5:15 PM - 6:45 PM
[PPS07-P18] Study on the transition of anisotropy in polygonal crack patterns of paste and its application to Martian terrain
Keywords:Mars, polygon, rheology
The gully found on the Martian slope was most likely formed by flowing water. However, there were various theories on gully formation, and the debate has not been settled yet. Therefore, we focused on the shape of thermal contraction polygons seen in gullies and hypothesized that the gully formation process could be restricted by the observed data. One of the effects that can induce anisotropic crack formation in the contraction fracture phenomenon, a shrinkage-induced fracture similar to thermal contraction fracture, is the memory effect of the paste. This effect is a phenomenon in which the paste remembers the direction in which it vibrates or flows during horizontal shaking due to plasticity, and cracks tend to propagate in the corresponding direction. When the solid volume fraction of paste is high and the paste vibrates during shaking, primary cracks propagate in the direction perpendicular to the shaking direction (memory of vibration). On the other hand, when the solid volume fraction is low and the paste flows during shaking, primary cracks propagate in the direction parallel to the flow direction (memory of flow). Since this effect can induce anisotropic crack fragments, we hypothesized that it could be applied to shape interpretation of Martian polygons. However, it was not clear by which parameter the transition between two memory effects occurs. Therefore, in this study, we experimentally investigated the conditions for the crack direction transition in the memory effect by using rheometer and examined whether the memory effect can be applied to the interpretation of Martian terrain.
First, to confirm the effect of the amplitude and frequency of oscillatory shear strain on the transition in memories, experiments were conducted in which the paste was directly subjected to oscillatory shear strain. Results revealed that, as long as a paste can be regarded as a visco-plastic fluid with plasticity, a transition from memory of vibration to that of flow occurs by changing the amplitude of oscillatory shear strain, and that the effects of frequency and solid volume fraction of paste are small. We also confirmed that memory of flow appears even when shear deformation in one direction is applied, indicating that memory of flow is likely to be found on planetary surfaces as well.
Next, based on the above results, a comparison was made between the ground-based desiccation fracture experiments and the analysis of Martian images. The paste was subjected to shear deformation in one direction by slope flow, which is assumed on planetary surfaces, and the desiccation crack patterns that appeared afterward were investigated. By comparing the experimental results with the results of analysis of Martian images, it was clarified that it is not contradictory to consider that the appearance of anisotropic fracture patterns is caused by the memory of flow due to sediment flow on the slope. By applying the interpretation based on the memory effect, it may be possible to place restrictions on the flow rate of sediment and other factors during formation of gullies. In the future, we would like to examine other factors that contribute to the anisotropy in the direction of cracks in the Martian polygons, and we discuss the applicability of memory effect to planetary surface terrain.
First, to confirm the effect of the amplitude and frequency of oscillatory shear strain on the transition in memories, experiments were conducted in which the paste was directly subjected to oscillatory shear strain. Results revealed that, as long as a paste can be regarded as a visco-plastic fluid with plasticity, a transition from memory of vibration to that of flow occurs by changing the amplitude of oscillatory shear strain, and that the effects of frequency and solid volume fraction of paste are small. We also confirmed that memory of flow appears even when shear deformation in one direction is applied, indicating that memory of flow is likely to be found on planetary surfaces as well.
Next, based on the above results, a comparison was made between the ground-based desiccation fracture experiments and the analysis of Martian images. The paste was subjected to shear deformation in one direction by slope flow, which is assumed on planetary surfaces, and the desiccation crack patterns that appeared afterward were investigated. By comparing the experimental results with the results of analysis of Martian images, it was clarified that it is not contradictory to consider that the appearance of anisotropic fracture patterns is caused by the memory of flow due to sediment flow on the slope. By applying the interpretation based on the memory effect, it may be possible to place restrictions on the flow rate of sediment and other factors during formation of gullies. In the future, we would like to examine other factors that contribute to the anisotropy in the direction of cracks in the Martian polygons, and we discuss the applicability of memory effect to planetary surface terrain.