Origin of discrepancies between crater size-frequency distributions of coeval lunar geologic units via target property contrasts

doi:10.1016/j.icarus.2016.11.040

Icarus

Volume 298, December 2017, Pages 49-63

https://doi.org/10.1016/j.icarus.2016.11.040 Get rights and content

Highlights

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New CSFDs on impact melts cover crater diameters in both strength- and gravity-scaling regimes.
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Craters from each regime yield discrepant absolute model ages.
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Calculations show target properties affect final crater diameters and the slope of the CSFD.
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Coeval materials with differing target properties may exhibit discrepant absolute model ages.

Abstract

Recent work on dating Copernican-aged craters, using Lunar Reconnaissance Orbiter (LRO) Camera data, re-encountered a curious discrepancy in crater size-frequency distribution (CSFD) measurements that was observed, but not understood, during the Apollo era. For example, at Tycho, Copernicus, and Aristarchus craters, CSFDs of impact melt deposits give significantly younger relative and absolute model ages (AMAs) than impact ejecta blankets, although these two units formed during one impact event, and would ideally yield coeval ages at the resolution of the CSFD technique. We investigated the effects of contrasting target properties on CSFDs and their resultant relative and absolute model ages for coeval lunar impact melt and ejecta units. We counted craters with diameters through the transition from strength- to gravity-scaling on two large impact melt deposits at Tycho and King craters, and we used pi-group scaling calculations to model the effects of differing target properties on final crater diameters for five different theoretical lunar targets. The new CSFD for the large King Crater melt pond bridges the gap between the discrepant CSFDs within a single geologic unit. Thus, the observed trends in the impact melt CSFDs support the occurrence of target property effects, rather than self-secondary and/or field secondary contamination. The CSFDs generated from the pi-group scaling calculations show that targets with higher density and effective strength yield smaller crater diameters than weaker targets, such that the relative ages of the former are lower relative to the latter. Consequently, coeval impact melt and ejecta units will have discrepant apparent ages. Target property differences also affect the resulting slope of the CSFD, with stronger targets exhibiting shallower slopes, so that the final crater diameters may differ more greatly at smaller diameters. Besides their application to age dating, the CSFDs may provide additional information about the characteristics of the target. For example, the transition diameter from strength- to gravity-scaling could provide a tool for investigating the relative strengths of different geologic units. The magnitude of the offset between the impact melt and ejecta isochrons may also provide information about the relative target properties and/or exposure/degradation ages of the two units. Robotic or human sampling of coeval units on the Moon could provide a direct test of the importance and magnitude of target property effects on CSFDs.

Section snippets

Introduction and background

Recent work on the lunar chronology using Lunar Reconnaissance Orbiter (LRO) Camera (Robinson et al., 2010) data has re-encountered a curious discrepancy in crater size-frequency distribution (CSFD) measurements between impact units that was observed, but not understood, during the Apollo era. For example, at Tycho, Copernicus, and Aristarchus craters, CSFDs of impact melt deposits give statistically significantly younger relative (e.g., Shoemaker et al., 1968, Strom and Fielder, 1968a, Strom

Approach

Here, we take a two-pronged approach to investigating the possible effects of target properties on CSFDs and to exclude major contributions by other geologic processes. First, we measured the size-frequency distributions of craters with diameters that cross the range at which crater scaling shifts from strength- to gravity-controlled on two large impact melt deposits: the large melt pond northwest of King Crater and the Tycho Crater floor impact melt sheet. Large impact melt deposits were

Data

For CSFD measurements and morphological observations on the Tycho Crater floor impact melt sheet (1500 km²) (Fig. 4a), we used a portion of the SELENE Terrain Camera ortho-mosaic with a resolution of 4096 pixels/degree or ∼7.8 m/pixel and a photometrically calibrated incidence angle of 30° (Haruyama et al., 2008) (Table 1). For new CSFD measurements and morphological observations on a 162 km² portion of the King Crater melt pond (Fig. 4b), we used LRO Narrow Angle Camera (NAC) (Robinson et al.,

Crater size-frequency distribution (CSFD) measurements

The measurement of CSFDs is described in detail elsewhere (e.g., Hartmann, 1966, Crater Analysis Techniques Working Group 1979, Neukum, 1983, Hiesinger et al., 2000). To obtain the relative or absolute age of a photogeological unit, one must (1) measure the surface area of the unit, and (2) measure the rim diameters of each primary impact crater within the unit. All craters within the count areas were included, regardless of morphology, except obvious secondary craters. The crater diameters are

King and Tycho impact melt deposit CSFDs

New measurements for a 162 km² portion (Fig. 4b) of the large impact melt pond to the northwest of King Crater follow the previously measured impact melt pond isochron (Ashley et al., 2012) at crater diameters <170 m, while crater diameters >∼315 m are similar to the ejecta blanket (Fig. 2a). Craters with diameters from 170 m to 315 m transition from the impact melt pond isochron to that of the ejecta blanket of Ashley et al. (2012). The R-plot shows that the shift from the younger to the older

Target layering

For our measurements on the King impact melt pond (Fig. 2), we considered whether the largest craters, which exhibit relatively older apparent ages, might simply penetrate through the impact melt into ejecta and/or bedrock units. Simple calculations of the excavation depths of craters in the range we measured (Table 4) reveal that it is likely that most of the craters measured on the impact units at both Tycho and King are not large enough to penetrate into the ejecta deposits. In the King melt

Implications and conclusions

Our study shows that coeval materials with differing target properties exhibit discrepant absolute model ages. Because the recent lunar chronology function (Neukum et al., 2001) is calibrated using CSFDs collected on the ejecta blankets of Copernicus, Tycho, North Ray, and Cone craters, stronger Copernican-aged units dated using the Neukum et al. (2001) chronology could give falsely young absolute model ages. This is the case for the discrepancy in ages between impact ejecta and melt units. The

Acknowledgments

C. H. vdB. and H. H. were funded by German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt) projects 50OW0901 and 50OW1504. We thank members of the LROC team for many intense discussions, as well as Carolyn Ernst, Zhiyong Xiao, and an anonymous reviewer for their helpful comments. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

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Origin of discrepancies between crater size-frequency distributions of coeval lunar geologic units via target property contrasts

Highlights

Abstract

Section snippets

Introduction and background

Approach

Data

Crater size-frequency distribution (CSFD) measurements

King and Tycho impact melt deposit CSFDs

Target layering

Implications and conclusions

Acknowledgments

Icarus

Icarus

Geochim. Cosmochim. Acta

Icarus

Icarus

Icarus

Icarus

Planet. Space Sci.

Icarus

Icarus

Earth Planet. Sci. Lett.

Icarus

Icarus

Earth Planet. Sci. Lett.

Icarus

Icarus

Earth Planet. Sci. Lett.

Modernization of the Integrated Software for Imagers and Spectrometers

Lunar Planet. Sci. Conf.

Geology of the King Crater region—New insights into impact melt dynamics on the Moon

J. Geophys. Res.

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Impact melt on lunar crater rims.

Ages of mare basalts on the lunar nearside

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