Lunar and Planetary Science XXIX 1866.Pdf
Total Page:16
File Type:pdf, Size:1020Kb
Lunar and Planetary Science XXIX 1866.pdf GALILEO AT CALLISTO: OVERVIEW OF NOMINAL MISSION RESULTS. J. E. Klemaszewski, R. Greeley, K.S. Homan, K.C. Bender, F.C. Chuang, S. Kadel;1 R.J. Sullivan;2 C. Chapman, W.J. Merline;3 J. Moore;4 R. Wagner, T. Denk, G. Neukum;5 J. Head, R. Pappalardo, L. Prockter;6 M. Belton;7 T.V. Johnson;8 C. Pilcher9 and the Galileo SSI Team. 1Dept. of Geology, Arizona State University, Tempe, AZ 871404; 2Cornell Univ., Ithaca, NY; 3SWRI, Boulder, CO; 4NASA Ames, Moffett Field, CA; 5DLR, Berlin, Germany; 6Brown Univ., Providence, RI; 7NOAO, Tucson, AZ; 8JPL, Pasadena, CA; 9NASA HQ, Washington, DC. The solid-state imaging (SSI) system on board the over 4000 and 1600 km in diameter; their Galileo orbiter acquired 118 images of the Jovian morphologies and theories concerning their origin satellite Callisto on 8 of its 11 nominal mission orbits. have been described by many investigators [e.g. On three of these orbits clear-filter imaging data were 2,3,6,8,9]. Whether or not Callisto is differentiated acquired at resolutions of 150 m/pxl or better, and on could not be determined from Voyager data [6]. five orbits color data were acquired at resolutions Galileo mission objectives for Callisto [10] include (a) between 13.7 and 1.1 km/pixel. Galileo images reveal the characterization of surface processes and that degradational processes have acted on the surface materials, (b) impact craters morphologies and of Callisto causing the erosion of crater walls and rims. evolution, (c) the search for evidence of endogenic This degradation is most likely responsible for the activity, (d) the determination of the origin and production of the dark material that appears to blanket morphology of multi-ring structures, and (e) Callisto’s surface, resulting in a lack of small (<10 km comparison of Callisto and Ganymede. diameter) impact craters. Coordinated monochromatic and multi-spectral SSI imaging with the near-infrared SSI OBSERVATIONS AND ANALYSIS mapping spectrometer (NIMS) is useful in determining The surface of Callisto as seen in moderate- to the distribution of candidate surface materials, such as high-resolution images appears to be smooth and CO2, SO2 and nitrogenous organic compounds. slightly undulating, containing a low spatial density Moderate- to high-resolution (800 - 40 m/pxl) imaging of small (<10 km/pxl) impact craters. This smooth of impact craters gives clues to impact processes and texture, first identified in high-resolution (40 m/pxl) impact evolution on icy targets. Degradational images within or near the Valhalla system, has also processes are seen to be effective in the modification been seen in moderate resolution (150-800 m/pxl) and removal of impact craters over time. Imaging of images near Callisto’s south pole and equatorial Callisto’s multi-ring structures, Valhalla, Asgard and regions. The wide spatial distribution of smooth Adlinda allows for the comparison of these structures terrains suggests that the smoothing process(es) are and provides insight into their formation and most likely global in extent. The dark non-ice evolution. Extension in at least one of Asgard’s rings material that appears to blanket the surface may appears to have been accommodated by several sub- become concentrated by the loss of volatile materials parallel fractures within the ring. There is a lack of during impact events, magnetospheric bombardment, clear geological evidence for the expression of and sublimation [11,12,13]. This material could be endogenic activity on Callisto’s surface in Galileo distributed by impacts and micrometeorite images. bombardment resulting in the infilling of low-lying topography. However, these processes are probably BACKGROUND inadequate to explain the redistribution of dark Callisto, the outermost of the Jovian satellites is material, with electrostatic levitation or other approximately 30% larger in diameter than Earth’s mechanisms further contributing to the mobilization moon. However, its lower mass, surface gravity, and of Callisto’s dark material [12]. Although higher albedo reflect its ice-rock composition. multispectral SSI imaging has not revealed clear Callisto’s surface is thought to consist of a mixture of subunits within Callisto’s cratered terrain, NIMS ice and dust or rocky materials [1,2]. Voyager images observations have allowed for the correlation of CO2 show the surface of Callisto to be heavily cratered deposits within several water-ice-rich craters, and with [3,4], but deficient in large (greater than ~50 km) moderate albedo features near the impact crater Lofn. impact craters [5,6] compared to the Moon and Other candidate materials that may correspond with terrestrial planets. Fresh craters on Callisto and NIMS spectra and show correlations with features Ganymede are 40% as deep as lunar craters of similar imaged by SSI include SO2 and tholins [14]. diameter [7], with central pit and dome craters being the dominant complex crater types for craters between Impact craters on Callisto exhibit a range of 35 and 175 km in diameter [6]. Seven ring structures degradation states, from fresh with clearly defined (multi- and single-ring) with diameters greater than ejecta blankets to craters that are barely 200 km have been identified on Callisto [2]. The two distinguishable. Mass wasting of crater walls appears largest of these, Valhalla and Asgard, are respectively to be a predominant process in this degradation. Lunar and Planetary Science XXIX 1866.pdf GALILEO AT CALLISTO: J. E. Klemaszewski et al. Deposits within craters that appear to be the result of measurements of Callisto during the Galileo mission inward debris slides are prevalent. However, quasi- have revealed that Callisto has undergone some circular debris slides are also evident within the floors differentiation. Schubert [17] proposed two of several craters . Several of these debris flows geophysical models that could explain the gravity originate on the west-facing walls of geologically results: either Callisto has undergone very limited recent craters. The steepening of the crater wall, differentiation that has resulted in a higher fraction of perhaps by sublimation, may result in a reduction of rocky material in its interior (without the development its stability. Collapse may be triggered by seismic of a core); or Callisto has a differentiated Fe-FeS core, energy from a nearby impact, or eventual ice-rich mantle, and rock-rich crust. Although imaging oversteepening of the wall leading to gravitational data are not able to completely distinguish between collapse. Continued sublimation erosion and mass these two models, the lack of features attributable to wasting might result in the preferential sun-facing rim endogenic tectonism or volcanism in SSI images retreat and crater-wall erosion seen in Galileo images, suggests that Callisto has undergone very limited leading to the progressive infilling of craters on differentiation. However, Galileo SSI imaging data do Callisto’s surface. not cover all of the candidate tectonic and volcanic features that were proposed based on low-resolution Moderate resolution (0.4 to 1.1 km/pixel) Galileo Voyager data [e.g. 6]. It is hoped that additional images show more detailed structural features within gravity data obtained during GEM can better constrain three multi-ring systems than seen in Voyager data. the state of Callisto’s interior. Galileo images of the Valhalla system reveal a variety of structural elements. Small sinuous ridges occur within the scarp zone sub-parallel to the outward- REFERENCES facing scarps, and in the trough zone. The spacing [1] Helfenstein, P. et al. (1995) LPSC XXVI 583- between the ridges decreases toward the outer edge of 584. the trough zone. The troughs, also imaged by Voyager [2] Bender, K.C. et al., (1996) Geology of Callisto [6,8], are now seen to be more complex. 1:15 million geologic map, USGS Misc Investigations Circumferential variations (e.g. areal density, width, Series, I-2581. and local orientation) seen in Galileo images may [3] Smith, B.A. et al. (1979) Science 204 951-972 reflect pre-existing near-surface heterogeneities. High- [4] Smith, B.A. et al. (1979) Science 206 927-206 resolution (88 m/pxl) of Asgard’s structural zones have [5] Chapman, C.R. and W.B. McKinnon (1986) in revealed discontinuous sub-parallel fractures within Satellites, p. 492-580. and adjacent to the innermost ring. Extension related [6] Schenk, P.M. (1995) J. Geophys. Res. 100 to the formation of Asgard was probably 19023-19040. accommodated by the rifting of Callisto’s “crust”, [7] Croft, S.K. (1981) LPSC abst. XII, 187-189 resulting in these fractures. The visibility of these [8] McKinnon, W.B. and H.J. Melosh (1980) fractures is enhanced by the presence of small pits Icarus 44 454-471. oriented along the fracture. These pits are probably [9] Wood, C.A. (1981) in Schultz, P.H. and Merrill, the result of near-surface materials falling into the R.B. Multi-Ring Basins p. 173-180. fracture; however their formation by degassing from [10] Carr, M. et al. (1995) J. Geophys. Res. 100 the sub-surface cannot be ruled out [15]. Moderate 18,935-18,955. resolution images of the multi-ring structure Adlinda [11] Chapman, C.R. et al. (1997) LPSC abst. reveal both positive and negative relief features: XXVIII, 217-218. discontinuous arcuate graben, fractures, and ridges. [12] Moore, J.M. et al. (1997) LPSC abst. XXVIII, The detailed morphology of Adlinda is obscured by 971-972. numerous impacts and the ejecta of Lofn, a nearby [13] Asphaug, E. et al. (1997) LPSC abst. XXVIII, younger impact. The highly degraded appearance of 63-64. Adlinda, together with the high spatial density of [14] McCord et al., (1997) EOS/supplement v78 superposed impact craters, leads us to interpret that it n46 p. F407. is the oldest of these three multi-ring systems.