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THE TIMING of VISCOUS RELAXATION on GANYMEDE. P. Schenk, Lunar and Planetary Insti- Tute, Houston TX 77586 ([email protected])

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THE TIMING of VISCOUS RELAXATION on GANYMEDE. P. Schenk, Lunar and Planetary Insti- Tute, Houston TX 77586 (Schenk@Lpi.Usra.Edu) 41st Lunar and Planetary Science Conference (2010) 2083.pdf THE TIMING OF VISCOUS RELAXATION ON GANYMEDE. P. Schenk, Lunar and Planetary Insti- tute, Houston TX 77586 ([email protected]) Introduction: With a return to the Jupiter system, and also apparent that craters as small as 7 km across can Europa and Ganymede in particular, under serious be relaxed. Unrelaxed craters follow the standard consideration, the geology and geophysics of these fresh crater d/D slope [6], but there is a defining limit bodies are being scrutinized anew. One of the earliest to relaxation. Most of the identified relaxed craters revelations dating back to Voyager and even before [1, have rim-floor relief of 250-400 meters, mostly a result 2] was that the surface morphology of Ganymede re- of the preservation of the narrow rim scarp feature. cords that satellite’s thermal history. This is manifest in the relaxation of (crater) topography, the study of which has evolved as our understanding of ice and relaxation physics [cf. 3, 4] has evolved slowly over the past three decades. In this report I look back on an issue that has been relatively neglected since the days of Voyager, the spatial distribution and sequence of crater relaxation, the timing and pattern of Gany- mede’s heat flow, and the degree to which it correlates with the mappable geologic history of Ganymede. Methods: Impact craters are the most suitable feature for mapping of relaxation on Ganymede (and Callisto) because of their relative abundance and their known (or assumed) shape prior to relaxation allows for meaning constraints. Mapping the distribution of re- laxed features on Ganymede requires regional topog- raphic mapping coverage at suitable resolutions. Ste- reo imaging is severely limited on Ganymede (mostly of the south pole), so we rely on photoclinometry (PC). PC has improved considerably since the early Voyager investigations, specifically though the use of more realistic photometric models (here the combined lunar-lambertian model [5, 6]). These data are also limited but several sites have been identified where we have sufficient areal coverage of both dark and bright terrain to obtain useful counts. The primary sites are at Enki Catena (500 m/pixel resolution, Tiamat Sulcus (500 m/pixel), Bubastis Sulcus (500 m/pixel) and Northern Marius Regio (950 m/pixel; Fig. 1). Addi- tional sites within Perrine Regio and other areas are being investigated to determine if topographic data can be obtained. Results: Although mapping coverage is severely lim- ited, several curious features are apparent in the new topographic maps. Relaxed craters show the classic bowed crater floors as well as the sharp crater rim in- dicative of the partial preservation of short-wavelength features (Fig. 2). However, rather than the continuum of crater depths reported by Passey [2, using an anti- quated photometric function], we find that craters tend Figure 1: Regional image (top) and PC-derived to- to be more clearly separated into relaxed and unre- pographic map (bottom) of Northern Marius Regio, laxed shapes (Fig. 3). This is evident from the imag- Ganymede. Resolution of base G8 image is 950 ing and topographic maps (Fig. 1), where fresh craters m/pixel. The study site is roughly evenly divided be- clearly stand out from the background cratering. It is tween dark and bright terrains. 41st Lunar and Planetary Science Conference (2010) 2083.pdf “short” global thermal outburst could have reset the topographic “clock” completely. Figure 2: Topographic profiles across craters on Ganymede. Top profile shows two relaxed craters (bold lines), bottom profile shows an unrelaxed crater. Figure 4: Areal density (N) of unrelaxed craters on dark terrain (black dots) and bright terrains (open boxes) at four unique sites on Ganymede. Error bars represent standard deviations. Did high heat flow decline rapidly and relaxation shut down as bright terrain formation ceased? Few relaxed craters are observed on bright terrain, suggest- ing that thermal decay rates were relatively rapid. The most glaring exception appears to be Bubastis Sulcus at the south pole, where several relaxed/modified cra- ters are observed. A small number of modified post- bright-terrain large impact features are also observed globally [7], the most obvious being penepalimpsests and penedome basins (e.g., Epigeus and Neith). These Figure 3: Combined depth/diameter statistics for cra- observations indicate that high heat flows persisted for ters on the 4 study sites for which we have regional a measurable time after bright terrain formed but not topographic coverage. Note the lack of craters with much longer (it may be possible to put a value on this intermediate d/D values, indicating that relaxation, time after a global survey of smaller craters on Gany- when it occurred, apparently was pervasive. mede). These data also indicate that all pre-bright ter- rain topographic features, including furrows, are re- The regional distribution of relaxed and unrelaxed laxed from their original state. craters is very distinct. Visual inspection of Fig. 1 The question of the thermal history on Callisto re- suggests that unrelaxed craters are equally distributed mains open. Galileo’s discovery of the erosionally across both bright and dark terrains. Statistics for the desiccated state of Callisto’s surface renders topog- 4 study sites (Fig. 4) confirm that the spatial density of raphic measurement of many craters meaningless in unrelaxed craters is identical on both terrains despite the context of relaxation. The observation of signifi- the factor of 4 or more difference in the spatial density cant numbers of “wreath” craters and pene-dome cra- of all craters on the two terrain types. ters [7] similar to those found on post-bright terrain Discussion: The uniform distribution of unrelaxed Ganymede suggests that Callisto too may have had a crater across all terrain types implies that all relaxation period of elevated heat flow. Topographic mapping of must have ceased after bright terrain was emplaced; both satellites is very limited, however, and global otherwise more unrelaxed craters would have accumu- mapping will probably be required before crater distri- lated on older dark terrain than on younger bright ter- bution and ages, and hence the detailed thermal histo- rain. This implies that elevated heat flows (>>10 ries of these satellites can be fully ascertained. mW/m^-2 [4]) occurred across all of Ganymede up to and through the period of bright terrain formation. References: [1] Johnson, T., and T. McGetchin, Icarus 18, Whether heat flow was persistently high throughout 612-620. [2] Passey, Q. (1983) PH.D. Thesis, Cal. Inst. Techn. [3] pre-bright-terrain epochs or only during a shorter ele- Goldsby, D, and D. Kohlstedt (2001) J.. Geophys. Res, 106, 11017- vated heat flow pulse in direct association with bright 11030. [4] Dombard, A, W. McKinnon (2006) J. Geophys. Res., 111, terrain formation is uncertain. It may not be possible E01001. [5] McEwen, A. Icarus, 92, 298-311. [6] Schenk, P. (2002) Nature,417, 419-421. [7] Schenk, P., C. Chapman, K. Zah- to determine this from the cratering record alone, as a nle, and J. Moore (2004) in Jupiter (F. Bagenal, T. Dowling and W. McKinnon, eds.) .
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