Physics
Scientific paper
Jan 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992phdt........20s&link_type=abstract
Ph.D. Thesis Arizona State Univ., Tempe.
Physics
1
Avalanches, Chutes, Mars (Planet), Planetary Geology, Slopes, Surface Properties, Geomorphology, Lunar Soil, Mars Photographs, Weathering
Scientific paper
The physical properties and evolution of steep slopes on the planet Mars are examined. Where martian slopes are steepest and of greatest relief, the slope morphology is distinctive and characterized by regular, alternating spurs and gullies. Previous workers identified these martian gullies as dry debris avalanche chutes, partially because a nonspecific 'dry mass-wasting' explanation seemed compatible, initially, with current surface conditions. An evacuated avalanche chute provides the opportunity to back analyze the stability of the mobile layer of the chute at the time of its failure. For the martian slope problem a new stability back-analysis technique was developed which incorporates the third dimension (chute width) into the back-analysis calculation, allowing unambiguous determination of the average cohesion and angle of internal friction of the mobile layer. A series of trials was performed to investigate the effects of gravitational acceleration, average slope angle, and average mobile layer density, cohesion, and angle of internal friction on the size and shape of the transverse chute cross-section. Results indicate that: (1) variations in density, gravitational acceleration, and cohesion affect the overall size of the cross-section but not its width:depth ratio, and (2) the contrast between average slope angle and angle of internal friction is most influential on the width:depth ratio of the cross-section. The difference between these two parameters is inversely proportional to the width:depth ratio. Application of the three-dimensional back-analysis technique to proposed martian avalanche chutes yields values of cohesion and angle of internal friction of the mobile layers that are similar mechanically (without genetic implications) to those of terrestrial glacial till, a poorly sorted material with moderate inter-grain cohesion, or lunar soils under normal stresses representative of (lunar) depths of between ten and twenty meters. Material of these strengths is not capable of supporting long, steep martian slopes visible on Viking Orbiter images. These results imply that weathering and weakening of initially stronger martian slope materials penetrated to depths of tens of meters before downslope failure of weathered material occurred (creating the observed avalanche chutes).
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