nCARUS 56, 15-37 (1983) Martian Rampart Crater Ejecta: Experiments and Analysis of Melt-Water Interaction ~ K. H. WOHLETZ* AND M. F. SHERIDANt *Earth and Space Science Division, Los Alamos National Lahoratot3', Los Alamos, New Mexico 87545 and ~fDepartment of Geology, Arizona State University, Tempe, Arizona 85276 Received October 18, 1982: revised May 31. 1983 Viking images of Martian craters with rampart-bordered ejecta deposits reveal distinct impact ejecta morphology when compared to that associated with similar-sized craters on the Moon and Mercury. Topographic control of distribution, lobate and terraced margins, cross-cutting relation- ships, and multiple stratigraphic units are evidence for ejccta emplacement by surface flowage. It is suggested that target water explosively vaporized during impact alters initial ballistic trajectories of ejecta and produces surging flow emplacement. The dispersal of particulates during a series of controlled steam explosions generated by interaction of a thermite melt with water has been experimentally modeled. Preliminary results indicate that the mass ratio of water to melt and confining pressure control the degree of melt fragmentation (ejecta particle size) and the energy and mode of melt-ejecta dispersal. Study of terrestrial, lobate, volcanic ejecta produced by steam-blast explosions reveals that particle size and vapor to clast volume ratio are primary parameters charac- terizing the emplacement mechanism and deposit morphology. Martian crater ramparts are formed when ejecta surges lose fluidizing vapors and transported particles are deposited cn masse. This deposition results from flow yield strength increasing above shear stress duc to interparticlc fric- tion. INTRODUCTION Carr and Schaber, 1977). Therefore, water is an important factor for consideration Studies by Head and Roth (1976) and when comparing the lobate ejecta patterns Cart et al. (1977) of Viking imagery point of Mars to the simple ballistic patterns on out that ejecta deposits surrounding large, the Moon and Mercury, where the environ- Martian impact craters, up to 50 km in ment has been essentially water free (Cin- diameter, are different from those around tala and Mouginis-Mark, 1980). To better similar-sized craters on the Moon and Mer- understand the relationship of target water cury. The Martian craters are surrounded to ejecta morphology, Johansen (1979), by lobate or multilobate ejecta deposits and Mouginis-Mark (1979a), and Blasius and have been termed "spiosh," "fluidized," Cutts (1980) attempt to correlate morphol- "rampart," or "pedestal" craters (Match ogy variation with latitude, elevation, and and Worronow, 1980; Mouginis-Mark, target lithology. However, the interpreta- 1979a). Their deposits appear to have been tion of morphology statistics has been lim- emplaced by ejecta flow over the surface ited by a lack of both information concern- away from the crater. The presence of ing distribution of water on the surface of abundant surface or near-surface water Mars as well as a generally accepted model early in Martian history is indicated by con- explaining formation of lobate ejecta de- siderable evidence (Toulmin et al., 1977; posits. A few recent papers relate water content t This work was performed under the auspices of the of target material to crater formation, dis- U.S. Department of Energy. The U.S. Government's right to retain a nonexclusive royalty-free license in persal of products, and types of deposits. and to the copyright covering this paper, for govern- Gault and Greeley (1978) and Greeley et al. mental purposes, is acknowledged. (1980) impacted targets of varying water 15 001% 1035/83 $3.00 Copyright ~ 1983 by Academic Press, Inc. All right~ of reproduction in any form reserved. 16 WOHLETZ AND SHERIDAN content using the NASA-Ames Research volcanoes where magma interacts with Center vertical gun. Preliminary results in- near-surface water (Colgate and Sigurgeirs- dicate a strong effect on crater and ejecta son, 1973; Peckover et al., 1973; Wohlctz, deposit morphology by yield strength and 1980; Sheridan and Wohletz, 1981). Numer- viscosity, which are dependent on water ous papers describe the phcnomenology content. Kieffer and Simonds (1980), in and products of such hydromagmatic erup- their review of the literature on terrestrial tions as reviewed by Wohletz and Sheridan impact structures, have shown that al- (1983). Usually flash vaporation leads to though impact melt is more readily formed strong fragmentation of the melt; dispersal in lithologies rich in volatiles, melts associ- of ejecta from the explosion center is by ated with such targets are widely dispersed ground-hugging, partly fluidized density from their craters by rapid volatile expan- currents termed