Lunar and Planetary Science XXXIII (2002) 1272.pdf

GANYMEDE FURROW SYSTEMS AS STRAIN MARKERS: IMPLICATIONS FOR EVOLUTION AND RESURFACING PROCESSES. L. M. Prockter1, G. C. Collins2, S. L. Murchie1, P. M. Schenk3, and R. T. Pap- palardo4. (1) Applied Physics Laboratory, MS 4-137, 11100 Johns Hopkins Road, Laurel, MD 20723, [email protected]. (2) Wheaton College, Norton, MA, (3) Lunar and Planetary Inst., Houston, TX, (4) LASP, U. Colorado, Boulder, CO.

Introduction: Multiringed basins are important offsets between regions. Later, utilizing improved geo- stratigraphic markers and their number, size, and sub- metric control of images, Schenk and McKinnon [15] sequent disruption provide insight into the early history expanded mapping of the anti-Jovian hemisphere and of a planet and its thermal and geological evolution. compiled the first database of sub-Jovian furrows, con- The initial sub-circular shape of a basin’s ring system cluding that there had been no offset of the Galileo- provides an ideal marker to investigate large-scale Marius furrows, and estimated the center of curvature strain. Ganymede is divided into two very different to lie near 21S, 180W, in a region deformed into surface units, dark and bright terrain [1]. Dark terrain is grooved terrain. They also concluded that the sub- Callisto-like and contains remnants of multiringed Jovian hemisphere contains two systems, one centered structures, or furrow systems, distributed over local- to in Perrine Regio and one in Nicholson Regio. Murchie regional-scale polygons separated by swaths of and Head [4] expanded the geographic coverage of younger, higher-albedo grooved terrain. Voyager-era mapped furrows further, and examined grooved terrain models of the breakup of dark terrain and its conver- for evidence of shear deformation. Their expansion of sion into grooved terrain [1, 2] invoked mostly exten- furrow mapping into eastern Marius Regio led them to sional deformation with low amounts of strain accom- conclude that centers of concentricity do suggest left- panied by cryovolcanism, to account for depletion of lateral shear offset from Galileo Regio, and that Marius' crater densities and the increased albedo relative to center of curvature lies within a giant, faint palimpsest dark terrain. Some researchers recognized the potential (15S, 168W) proposed previously to be the scar of the role of shear [3], while others cited evidence for offsets impact responsible for the furrows' formation [3]. Mur- in the furrows’ centers of concentricity as evidence that chie and Head also showed evidence for a zone of shear played a major role [4]. lesser magnitude, right-lateral shear south of Marius. In Some studies of grooved terrain using Galileo im- both cases, deformational patterns in grooved terrain ages have concluded that grooved terrain is primarily were interpreted as evidence that shear contributed to extensional in nature with pervasive but locally small grooved terrain deformation. amounts of shear, and little evidence for associated New datasets: Combined imaging from Voyager volcanism [5-8]. In contrast, recent studies by Schenk et and Galileo covers 98% of Ganymede at resolutions of al. [9] provide evidence for an origin of smooth 3.5 km/pixel or better, providing for the first time the grooved terrain segments through cryovolcanic flood- basis for truly global analysis of the configuration of ing of graben, a result that would imply relatively small structural features. Specifically, the Galileo images fill amounts of strain. Collins et al. [10] suggested full- gaps in Voyager coverage, linking the sub- and anti- scale separation of dark terrain lithosphere has oc- Jovian hemispheres covered by Voyager, and many of curred, with new grooved terrain material filling the the images are targeted at key parts of the furrow sys- gap, a process which has been shown to occur on Eu- tems (e.g., eastern Galileo Regio, western Marius Re- ropa [e.g. 11, 12], and which would imply larger gio). The United States Geological Survey (USGS) in amounts of strain. McKinnon [13] showed that the larg- Flagstaff has generated a new coordinate-controlled est coherent piece of dark terrain remaining on Gany- Ganymede basemap using both Voyager and Galileo mede, Galileo Regio, could be used to constrain the imaging data [16]. The basemap allows locations of amount of planetary expansion to less than 1% in ra- features to be determined with much improved accu- dius. Further understanding of the exact nature of dark racy. Using this map, we have compiled a global Geo- terrain breakup would aid in constraining models of graphic Information Systems (GIS) database of grooves Ganymede’s thermal evolution. [17] and furrows, and present the complete map here Previous work: The first Voyager era analyses of for the first time (Fig. 2). The grooves and furrows grooved terrain noted an apparent discontinuity in fur- have been mapped using the ArcView GIS software row curvature across the Uruk Sulcus region, separat- package. ing Galileo Regio from Marius Regio. Several hundred Current work: These products provide the capabil- kilometers of left-lateral shear was suggested as the ity for investigation of regional deformation of grooved cause [3]. This hypothesis was tested by Zuber and terrain and whether and how it is related to dark terrain Parmentier [14] who fitted the furrow systems in sepa- breakup. Specifically, we are extending the work of rate anti-Jovian dark terrain regions (Galileo and Schenk and McKinnon [15], and Murchie and Head Marius Regiones) to small-circle systems. They found [4], by using furrow system locations from the new deviations from a concentric geometry within single, database as strain markers. Figure 1 shows a newly undeformed regions to be at least as large as any due to identified small furrow system that apparently lies Lunar and Planetary Science XXXIII (2002) 1272.pdf

