Award Date

12-2010

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Geoscience

Department

Geoscience

First Committee Member

Eugene Smith, Chair

Second Committee Member

Elisabeth Hausrath

Third Committee Member

Rosaly Lopes

Fourth Committee Member

Adam Simon

Graduate Faculty Representative

Stephen Lepp

Number of Pages

159

Abstract

Crater and ejecta morphology provide insight into the composition and structure of the target material. Martian rampart craters, with their unusual single-layered (SLE), double-layered (DLE), and multi-layered ejecta (MLE), are the subject of particular interest among planetary geologists because these morphologies are thought to result from the presence of water in the target. Also of interest are radial lines extending from the crater rim to the distal rampart of DLE craters. Exactly how these layered ejecta morphologies and radial lines form is not known, but they are generally thought to result from interaction of the ejecta with the atmosphere, subsurface volatiles, or some combination of both.


Using the shock tube at the University of Munich, this dissertation tests the hypothesis that the decompression of a rock-water mixture across the vaporization curve for water during the excavation stage of impact cratering results in an increased proportion of fines in the ejecta. This increase in fine material causes the ejecta to flow with little or no liquid water. Also tested are the effects of water on rock fragmentation during shock decompression when the vaporization curve for water is not crossed.


Using results from these experiments, a hybrid model is proposed for the formation of fluidized ejecta and suggests that the existing atmospheric and subsurface volatile models are end members of a mechanism resulting in ejecta fluidization. Fluidized ejecta can be emplaced through interaction with an atmosphere (atmospheric model) or through addition of liquid water into the ejecta through shock melting of subsurface ice (subsurface volatile model). This dissertation proposes that these models are end members that explain the formation of fluidized ejecta on Mars.


When the vaporization curve for water is crossed, the expanding water vapor increases the fragmentation of the ejecta as measured by a significant reduction in the median grain size of ejecta. Reducing the average particle size in the ejecta curtain reduces the height above the ground at which the advancing curtain becomes permeable to the atmosphere it is compressing. This allows a vortex ring to form behind the curtain and deposit fine ejecta in a fluidized fashion. When the vaporization curve for water is not crossed, water within open pore space increases the fragmentation threshold of rocks, shifting the median grain size to larger sizes. If the amount of water within open pore space is sufficiently large and the vaporization curve is not crossed, the ejecta may contain very large blocks. In the model proposed in this dissertation, the inner layer of DLE forms when there are very large blocks at the base of the ejecta curtain and much finer particles toward the top. In this situation, the larger blocks fall out first and produce the inner ejecta layer. A ring vortex is still formed where the ejecta curtain becomes permeable to the atmosphere. This vortex deposits finer grained material behind the advancing ballistic ejecta and deposits the outer layer. At discrete locations within the ejecta curtain, some of the larger blocks extend outside the average curtain width. At these points Raleigh-Taylor or Kelvin-Helmholtz instabilities (Chandrasekhar, 1981; Boyce et al., 2010) form, punching holes in the curtain and forming scouring jets below the ring vortex. These jets carve out the radial lines in the inner and outer ejecta blanket.

Keywords

Fluidized ejecta; Impact crater; Mars (Planet); Martian craters; Planets – Crust; Rampart Crater; Shock tubes

Disciplines

Geology | Other Earth Sciences | Physical Processes

Language

English


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