surges (Moore, 1966: Wa- sion. Hence dry terrestrial targets contain ters and Fisher, 1971; Wohletz and Sheri- thick melt sheets, whereas wet targets lack dan, 1979). melt sheets but are surrounded by suevite Also of importance to ejecta emplace- blankets. ment is the affect of shock waves. The af- This paper describes possible effects that fect of shock waves on the atmospheric explosive water vaporization might have on flow field produced by the projectile bow ejecta emplacement after impact into a wet shock, vapor explosion, or other mecha- target. A general model (Fig. !) is formu- nism is complex and beyond the scope of lated from analysis of Viking imagery and this paper (see Jones and Kodis, 1982: experimental vapor explosions as well as Schultz and Gault, 1982). It is worthwhile consideration of fluidized particulate trans- to note. however, that the experimental port and lobate volcanic deposits. The work to bc discussed indicates that shock model contends that as target water content phenomena trigger and enhance vapor ex- increases, the effects of vapor expansion plosion processes. due to impact increasingly modify thc bal- listic flow field during crater excavation. VIKING IMAGERY DESCRIPTION This modification results in transport by Observations of Martian rampart crater gravity driven surface flowage, and is simi- deposits based on interpretation of Viking lar to that of atmospheric drag effects on imagery tbrm the principal constraints on ejecta modeled by Schuitz and Gault our models of ejecta emplacement. Twelve (1979). Martian rampart craters were studied to Critical to this problem is the mechanism quantitatively characterize their deposit by which water and impact melt become morphology. Maps of three of the craters intimately intermixed so that efficient en- (Figs. 2-4) are presented to show represen- ergy transfer is possible. We consider sev- tative deposit characteristics, aereal extent eral other situations of melt-water interac- of depositional units, and spatial relation- tion, albeit quite different physically. ships of rampart lobes. The following map however, instructive in the dynamics of va- units are based in part on interpretations of por explosions. The dynamic interaction of Mouginis-Mark (1978, 1979b). molten materials and water has been stud- Wall material (14/). This material com- ied by numerous workers concerned with prises deposits that form discontinuous, vapor explosions at foundaries (Lipsctt, concentric scarp ridges parallel to the cir- 1966). The literature on such industrial cumferencc rim on the inner slopes of cra- melt-coolant explosions is summarized by ters. Some craters have multiple rings of Sandia Laboratories (1975). Another wall material that form concentric tilted ter- source of data comes from natural steam races. The inward decrease in elevation of explosions which are common in terrestrial multiple ridges suggests an origin by col- MARTIAN RAMPART CRATER EJECTA 17 INTERACTION OF IMPACT / MELT AND WATER Hz0 ~ H~0 $H~ 0/MELT RATIO ,CONFINEMENT $MIXING AND FRAGMENTATION BALLISTIC EJECTION FOLLOWED BY VAPOR EXPLOSION *VAPOR TO PAIITICI,E ,,,--7e--7--..,z.~ RATIO oVAPOB ENTROPY f-.~,, k'~k~.~:U-,.~/. BALLISTIC (SUPERllEATING) 3. ~(. t' " "-~/.~ff CURTAIN INITIAL SURGE FOLLOWED BY REPEATED VAPOR EXPLOSIONS OF DECREASING ENERGY • AERODYNAMIC EFFEC'I ,'S -~'~ VISCOUS-- / INERTIAL-- UPON BAI,I.ISTIC MATFBIAL % .. • . ",- .'."" ..- : tRANSPORT-tRANSItION 4. FROM VAPOR-SUPPORTED VISCOUS TO INERTIAL FLOW eE]FCTA SIZE oVOID SPACE C R H La L~ *"WETNESS" 5. MULTI-LOBED EJECTA FLOW DEPOSIT Fn(;. I. Schematic model of rampart ejecta emplacement. lapse of slump blocks. Such terraces are Hummocky material (H). The outer common in large impact craters regardless slopes of rampart crater rims are character- of whether the target was dry, such as the ized by hummocky ridged material that ex- Moon or Mercury, or volatile rich, as ap- tends out to the surrounding plain. This ma- pears to be the case for many Martian cra- terial resembles short and stubby stacked ters. We suggest that in a volatile-rich envi- platcs or "shingles" of ejecta layers that ronment these features could also be may be related to an overturned flap of rim explained by rapid, local withdrawal of sup- materials. Multiple layers are likely the port related to vapor explosions in the cra- result of excavation through a crustal stratig- ter during and shortly after initial excava- raphy composed of several different litbo- tion. Terrestrial analogs of such features logic units. are common within maar craters produced Lobate ejecta (L). These relatively thin by hydrovolcanic steam-blast explosions ejecta sheets extend beyond the hummocky (Lorenz, 1970). material to the limits of continuous ejecta. 18 WOHLETZ