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Title: The Morphology and Mode of Formation of Martian Rampart Craters.
Author: Mougins-Mark, P. J.
ISNI:       0000 0001 3427 8930
Awarding Body: University of Lancaster
Current Institution: Lancaster University
Date of Award: 1977
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The Viking Missions to Mars revealed many features that had hitherto escaped identification either due to insufficient camera resolution or due to the poor viewing conditions induced by atmospheric dust during the Mariner 9 mission in 1971 (Carr et al., 1976). Landforms exhibiting obvious similarities with features on the Moon and Mercury were seen alongside totally new types of terrain and geological features. In particular, Carr et al. (1977) drew attention to the unique character of a certain type of crater morphology which does not exist anywhe,re else in the known solar system. They refered to this crater type as a "rampart" or "pedestal" crater, since the crater appears to be sitting on top of a deposit which raises the crater above the pre-existing topography. The areally extensive deposits that surround these rampart craters appears significantly different from the morphology of lunar craters with wide rim deposits (e.g. Cassini), since the martian deposits extend several times further from the crater's rim crest than for the lunar examples. Lunar craters have been found to show no direct comparisons with superficially similar features on Mars. This is attributed to their different modes of formation; namely, the lunar examples are produced by the burial of the discontinuous ejecta blanket of a morphologically .l" mature crater by mare lavas to give the appearance of an unusually wide crater wall. Close inspection of the ejecta blankets of rampart craters, however, reveals that they consist not of the normal blocky ejecta which characterises impact craters on Mercury and the Moon, but rather the material appears to have flowed radially away from the crater as a high density ground surge, similar to a pyroclastic flow or viscous mudflow. Few papers have yet been published that describe the morphology of this new catagory of martian crater, and any attempts to explain their mode of formation have little theoretical backing to support them. A re-examination of pre-Viking literature demonstrates that this crater type had already been identified from Mariner 9 photography (Head and Roth, 1976), but had generally been incorrectly attributed to wind erosion scouring typical blocky ejecta blankets (McCauley, 1973; Arvidson, 1976; Mutch et al., 1976A). The lack of sufficient time since the collection of' the data from Viking has also meant that the vast amount of new information from the two Viking Orbiters has only been superficially scanned for the most interesting features. As a result of this, previous work on a limited data base has proved inadequate in the presence of the high quality imagery from Viking. Chapter One therefore presents a comprehensive (but by no means exhaustive!) morphological study of these craters in order to describe this new landform adequately. A total of 63 craters illustrating a variety of new surface features enable the "rampart" or "pedestal" craters to be classed as a separate group of planetary crater. The diameters of the rampart craters included within the sample presented here range from 1.3 to 37.5 km. Various morphological parameters were measured from Viking imagery covering the photographs taken from Rev. 1 to Rev. 210 of Orbiter One and from Rev. 1 to Rev. 160 of Orbiter 2. A classification scheme for rampart craters is adapted from that of Head and Roth (1976) and this is used in subsequent work .l" on crater size distribution and central peak formation. The maximum range and surface area of the lobate flows surrounding the primary crater, with respect to the primary's diameter, are also investigated. A comparison'is also made from the photographs between the circularity of these craters, the circularity of craters of presumed impact origin on the Moon, 'and terrestrial volcanic craters~ Based on the data presented here, it is concluded that the sample of rampart craters is one primarily, if not exclusively, of craters formed by impacting meteorites. Ejecta from there impacts appears to have undergone some form of mobilisation at an as yet unspecified stage in the evolution of the crater. Chapter Two provides a close investigation of several terrestrial volcanoes in order to support the suggestion that the rampart craters on Mars are of impact, rather than volcanic, origin. Further consideration is given to the hypothesis that rampart craters are of impact origin in Chapter Three, in which an interplanetary comparison between the ejecta blankets of impact craters of various sizes on the Moon, Earth and Mercury is made. The material presented at this stage is used to support further the idea that martian rampart craters are of impact origin, but also draws attention to the different types of crater morphology which' are produced under different planetary environments. Chapter Four then returns exclusively to the martian rampart craters. Several aspects of the craters' morphology are presented alongside possible terrestrial analogues in an attempt to first identify plausible modes of formation and then describe possible processes by which specific lobate layers around the outer walls of rampart may have been formed. The final two chapters explore the possible formation mechanism of rampart craters from a theoretical point of view. Using a series of specially developed computer models, an attempt is made to show that, for a range of likely parameters, a layer of permafrost within the target material (the existence of which is considered likely by Sharp, 1973; Carr et al., 1976; Mutch et al., 1976B) would first be boiled off from an expanding ejecta cloud during the formation of the crater cavity. The dry ejecta would then be emplaced in a dehydrated state upon the surrounding terrain. Blocks within the ejecta are predicted to be sufficiently hot to liberate large quantities of melt water from the underlying layers of permafrost. It is concluded that, with only minor variations of this model, virtually the entire 'range of landforms and surface features associated with the martian rampart craters can be explained adequately by selective fluidisation of the ejecta by this melt water and that radial movement of this material in a form similar to a lahar has produced the characteristic lobate flows around this type of martian crater.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available