MARTIAN LARGE CRATER AND BASIN DEPOSITS: IMPLICATIONS FOR THE THICKNESS OF A SUBSURFACE VOLATILE LAYER AND SITE GEOLOGY AT VIKING LANDER 2. P.J.Mouginis-Mark Dept. Geological Sciences, Brown University, Providence RI 02912; and B.R.Hawke Hawaii Institute Geophysics, Honolulu Hawaii 96822.

Introduction: Recent analyses of martian impact craters and basins larger than 50 km diameter have emphasized the identification and description of the ejecta materials, together with an issessment of the emplacement sequence for these deposits (1,2,3) . The objectives of such studies were to investigate the role that target volatiles may have played in the fluidization of large ejecta deposits, given the previously identified ground-flow morphology of the materials surrounding craters smal ler than 30 km (4). Theoretical models (5) and crater morphometry (6) have suggested that the thickness of this volatile layer may be of limited extent - the total volume possibly equiv- alent to a layer about 100 meters thick (7). Boyce and Witbeck (8) have identified a minimum crater diameter (2-4 km) before ejecta deposits i1 lus- trate the characteristic fluidized morphology, implying that at the time of crater formation the near-surface layers of the target were rel-atively volatile poor. Similarly, for craters larger than about 50 km, depth/diameter values for fresh martian craters (6) suggest that target volatiles may have been absent at depth, and so were not able to control the final crater mor- phology. This investigation addresses this inferred finite thickness of the target volatile layer from the interpretation of craters in the 50-200 km diameter range, and utilizes the morphology of fresh craters of this size to consider the possible influence that the crater Mie (105 km diameter) may have had on the site geology at the Viking Lander 2 site in Utopia Planitia.

Observations: A variety of ball istic and ground-fl ow ejecta deposits have been recognized around the fresh martian craters Bamburg (55 km; 30W, 40'~) , Curie (120 km; OW, 280N), Lowell (190 km; 81°W, 52's) and Lyot (200 km; 330°w, 50°N) (refs. 1,2,9). A1 though different in detailed morphology, each of these craters possess : 1) A secondary crater field of ballistic origin; 2) Extensive lobate deposits emplaced by a ground-flow mechanism; and 3) Mass- flow materials close to the crater rim that were probably produced bj the failure and subsequent flow of volatile-rich ejecta originally emplaced on the crater rim (1,9). The recognition of partially buried secondary craters at Bamburg (1) and Lyot (2) imp1 ies that these craters were produced prior to the arrival of the ground-flow deposits, and that fragments of sufficient size to create 5 knl secondary craters were excavated from the cavity together with the more fluid ground-flow materials. Maximum travel distances for the lobate ejecta deposits (normalized to the diameter of the parent crater) are found to be in approximate inverse relationship to the size of the parent crater. At Bamburg, fluidized ejecta extends 2.1 D from the crater center (where D .is the crater diameter), at Curie to 1.9 D, and at Lyot to 1.4 D. Mass-flow materials, a1 though present at all the craters investigated, are most extensive around Curie, where additional evidence for post-emplacement ejecta flow is seen in the abnormally wide set of terraces within the crater (9). Aspects of the ejecta morphology indicate that the fluidization of this material was primarily the product of target properties, rather than the proposed atmospheric effects (10) . Channels on the surface of the ejecta deposits around Bamburg and Cerulli (110 km; 338O~, 32'~) indicate that the ejecta was volatile-rich at the time of deposition (1). In addition, a lobate flow south of Curie appears to have

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been created by the slumping of the mass-flow materials after their initial emplacement (9), indicating that ejecta fluidization could be achieved on Mars even with low-velocity materials that fa1 1 (at most) a few hundred meters,