GANYMEDE FURROW SYSTEMS: L. M. Prockter et al.

partly in Galileo Regio, and partly in Perrine Sulcus. neighboring moons. Evidence of extensive horizontal We will present results for this basin in which we esti- motion will naturally lead to comparisons with Europa, mate the location of the pole of the system, and deter- and may constrain models of a Ganymede ocean. mine any deviations from concentricity. The latter is References: [1] Shoemaker E.M., et al., (1982) in Satellites of necessary in order to make a significant estimation of Jupiter, pp. 435-520, Univ. of Ariz. Press, Tucson. [2] Golombek strain across Xibalba Sulcus, as furrow systems may be M.P., (1982) Proc. Lunar Planet. Sci. Conf. 13th, JGR., 87, A77- non-circular in their outer reaches, probably as a result A83. [3] Lucchitta B.K., (1980) Icarus, 44, 481-501. [4] Murchie of lithospheric homogeneities [14, 15]. In this way, we S.L. and J.W. Head, (1988) JGR, 93, 8795-8824. [5] Head, J. et al., (1988) LPSC XXIX [CD-ROM], 535-536. [6] Collins G.C. et al., can determine the nature and extent of the separation of (1998) Icarus, 135, 345-359. [7] Pappalardo R.T., et al. (1998) different segments of dark terrain in order to constrain Icarus, 135, 276-302. [8] Prockter L.M. et aL., (2000) JGR, 105, the style of grooved terrain formation. The magnitude 22519-22540. [9] Schenk P.M. et al., (2001) Nature, 410, 57-60. and amount of offset, if any, between partial systems of [10] Collins G.C. et al., (2001) LPSC XXX1, [CD-ROM] 1034. [11] Schenk P. and W. McKinnon, (1989) Icarus, 79, 75-100. [12] Sulli- furrows can be determined in order to understand the van R. et al., (1998) Nature, 391, 371-373. [13] McKinnon W.B. mobility of the lithosphere and compare global with (1989) Proc. LPSC, 12B, 1585-1597. [14] Zuber M. and E.M. Par- local scale tectonics. This study will allow direct com- mentier (1984) Icarus 60, 200-210. [15] Schenk P.M. and W.B. parisons to be made with multiringed basins on Cal- McKinnon (1987) Icarus 72, 209-234. [16] Becker T. et al. (1999) LPSC XXX [CD-ROM] #1692. [17] Collins, G.C. et al. (2000) LPSC listo, and will contribute directly to an understanding of XXXI [CD-ROM] #1034. the evolutionary divergence between these two

Figure 1: Left: Galileo Regio and Perrine Regio, each of which contains a small furrow system and which are separated by Xibalba Sulcus. Center: Closer view of furrow system in eastern Galileo Regio identified in Galileo images; projection is simple cylindrical. Right: Similar sized furrow system found in Perrine Regio identified in Voyager images, reprojected around its inferred pole by [15] (polar sterographic projection). Dashed yellow lines indicate the approximate outer extent of each system. The two systems may have originally been part of the same multir- inged basin; if so, they have been offset by the formation of the intervening grooved terrain swath, Xibalba Sulcus.

Figure 2: GIS database of all furrows and grooves on Ganymede. Database is compiled from global map of Ganymede [17].