Lunar and Planetary Science XXXVI (2005) 1472.pdf CRATER GRADATION IN GUSEV CRATER, MERIDIANI PLANUM, AND ON THE EARTH. J. A. Grant1, M. P. Golombek2, A. F. C. Haldemann2, L. Crumpler3, R. Li4, and the Athena Science Team 1Center for Earth and Planetary Studies, National Air and Space Museum, Smithsonian Institution, Washington, DC 20560, 2Jet Propulsion Labora- tory, California Institute of Technology, Pasadena, CA 91109, 3New Mexico Museum of Natural History and Science, Albuquerque, NM 87104, 4Department of Civil Engineering and Remote Sensing, The Ohio State University, Colum- bus, OH 43210. Introduction: The Mars Exploration Rovers Spirit diameter Bonneville crater are debris -mantled and slope and Opportunity have examined multiple impact craters an average 11 degrees, but there is little evidence of since landing in Gusev Crater (14.569oS, 175.473oE) and downslope movement (e.g. debris chutes or talus Meridiani Planum (1.946oS, 354.473oE), respectively [1- aprons) and eolian infilling is generally only a few me- 3]. Craters at both locations are in varying states of ters based on observations of protruding rocks. preservation [4] and comparison between their evolved Basaltic ejecta fragments around Bonneville and gradation signatures and those around simple, unglaci- nearby 150 m diameter Missoula crater possess a size ated terrestrial craters provide clues to the processes and spatial distribution consistent with that expected and amount of Martian crater modification. for pristine deposits [2]. Eolian deposits are local and Summary of Terrestrial Crater Gradation: With generally <50 cm thick, whereas exposed surfaces likely few exceptions and even in arid settings, fluvial grada- experienced no more than 60 cm deflation [9]. tion dominates at terrestrial craters and enables defini- Smaller (<20 m in diameter) and generally more tion of a first-order gradation sequence [5-7]. Early modified impact structures referred to as hollows are backwasting via mass-wasting dominates modification distributed across the Gusev plains. These craters are of steep walls to form debris chutes and aprons that mostly sediment-filled and surrounded by abundant then are incised by runoff as more comp etent steep fractured and perched rocks [2]. Trenching with the wall-rock is exposed. Continued gradation leads to coa- rover wheels reveals uniform sediments capped by lescing fans on the crater floor as wall drainages erode dust, but devoid of any detectable dust interbeds. headward and eventually breach the rim and pirate Impact Structures in Meridiani Planum: Craters headward reaches of exterior networks. In this se- explored at Meridiani are fewer and farther between quence, wall slopes decrease from generally above to than at Gusev and all are formed into bedrock. The Me- below the angle of repose and associated drainage den- ridiani craters have depth-to-diameter ratios >0.10, sities decrease from ~13 km/km2 (Meteor Crater), to ~5 thereby suggesting they may be modified primary cra- km/km2 as walls are stripped (Lonar Crater, India), then ters, and they preserve walls sloped generally between increase to ~7 km/km2 (Talemzane Crater, Algeria) as 10 degrees (Eagle crater, 22 m diameter) and 15-30 de- the rim is breached and basin area increases. Rim grees (Endurance crater, 140 m in diameter) that locally breaching is associated with development of 10’s m’s exceed the repose angle in Endurance. Craters are relief that persists after significant eolian deposition variably infilled, but an absence of protruding rocks (Roter Kamm, Namibia). precludes precise constraint of thickness. Profiles By contrast, gradation of ejecta is more subtle, as across Eagle crater reveal smoothly varying slopes surfaces are first modified to form a lag whose incis e- (except over outcrop), whereas profiles across Endur- ment is limited by a higher infiltration capacity and ance generally display an inflection halfway up the lower slopes than on the walls . As a result, drainage walls corresponding to the occurrence of large rocks scale and density is lower, with density averaging ~3-5 and transition to lower slopes immediately above. km/km2 around most craters examined. Except for the 7 m diameter Fram crater, little ejecta Overall effects of these processes are dependent on are definable, though large rock “plates” are seen along crater size and local slopes, but a ~1 km in diameter the flank and near-rim of Endurance crater. None of the crater with a fluvially breached rim likely experienced craters show evidence of incis ement. 