Abstract DGP2026-18

Introduction

Moretus is a ~116 km-diameter crater at the southern lunar highlands whose smooth, mostly flat interior has been interpreted as impact-melt deposits [1]. Its formation age is uncertain: previous studies alternately suggested an Eratosthenian or late Imbrian age [2,3], or an origin even as early Imbrian [4]. To address those contradicting ages, we apply crater size-frequency distribution (CSFD) analysis to the smooth-floor area of Moretus crater leveraging our geomorphological map [1]. We aim to improve the absolute model age (AMA) of Moretus crater and clarify its position in the lunar stratigraphy.

Datasets and Mapping

CSFD measurement areas were delineated using multi-instrumental datasets: Kaguya Terrain Camera (TC) orthophotos (~10 m/px) [5] and Chang’e-2 stereo-derived topographic products (DEM/DTM, ~7 m/px) [6,7]. DEM-derived hillshade, slope, and surface ruggedness guided geomorphologic mapping at 1:100.000 scale. The Moretus smooth-floor region was selected for homogeneity and low relief; areas showing obvious ray coverage or anomalous high-albedo material were noted and treated separately to avoid misattribution.

Methods

Crater diameters were manually measured with OpenCraterTool [8] in QGIS [10] on the Chang’e-2 DEM (~7 m/px) [6] hillshade and cross-checked on Kaguya TC imagery (~10 m/px) [5]. Count areas were restricted to geologically homogeneous, shallow-sloped (<5°) terrain to minimize slope degradation effects [10]. Obvious secondary craters (chains, clusters) and ambiguous depressions were excluded after multi-dataset verification. Counts were exported and analysed with CraterStats [11] using pseudolog binning, R-plot inspection, build-in randomness analysis, and isochron fitting with the Neukum production and chronology functions [12] to derive AMAs.

Results

In total, 1,435 craters were counted over a cumulative area of 1,030 km². The CSFD was fit over the diameter range of 0.40 – 1.30 km; diameters < 0.40 km deviate toward equilibrium and were excluded from age fits. Randomness analysis indicates that the selected diameter range is not influenced by secondary crater. The cumulative fit yields an AMA of 3.63 Ga with asymmetric uncertainties of +0.015/-0.017 Ga (cumulative crater density N (1) = 7.53 x 10⁻³ km ⁻²).

Conclusion

Our CSFD age for Moretus crater smooth-floor (~3.63 Ga) is younger than prior dating by Krasilnikov et al. (2023) with 3.75 Ga [3]. We used the stratigraphic definition of the late Imbrian Epoch boundary of Wilhelms et al. (1987) [13] to place and certify Moretus crater within the Late Imbrian Period. Areas previously mapped as light plains [4] are better explained as bright Tycho-ejecta overprints rather than light plains material originating from the Orientale basin [14] at ~3.82 Ga. Thus, the chronology supports interpretation of Moretus as a Late Imbrian impact with subsequent modification by younger ejecta, providing a tighter stratigraphic constraint for regional lunar mapping and chronology calibration.

References

[1] Klingenberg et al. (2024) LPSC 55, Abstract#1932. [2] Fortezzo et al. (2020) LPSC 51, Abstract#2760. [3] Krasilnikov et al. (2023) Icarus Vol.394, #115422. [4] Giuri et al. (2024) Planet. Sci. J. 5, #239. [5] Haruyama et al. (2009) LPSC 40, Abstract#1553. [6] Chunlai et al. (2018) Geomatics Inf. Sci. Wuhan Univ. 43, Abstract#4. [7] Chang’e-2 global digital orthophoto dataset, NAOC (2019). [8] Heyer et al. (2023) Planet. & Space Sci. Vol.231, #105687. [9] QGIS Dev. Team (2025), (Ver. 3.30.0). [10] van der Bogert et al. (2018) Icarus Vol.306, P.225-242. [11] Michael & Neukum (2010) EPSL, Vol. 294 (3-4), 223-229. [12] Neukum (1983) Habilitation Dissertation, LMU München. [13] Wilhelms et al. (1987) USGS Prof. Pap. 1348, 302 p. [14] Orgel et al. (2018) JGR: Planets Vol.123 (3), P.748-762.