Terra-mare comparison of small young craters on the Moon

Several characteristics of very small impact craters of lunar surface have recently been improved by high resolution datasets. Lunar impact craters with diameter below 100 m and especially below 10 m were surveyed and compared at mare and terra surfaces in this work, altogether at 0.6 km(2) area with 4866 craters there. The surface density values (of all craters/km(2)) at the mare regions were 1.5-2.0 times larger than at the terra regions, and this difference came mainly from < 10 m diameter craters. Although the crater retention and stability might have been a reason for such a difference causing shorter crater lifetime on steeper terra units, similar spatial density was found at sloping and nearly horizontal parts of the terra area, and no signature of partly destroyed craters was observed there, leaving the resolution of the difference open. The difference in the properties of target material is a relevant candidate reason for it, however based on shadow length measurements no depth difference was identified between terra and mare craters. There is an ongoing debate on the reliability and usage of small craters for age estimation (Xiao and Strom, 2012), this work provides some further information to evaluate this aspect. No correlation was found between the areal density values of craters and ages determined from size-frequency distributions, confirming (using examples of smaller craters than analysed in most earlier works), that large sized craters dominate the age estimation despite the larger number of < 10 m craters. Characteristic age values for < 10 m diameter crater size category ranged between 65 and 100 Ma, while for < 100 m craters the estimated ages were 76-250 Ma. Substantial fluctuations in age within terra and mare regions were observed in the absence of nearby secondary craters, suggesting that the role of nearby secondaries might be minor in the production of the observed areal heterogeneity. Although the role of distant secondaries could not be excluded, the results suggest natural random fluctuations should be more carefully considered in the future. Analysis of the depth of regolith layer that different size groups of craters influence, roughly the top 1-2 meter layer could be mixed during 30-60 Ma age, and this age range is close to recent estimations by other authors.