Water Or Ice in the Martian Regolith?: Clues from Rampart Craters Seen at Very High Resolution R PETER J
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ICARUS 71, 268-286 (1987) Water or Ice in the Martian Regolith?: Clues from Rampart Craters Seen at Very High Resolution r PETER J. MOUGINIS-MARK Hawaii Institute of Geophysics, University of Hawaii, Honolulu , Hawaii 96822 Received October 23, 1986 ; revised March 3, 1987 Very high resolution Viking Orbiter images (8-17 m per pixel) have been used to investigate the morphology of Martian rampart crater ejecta blankets and the crater interiors, with the objective of identifying the fluidizing medium for the ejecta and the physical properties of the target rock. The occurrence of well-preserved, small-scale pressure ridges and scour marks, evidence for subsidence around isolated buried blocks in partially eroded ejecta lobes, and the stability of crater walls and distal ramparts argue for ground ice being the dominant state for volatiles within the target rocks at the time of impact. Rare examples of channels (190-650 m wide) on the surfaces of ejecta blankets, and on the inner walls of the crater Cerulli, indicate that in some instances liquid water was incorporated into the ejecta during its emplacement. No morphological evidence has been found to discount the idea that atmospheric effects were partially responsible for ejecta fluidization, but it is clear that these effects were not the sole reason for the characteristic lobate deposits surrounding at least some rampart craters on Mars. © 1987 Academic Press, Inc. INTRODUCTION laboratory experiments (Schultz and Gault 1981, 1984) have shown that rampart From the return of the first Viking bordered ejecta facies and ejecta flow lobes Orbiter images of Martian impact craters it can develop without the presence of water. was evident that the morphology of the Laboratory studies have also demonstrated ejecta blankets surrounding these craters that targets of varying viscosities can be was different from that of their lunar and used to investigate the fluidization of Mar Mercurian counterparts insofar as much of tian rampart craters (Gault and Greeley the ejecta appeared to have been emplaced 1978). Decreasing the viscosity of mud tar by surface flow (Carr et al. 1977). Over the gets promoted postdepositional flow of the last 10 years, considerable debate has ejecta and increased the radial extent of the focused on the possible cause(s) for this "continuous" ejecta deposit. In several ejecta fluidization and flow. Possible forms instances, these laboratory craters were that the fluidizing medium may have taken also seen to possess multiple flow units include water ice or liquid water within the similar in kind to the multilobed craters on target material (Carr et al. 1977, Boyce Mars (Carr et al. 1977). 1979, Johansen 1979) or atmospheric gases A groundwater system has been pro interacting with suitable particle sizes posed for Mars on a global scale (Carr 1979, within the ejecta curtain (Carr et al. 1977, Clifford 1986), and it is believed likely that Schultz and Gault 1979, Schultz 1986). such a system would influence the Indeed, Schultz (1986) draws attention to occurrence and physical state of impact the fact that the fluidized ejecta facies only crater ejecta. A major difficulty in correlat indicate fluid-like emplacement, and ing these inferences about ground volatile 268 0019-1035/87$3.00 Copyright © 1987 by Academic Press, In c. All rights of reproduction in any form reserved. MARTIAN CRATERS 269 content with the observed properties of cation (or lack thereof) of meltwater chan crater ejecta blankets lies in knowing the nels and small-scale topography on the extent to which the groundwater system surface and edges of the ejecta blankets and (located within the deep megaregolith) was the degree of stability of the ejecta lobes filled. Many of the large outflow channels and parent crater walls (Mouginis-Mark seen on Mars may have formed through the 1986). Diagnostic features are likely to be eruption of groundwater under pressure physically small in size (probably less than from the megaregolith beneath the per a kilometer in extent) and so have not mafrost, which is estimated to have been previously been considered in either global about 1 km thick (Carr 1979). Carr (1986) studies of rampart craters (e.g., Johansen concluded that the megaregolith below this 1979, Horner and Greeley 1986) or permafrost layer, planetwide, might con cratering models (e.g., Schultz and Gault tain no less than the equivalent of a water 1984, Schultz 1986). This paper therefore layer 350 m thick. Clifford's (1986) cal describes morphologic features seen in culations of outgassed H 20 on Mars would very high resolution (8 to 17 m per pixel) suggest that if more than a few percent of Viking Orbiter images of crater ejecta blan the quantity of water required to saturate kets and interiors in order to constrain the the pore volume of the cryosphere were most likely target properties at the ti me of present, then a subpermafrost water system crater formation. of substantial proportions would