Lunar regolith thickness deduced from concentric craters

Yue et al., 2019

This paper focuses on estimating the depth of the lunar regolith in China’s Chang’E-5 landing site using the traditional crater morphological method. The authors focused on only one type of morphology in their study-concentric craters. This work is inspired by the previous analysis done by Quaide & Oberbeck (1986) and Bart (2014) and the authors use the same regolith thickness equation used in these previous works (t = (k − DF /DA)DA tan(α)/2). For their work, they’ve used high-resolution LROC NAC images with similar incidence angles, ArcGIS software, and CraterTools toolkit to map the craters. Their initial results show a uniform distribution of these craters over the region. The derived lunar regolith depth range from 0.74 m to 18 m with a mean of 7.15 m. The regolith is generally thinner in the SE study area but overall remains thicker than 2 m over most of the CE-5 landing area. The authors also observe that around 73 concentric craters that show regolith thickness of <4 m, indicating that the coherent substratum is relatively shallow.

Concentric crater delineation using CraterTools. (A) A typical concentric crater (59.69°W, 44.23°N) with perfect outer and inner crater rims. (B) A concentric crater with an irregular inner crater rim (65.86°W, 43.06°N). (C) A concentric crater with a partially obscured inner crater rim (61.31°W, 42.98°N). (D) A concentric crater (50.52°W, 42.94°N) with a central mound, which is also measured

Discussion: The authors discuss briefly the uncertainties in regolith thickness due to conventional error propagation. Their calculation re that the uncertainty is irrelevant to the size of the circle (using CraterTools) but is rather related to the distribution of the selected three points. They conclude that using the same values for parameters k and α the uncertainty of thickness would be ± 0.24 m. The authors also classify the area in 10 geologic units defined in the CE-5 landing area, where they observe at least five concentric craters in each unit referring to ages Eratosthenian and Imbrian (i.e. generally, lunar regolith thickness increases with geologic time).

Distribution of concentric craters and median regolith thickness over CE-5 landing area. (A) concentric craters and derived thickness of lunar regolith; the size of the dots are scaled with the thickness of lunar regolith, (B) lunar regolith thickness map derived using the natural neighbor interpolation method for every 15 km×15 km in the CE-5 landing area.

Additionally, the authors compare their results to the previously estimated regolith depths for Oceanus Procellarum which are shown in the table below. Their estimated values for regolith thickness using only the concentric craters is closest to work done by Kobayashi et al. 2010. The authors have also included central mound craters in their observations despite the ambiguity about whether they’re secondary craters or not. When they include the regolith depth calcuations for these central mound craters in their overall results, the average depth is estimated at 44.56 ± 28.06 m which is much higher than all the contemporary studies they refer to. This is a strong indication that a significant number of central mound craters can’t be used to estimate the regolith thickness in their study area. They also point out that this further proves that concentric craters in this particular area are formed through layered targets and their results are reliable.

I already discussed in my last blog how I’m implementing this crater morphology method in my own research to estimate the regolith thickness in and around the radar-dark halo craters. After reading this paper, I may have to re-evaluate the flat-bottom craters I already looked at/taken into consideration for my estimates to see whether they’re a result of secondary impacts. Once I have looked around more than one radar-dark halo crater, my results may be able to tell me more about the unusual mechanism of these radar-dark haloes.

Until next time!

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