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Lofn Crater, Callisto: a Large Flat-Floored Impact Crater Observed by Galileo

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Lofn Crater, Callisto: a Large Flat-Floored Impact Crater Observed by Galileo Lunar and Planetary Science XXIX 1884.pdf Lofn Crater, Callisto: A Large Flat-Floored Impact Crater Observed by Galileo. S. E. Heiner, J. F. McHone, J. E. Klemaszewski, R. Greeley, K. C. Bender, K. S. Homan, Department of Geology, Box 871404, Arizona State Univ., Tempe, Arizona, 85287-1404, USA. T. B. McCord, C. A. Hibbitts, G. B. Hansen, Hawaii Inst. of Geophys. and Planetology, Univ. of Hawaii, Honolulu, Hawaii, 96822, USA. E-mail: [email protected] Lofn crater is a 125 km diameter circular height. However, it is difficult to separate these structure on the surface of Callisto. It is centered at measurements for Lofn, so 0.6 km can be used as the -56°, 23° W, and imaged at 867 m/pixel by the Galileo maximum possible value for Lofn’s current depth. spacecraft. A bright, flat, circular zone of 100 km This value is shallower than is predicted by the depth- diameter is surrounded by a continuous ring of apparent diameter relationship formulated by Schenk [1]. The ejecta. The ring width ranges from 75 km in the equation (d=0.267*(D^0.358), where d=depth and northwest to 125 km in the southeast. It appears D=diameter) was based on depth and diameter especially youthful in the southeast portion, where it measurements of fresh complex craters on Callisto, and contains massifs 15 to 20 km across. The outermost predicts a depth of about 1.5 km for a crater of Lofn’s zone consists of discontinuous ejecta, which contains size (125 km). both dark material and patches of bright material that In general, craters on Callisto are shallower are concentric to Lofn. Bright rays are also found in than those on Earth’s moon. This is likely due in part the outer zone, and radiate as far as 450 km from the to Callisto’s relatively weak, ice-rich, near-surface crater’s center. Very few impact craters are superposed target materials, which are unable to support steep on Lofn and its ejecta, an indication of the structure’s crater walls [1]. Yet Lofn seems too shallow for this relative youth. to be the sole explanation. Post-impact viscous NIMS data indicate that Lofn’s bright floor, relaxation is possible (e.g. [2]), although the presence and both the continuous and bright discontinuous of two older dome craters nearby (one 60 km diameter ejecta, have strong absorption properties at about 1.48 crater to the northeast of Lofn, the other a possible 120 microns, the spectral region corresponding to water-ice km diameter crater to the southeast) indicates that concentrations. Thus, these areas are icier than their viscous relaxation does not seem to be significant in surroundings. Ices in the central floor and continuous this area [3]. Another possibility is an impact into a ejecta likely represent impact melts. The bright, ice- differentiated body that has a thin crust overlying a rich discontinuous ejecta is more difficult to explain, as more mobile zone, which also would not be able to it lies within the dark discontinuous ejecta. This support steep walls. However, this is unlikely due to pattern could represent the deposition by the ejecta of Callisto’s limited differentiation. different layers during the impact, with the bright Alternatively, the incoming projectile that material coming from an icier layer. formed Lofn may have fragmented before impacting the Two concentric scarp segments, 200 and 175 surface. Comet Shoemaker-Levy 9 demonstrated that km in length, respectively, are located at radii of 200 cosmic bodies can be fragmented by Jupiter’s powerful and 250 km, northwest of Lofn’s center and near the tidal forces. These fragments could form a crater chain, margin of the Adlinda multiring basin. These scarps a multiple crater cluster, or (if the fragments are could have formed by one or more of the following clumped close together, as Asphaug and Benz [4] processes: the incipient formation of a multiring basin, theorized was possible) a single crater on impact. a slightly oblique impact (perhaps responsible also for Simulations of clustered impacts by Schultz and Gault the observed asymmetric ejecta pattern), or weakened [5] showed that ‘open’ clusters of fragments (diameter near-surface materials related to Adlinda. of fragments/diameter of single body of equal mass ~9) Lofn has remarkably low relief. Although its produce a single crater with a shallow, flat floor. outer rim is very difficult to discern, a portion along Clustered impacts of fragmented projectiles were the western edge of the smooth floor is possibly proposed to explain the low-relief topography of the 23 marked by a series of 0.5 to 1 km-sized rubble blocks. km diameter Rochechouart crater in France [6], as well Shadow measurements indicate block heights range as surface ‘splotches’ on Venus [7]. Lofn crater on from 0.25 to 0.77 km, and average about 0.6 km. If Callisto may be another example of such a clustered these blocks are related to the rim, their height projectile impact event. probably represents both the crater depth and the rim Lunar and Planetary Science XXIX 1884.pdf LOFN CRATER, CALLISTO: A LARGE FLAT-FLOORED IMPACT CRATER OBSERVED BY GALILEO. S.E. Heiner, et al. References: [1] Schenk, P., JGR, 96, E1, 15,635-15,664, 1997. [2] Passey, Q.R., and E.M. Shoemaker, in Morrison, D., Satellites of Jupiter: Tucson, University of Arizona Press, 379-434, 1982. [3] Heiner, S.E., et al., Eos, 78, 46, F419, 1997. [4] Asphaug, E., and W. Benz, Nature, 370, 120-124, 1994. [5] Schultz, P.H., and D.E. Gault, JGR, 90, B5, 3701-3732, 1985. [6] Lambert, P., Meteoritics, 17, 4, 240-241, 1982. [7] Phillips, R.J., et al., JGR, 97, E10, 1992..
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