Physical Properties of Basalt Ejecta Boulders at Lonar Crater, India: Insights Into the Target Heterogeneity and Impact Spallation Processes in Basalt With Application to Mars

摘要

The 1.88-km-diameter Lonar crater emplaced in the Deccan basalt is an excellent terrestrial analog of bowl-shaped impact craters on Mars. It contains a pristine ejecta blanket with abundant basalt boulders. We measured the density, porosity, P and S wave velocities and attenuation, magnetic susceptibility, uniaxial compressive strength, and tensile strength of 313 core specimens from 65 basalt ejecta boulders. Using these values, we quantified the gross physical properties of preimpact target and applied them in the cratering equations and estimated the spall plate dimensions and ejection distances. The impact of 35-m-radius projectile with 20 km/s velocity on the basaltic target with 77.2-MPa dynamic tensile strength, 4,163 m/s P wave velocity, and 2.56 g/cm~3 density gives a spall thickness range (0.2 to 9.7 m) similar to the observed boulder sizes at Lonar crater. Because aqueous weathering and vesicularity are present in the target, the Lonar impact produced smaller spall sizes than those expected from a massive basalt target. We also measured the ejecta boulder sizes and shapes around a fresh impact crater (of similar size and target to Lonar) in Arcadia Planitia on Mars and calculated the spall thicknesses. The low-velocity impact (5.5 km/s) of 50-m-radius projectile on the target similar to Lonar basalt gives a spall thickness range (1 to 25 m) similar to the observed boulder sizes at the Martian impact crater. The impact craters of similar sizes and targets on Earth and Mars produced different spall boulder size distributions reflecting the distinct impact conditions on these planets.