacecraft's prime mission, in which I participated in initial documentation and validation of the SSI data (see reference below). Pappalardo, R. T., J. W. Head, G. C. Collins, R. L. Kirk, G. Neukum, J. Oberst, B. Giese, R. Greeley, C. R. Chapman, P. Helfenstein, J. M. Moore, A. McEwen, B. R. Tufts, D. A. Senske, H. H. Breneman, and K. Klaasen. Grooved terrain on Ganymede: First results from Galileo high-resolution imaging. Icarus, in press.