Parameters critical to the morphology of fluidization craters

The anomalously high number of craters with diameter less than 2.8 km, the igneous nature of rocks from the Apollo landing sites, and the possibility of outgassing magmas in the lunar crust, suggest that fluidization may be a viable mechanism for producing many of the smaller lunar craters, Fluidization craters were formed in the laboratory by blowing gas through various thicknesses of particulate material. Gas pressure, regolith thickness, and the duration of gas streaming were controlled over practical experimental limits and compared with the resultant crater morphology. Low to moderate fluidization pressures on coarsely crushed limestone (Mø = 0.40, So = =0.50) with low cohesion (ø - 43°) produced bowl shaped, basin shaped, and flat bottomed craters. Bowl shaped craters change into basin shaped and/or flat bottomed craters with long durations of gas streaming. Cone, funnel, and flat-funnel shaped craters are indicative of high fluidization pressures. Craters formed in finely crushed limestone (Mø - 1.55, So - 0.85) that is electrostatically charged by the streaming gas, are flat bottomed. Terraced craters develop from slumping during and after the discontinuation of gas flow. Central mounds inn terraced craters result from slumping into a confined space. In particulate material, fluidization craters have high circularity and axial symmetry, similar to those produced by impact. The use of an impact model and crater morphology (normal, flat bottomed, and concentric) for estimating lunar regolith depth is questioned because similar craters can be produced by fluidizationn processes in a thicker regolith.On leave at the Earth Physics Branch, Dept. of Energy, Mines and Resources, Ottawa, Canada.