Surface exposure duration of individual lunar soil grains : results from Luna-24 core samples

Individual soil grains taken from six Luna-24 drill core samples have been analysed for their records of surface exposure correlated features e.g., solar flare tracks, microcraters and accretionary discs. About 40 percent of the grains having solar flare tracks are devoid of microcraters and accretionary discs indicating their shielded exposure on the lunar surface during their solar flare exposure durations. Based on the observed microcrater data ''effective'' exposure durations of individual grain surfaces in the topmost layer of the lunar regolith with varying amount of shielding (up to one micron) have been estimated. The average exposure ages are approximately 1000yrs, approximately 8000yrs and approximately 40,000yrs for zero shielding, <= 0.5 micron shielding and <= 1 micron shielding conditions respectively. The range in the exposure ages of individual grain surfaces, however, vary by more than a factor of ten in each case. The ''effective'' zero-shielding exposure ages obtained here are much lower than those predicted on the basis of Monte Carlo simulation of the lunar regolith processes. (auth.)