result. After saturation of the pore volume of the PREVIOUS GLOBAL OBSERVATIONS cryosphere, the equivalent of an additional Johansen (1979) and Kargel (1986) have 100 m of H 20 would be sufficient to create suggested that the presence or absence of a an aquifer nearly 4.3 km deep. That this distal ridge on an ejecta flow may be an small volume of water is all that is required indicator of water (ridge present) or ice to produce the aquifer is due to the pre (ridge absent) within the target material. dicted low pore volume at depth (Clifford Although there is a predominance of ridged 1986). It is further predicted that much of craters at low latitudes and ridgeless craters the water originally at depths of less than 1 at high latitudes (Kargel 1986), no corrobo km would remain in situ for most of Mar rating models for ejecta emplacement, or tian history (Fanale et al. 1986), thus con theoretical or experimental data, were stituting a possible medium for fluidizing presented by these investigators to support impact crater ejecta. their ideas on ejecta fluidization. On the Because of the importance that the basis of finite-element studies of the stress presence of liquid water, liquid brines, or magnitudes, distributions, and directions in ground ice close to the surface would have hypothetical Martian ejecta blankets, for global models of volatile abundance and Woronow (1981) concluded that rampart physical state (e.g., Fanale and Jakosky craters had water contents between 16 and 1982, Fanale et al. 1986), more precise 72 volume percent soon after emplacement. observational information is needed to help However, Woronow hypothesized that the constrain these theoretical models of fluidized ejecta deposits were formed by volatile distribution and state. One the mechanical failure and subsequent approach to identifying both the former radial flow of lunarlike ejecta deposits; such presence (or absence) and physical state of a mechanism is not believed to be ap subsurface volatiles in the Martian past propriate for describing the ejecta comes from their inferred influence in con emplacement process for these Martian trolling the morphology of rampart craters craters (Gault and Greeley 1978, Schultz and crater ejecta blankets. Key testable and Singer 1980, Mouginis-Mark 1981, morphologic indicators are the identifi- Wohletz and Sheridan 1983). 270 PETER J. MOUGINIS-MARK At issue is the relative importance of of atmospheric and target properties. atmospheric effects and target volatiles in However, Wohletz and Sheridan (1983) controlling the morphology of ejecta sur noted that since the formation of distal rounding Martian impact craters. Schultz rampart ridges around Martian craters is and Gault (1979) demonstrated quantita affected by local preexisting topographic tively that, during the emplacement of an obstacles, the formation of the ramparts ejecta cloud, ejecta deposition would be probably resulted from the deposition of controlled by the particle size distribution ejecta as the yield strength of the ejecta of the clasts and that, as a result of the size flow increased above a critical shear stress distribution of particles, a multiphase due to interparticle friction. This suggests emplacement sequence would probably that the morphology of the ejecta blanket result. Subsequent laboratory experiments was strongly related to the initial yield by Schultz and Gault (1981, 1984) have strength of the flow and, hence, the degree shown that under conditions scaled to of ejecta fluidization. simulate the Martian environment, craters A method for assessing the "average" with distal ridges can be produced without rheology, or degree of fluidization, of the any volatiles being present in the target. ejecta flows for crater populations found on These experiments suggest that craters uniform target materials has been the exam with contiguous ramparts may represent ination of the mechanical strength of the the products of relatively low-velocity ejecta lobes, based on their topographic impacts into target lithologies pqssessing relief and areal extent. Wohletz and Sheri lower volatile contents than the targets dan (1983) observed that the ejecta deposits associated with craters possessing highly appear to be composed of unconsolidated, fluidized ejecta facies. Increased volatile fine-grained particles that can be trans contents are predicted to decrease ejecta ported by Martian winds, and that the size by comminution and by aerodynamic thickness of the ejecta material is to breakup, thereby increasing the lateral pographically controlled. Ejecta lobe thick extent of the rampart. Ifthe impact velocity ness appears to be limited by some physical or volatile content were sufficiently high, aspect of the deposit, inferred to be the the energy trapped in the ejecta cloud maximum shear strength of the fluidized would result in long run-out flows or radial medium (Mutch and Woronow 1980). scouring. Thus, Schultz and Gault (1981 , Based on observations of ejecta flow 1984) concluded that volatiles within the around low topographic obstacles (Carr et Martian regolith were sufficient, but not al.