Imp1 ications for volatile layer thickness : A1 though the depth of excavation for martain cratering events is unknown, certain inferences can be drawn about the volatile layer from the observed depths of fresh martian craters and the characteristics of their ejecta materials. Fresh craters in the size range 50 - 200 km are predicted to be 2.3 - 4.9 km deep from morphometric measurements (6). In addition, each transient cavity probably excavated to a considerably greater depth (10 - 20 km ?), indicating that target materials from depths of several kilometers were probably incorporated within the ejecta. The observed occurrence of lobate ejecta flows around 200 km diameter craters (e.g. Lyot and Lowell) demonstrates that even for small basins on Mars a sufficient amount of volatile-rich material was excavated to fluidize the ejecta. The decrease in the normalized ejecta range, and the increasing frequency of secondary craters, probably indicate the contamination of the near-surface volatile-rich material with ejecta from strata sufficiently deep to be depleted in volatiles for these larger impact events. Site geology at Viking Lander 2: Fig.1 illustrates the inferred position of the Viking Lander 2 spacecraft (VL-2), which lies approximately 190 km to the west of the crater Mie (105 km; 220°W, 48ON). Morphologically, Mie is a large central-peaked crater with terraced walls and subdued interior deposits suggestive of an eolian mantle. Although poorly visible on the available Viking images, several ejecta deposits comparable to those described in this analysis for other craters can also be identified around Mie (11). The precise role that Mie may have played in controlling the local surface deposits at VL-2 is, however, poorly understood. Analyses of the boulder field observed from the lander show that a greater diversity of boulder morphologies (11, 12) and colors (13) exist at the Utopia Planitia site than have been observed at Viking Lander 1 in Chryse Planitia (13, 14). Such a range of boulder types may be attributable to the influence of Mie, which is more than an order of magnitude larger than any crater close to VL-1. From our investigations of other large craters on Mars, we would predict that the terrain surrounding Mie would have experienced secondary cratering and the emplacement of lobate ground-flows. Indeed, such fluidized deposits have been tentatively recognized close to the lander (11). If such an interpretation is val id, the variety of the VL-2 boulder shapes (12) may be a consequence of multiple modes of emplacement for materials derived from depths ranging from near-surface to more than 5 km. In collaboration with a detailed investigation of the VL-2 boulder field .(12), we are consequently pursuing the interpret- ation of the Utopia site as a further constraint to our analysis of large martian craters. References: 1) Mouginis-Mark P.J. (1979) PLPSC loth, 2651-2668. 2) Hawke B.R. and Mouginis-Mark P.J. (1981) NASA-TM 82385, 152-154. 3) Mouginis-Mark P.J. et al. (1980) Proc.Conf.Multi-ring Basins, submitted. 4) Carr M.H. et a1 . n977) J.Geophys .Res. 82, 4055-4065. 5) Fanale F.P. (1976) Icarus 28 179-202. 7) Judson S. and Kssbacher L.A. (1980) Proc.3rd Conf. Water plan. Regoliths, in press. 6) Cintala M.J. and Mouginis-Mark P.J. (1980) Geophys. Res. Lttrs. -7, 329-332. 8) Boyce J.M. and Witbeck N.E. (1981) NASA-TM 82385-9

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Fig.1: The location of the Viking Lander 2 site ("+" in the center of the two landing ellipses) is within two crater diameters of the 105 km crater Mie (top right of image). Scaled to the craters Bamburg ("B") and Curie ("C") this places the landing site within the region where both ballistic and ground-flow ejecta emplacement are expected. Scale bar is 100 km. Part of JPL photomosaic P-17676.

140-143. 9) Mouginis-Mark P.J. and Head J,W, (1979) LPS X, 870-872. 10) Schultz P.H. and Gault D.E. (1979) J.Geophys.Res. 84,-7669-7687. 11) Mutch T.A. --et al. (1977) J.Geophys.Res. -82, 4452-4467. 12) Garvin J.B. --et al. (1981) The Moon and Planets, in press. 13) Strickland E.L. (1979) PLPSC loth, 3055-3077. 14) Binder A.B. --et al. (1977) J.Geophys.Res. -82, 4439- 4m

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