S. V. S. Murty

Modelling of lunar crater ejecta extent for highland and mare using Mini-RF data

Previously using radar it was for the first time established that it is possible to quantify lunar ejecta in terms of spatial extent and can be characterized into fine and coarse based on radar backscatter effects. We also described that ejecta extent consistently increases with increase in crater diameter and is best related using power law equations. Here we describe utility of SAR to measure spatial ejecta extent for small sized (0.2 to 6 km) fresh and degraded craters and further extend our investigations to understand the variability in spatial ejecta extent derived in response to the change in target rock strength. We also analyze relation between ejecta distribution and crater diameter. Comprehensive analysis of 98 fresh and degraded craters from mare and highland regions composed of two entirely distinct rocks namely basalt and anorthosites respectively was carried out. Our observations show that spatial ejecta extent consistently increases with increase in crater diameter and is best related by power law equations. We also observe the amount of ejecta expelled and its subsequent surface deposition to be highly dependent on the physical attributes of the target rock. Extent tends to be greater for highland than compared to mare owing to the difference in rock strength. Based on this fact, highland fresh craters are thus observed to have highest amount of expelled and preserved ejecta.

Evidence of shock pressure above 600 kilobar and post-shock annealing in Nyaung IIIAB octahedrite

Nyaung iron is structurally a medium octahedrite (IIIAB) with several large inclusions of troilite-daubreelite aggregates and rare small inclusions of rhabdites. Primary cooling related textures include Widmanstätten structures on mesoscopic scale and large variety of plessites (net, comb and black variety); the latter formed under low temperature through martensitic transformations. Neumann lines (<130 kb) and widely spread matte kamacites (>600 kb) refer to shock- induced secondary changes. Post- shock annealing event is well documented with a large number of microtextures which include disappearance of older Neumann lines and development of decorated Neumann lines, sub-grain boundaries associated with triple point bearing polygonised kamacite, polygonised plessite and spherodised taenite particles; local recrystallisation coupled with degeneration of e — kamacite, plastic flowage of taenite lamellae and moving grain boundary pinned with taenite beads. Mean cooling rate for Nyaung octahedrite is estimated ~385°C/ Ma with cooling rate variation by a factor of 3.2. Rapid cooling rate is explained in terms of undisturbed fractional crystallisation of small molten core insulated within a very thin silicate mantle.