10’s of m’s erosion in steep near-rim areas while retain- Martian Crater Gradation: Unlike most terrestrial ing up to 25% of the more distal continuous ejecta. craters, the Martian craters lack evidence for apprecia- Impact Structures in Gusev Crater: Craters and ble modification by water, thereby enabling processes their associated ejecta deposits dominate the surficial that are typically subordinate on Earth to dominate. As landscape on the Gusev Plains [2]. The craters have on Earth, gradation on Mars is highly slope and scale depth-to-diameter ratios generally <0.10 and many may dependent and may be predictable. For example, craters be secondary craters [8]. Most possess raised rims and >100 m in diameter at Gusev likely experienced variable, obvious ejecta deposits. Walls bounding the 210 m-in- but limited (meters) of infilling and backwasting of walls Lunar and Planetary Science XXXVI (2005) 1472.pdf by eolian and mass-wasting activity (Fig. 1), whereas with fines swept in from the surrounding plains con- ejecta are little modified by mostly eolian processes tributes to crater infilling. Lower portions of crater walls that account for ~1 m gradation. By contrast, similar experience lesser backwasting due to a combination of amounts of gradation of the hollows produce more reduced exposure to winds, protection by remnant ta- modified forms . lus, and eolian deposition (Fig. 1). If Eagle and Endur- Impact excavation of the craters and hollows and ance are primary craters, these processes collectively emplacement of associated ejecta disrupted a surface in account for ~30-40% infilling/backwasting, or some equilibrium with the gradational environment, thereby combination of up to 10 m infilling/25 m backwasting resulting in deflation of ejecta to expose perched rocks and 1.5 m infilling/3.5 m backwasting at Endurance and and infilling (nearly complete for many hollows) as more degraded Eagle crater, respectively. sediment is redistributed by the wind. The absence of The predicted amounts and processes or crater gra- dust interbeds in the fill suggests that early gradational dation at Gusev and Meridiani is consistent with in- activity inside and around the craters in Gusev was ferred erosion rates at both sites since the Hesperian initially more important as newly exposed surfaces [10]. It is unclear, however, how representative the cra- equilibrated with the geologic setting (Fig. 1). ters investigated at Meridiani to date are of regional Preserved morphology of craters at Meridiani indi- gradation. For example, orbital data reveal craters to the cates most experienced relatively greater gradation than south that may be mantled and/or partially exhumed. at Gusev. Gradation is accomplished as less competent Hence, exploration of these craters may lead to modifi- bedrock along crater walls is subject to more eolian cation of the outlined gradational sequence. stripping (especially along the upper wall) that together References: [1] Squyres, S. et al. (2004) Science, 305, with mass-wasting causes backwasting, crater enlarge- 794-799, 2004. [2] Grant J. et al. (2004) Science, 305, 807- ment, and some infilling that proceeds without devel- 810, 2004. [3] Squyres, S. et al. (2004) Science, 306, 1698- opment of debris chutes characteristic of mass wasting 1703. [4] Haldemann, A.F.C et al. (2004) 7th Mars Crater on Earth (Fig. 1). At Endurance, backwasting of the wall Consortium, Flagstaff, AZ. [5] Grant, J.A. (1999), Int. J. is slowed as large rock “plates” along the rim are un- Impact Engin., 23, 331-340. [6] Grant, J.A. et al. (1997) J. dercut and slide into the crater, thereby helping to ar- Geophys. Res., 102, 16,327-16,388. [7] Grant, J.A. and mor the wall in much the same manner that a lag slows Schultz, P.H. (1993) J. Geophys. Res., 98, 11,025-11,042. [8] erosion of terrestrial crater ejecta. At Eagle crater, more Hurst M. et al. (2004) LPS XXXV, Abs. #2068. [9] Greeley, advanced gradation creates a fairly stable profile, with R., et al. (2004) Science, 305, 810-821. [10] Golombek, M. et active backwasting largely limited to areas with ex- al. (2005) LPS XXXVI, Abs. (this volume). posed outcrop. Some of the mobilized sediment along Figure 1. Summary of relative importance of gradational processes in and around craters explored at Gusev crater and Meridiani Planum. At Meridiani, the relative importance of eolian deflation and mass-wasting along crater walls may vary from crater to crater and over time. The scale and slope dependent nature of gradation and resultant signatures necessitates the qualitative summary shown (see text for specific, quantitative examples). .