Constraints on the recent rate of lunar ejecta breakdown and implications for crater ages

Ghent, R. R.; Hayne, P. O.; Bandfield, J. L.; Campbell, B. A.; Allen, Carlton C.; Carter, Lynn M.; Paige, D. A.

doi:10.1130/g35926.1

Cited by 98 publications

(112 citation statements)

References 18 publications

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“…Thus, individual ejecta clasts deposited at larger ranges are not expected to be part of the bulk of a thicker deposit, but would be deposited at the surface or quite shallowly mixed into preexisting regolith (e.g., Oberbeck, 1975). We reiterate that the importance of subsurface clasts to the CPR evolution of initially rock-rich ejecta has been suggested previously (e.g., Bandfield et al, 2011;Ghent et al, 2014Ghent et al, , 2016, but our data provide independent support for this hypothesis. Nearer to the rim, where R~1 to 1.5, ejecta thicknesses are multiple meters, rocks are buried during ejecta emplacement in this thicker deposit, and partially shielded from the destructive environment at the surface.…”

Section: Cpr Enhancement In Crater Ejectasupporting

confidence: 73%

Temporal Evolution of S‐Band Circular Polarization Ratios of Kilometer‐Scale Craters on the Lunar Maria

et al. 2018

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Circular polarization ratio (CPR) measurements from the Miniature Radio Frequency (Mini‐RF) instrument provide information about how lunar craters evolve with time. In particular, S‐band CPR data are sensitive to the rockiness and/or topographic roughness of the uppermost ~1 m of the subsurface. We extracted CPR as a function of radial range for 6,206 unique craters superposed on the lunar maria with diameter D = 0.8–2 km and constructed median profiles aggregating craters into 13 different age classes. These aggregate profiles show systematic evolution of craters' CPR with time. The freshest craters (<200 Ma) have ejecta exhibiting elevated CPR compared to the background maria out to distances of more than three crater diameters beyond the craters' rims. The extent and magnitude of this enhancement declines as craters age. Within crater interiors, the CPR signature initially increases for 0.4–0.6 Ga and then declines. These observations provide constraints on rock breakdown and regolith development after crater formation. Additionally, our results demonstrate that the CPR evolution of crater interiors and ejecta are significantly decoupled. The CPR enhancement in crater ejecta fades faster than crater interiors, causing their overall CPR signature to look similar to anomalous craters whose interior CPR anomalies have been attributed in past work to the presence of water ice. Craters in this study became more anomalous‐looking as they reach middle age (~1.5–2.5 Ga), as their interior and exterior regolith differ in rockiness as time passes. Our results support, but do not prove, that anomalous craters' CPR signatures can arise without requiring water ice.

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Section: Cpr Enhancement In Crater Ejectasupporting

confidence: 73%

Temporal Evolution of S‐Band Circular Polarization Ratios of Kilometer‐Scale Craters on the Lunar Maria

et al. 2018

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“…Using the rock abundance signature of the ejecta from Copernican‐age lunar craters D > 5 km derived from the Diviner instrument on LRO as a proxy for age (Ghent et al. ), Mazrouei et al. () and Ghent et al.…”

Section: Impact Ratesmentioning

confidence: 99%

Dating very young planetary surfaces from crater statistics: A review of issues and challenges

Williams

Bogert

Pathare

et al. 2017

Meteorit & Planetary Scien

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Abstract-Determining the ages of young planetary surfaces relies on using populations of small, often sub-km diameter impact craters due to the higher frequency at which they form. Smaller craters however can be less reliable for estimating ages as their sizefrequency distribution is more susceptible to alteration with debate as to whether they should be used at all. With the current plethora of meter-scale resolution images acquired of the lunar and Martian surfaces, small craters have been widely used to derive model ages to establish the temporal relation of recent geologic events. In this review paper, we discuss the many factors that make smaller craters particularly challenging to use and should be taken into consideration when crater counts are confined to small crater diameters. Establishing confidence in a model age ultimately requires an understanding of the geologic context of the surface being dated as reliability can vary considerably and limitations of the dating technique should be considered in applying ages to any geologic interpretation.

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“…Thermophysical mapping by the LRO Diviner thermal radiometer revealed that Sosigenes and several other IMPs had much lower rock abundances than the ejecta of some late Copernican‐aged craters (e.g., ~170 Ma Aristarchus and ~85 Ma Tycho, actually only slightly higher than typical lunar surfaces; Ghent et al. ) and an interpreted surface regolith layer thicker than 10 cm, suggesting that either lunar IMPs were older than the crater‐count dating results, or there was an unusually rapid development of regolith materials on IMPs compared with blocky ejecta blankets (Elder et al. ).…”

Section: Introductionmentioning

confidence: 95%

The role of substrate characteristics in producing anomalously young crater retention ages in volcanic deposits on the Moon: Morphology, topography, subresolution roughness, and mode of emplacement of the Sosigenes lunar irregular mare patch

Qiao

Head

Xiao

et al. 2017

Meteorit & Planetary Scien

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Lunar irregular mare patches (IMPs) comprise dozens of small, distinctive, and enigmatic lunar mare features. Characterized by their irregular shapes, well‐preserved state of relief, apparent optical immaturity, and few superposed impact craters, IMPs are interpreted to have been formed or modified geologically very recently (<~100 Ma; Braden et al. ). However, their apparent relatively recent formation/modification dates and emplacement mechanisms are debated. We focus in detail on one of the major IMPs, Sosigenes, located in western Mare Tranquillitatis, and dated by Braden et al. () at ~18 Ma. The Sosigenes IMP occurs on the floor of an elongate pit crater interpreted to represent the surface manifestation of magmatic dike propagation from the lunar mantle during the mare basalt emplacement era billions of years ago. The floor of the pit crater is characterized by three morphologic units typical of several other IMPs, i.e., (1) bulbous mounds 5–10 m higher than the adjacent floor units, with unusually young crater retention ages, meters thick regolith, and slightly smaller subresolution roughness than typical mature lunar regolith; (2) a lower hummocky unit mantled by a very thin regolith and significantly smaller subresolution roughness; and (3) a lower blocky unit composed of fresh boulder fields with individual meter‐scale boulders and rough subresolution surface texture. Using new volcanological interpretations for the ascent and eruption of magma in dikes, and dike degassing and extrusion behavior in the final stages of dike closure, we interpret the three units to be related to the late‐stage behavior of an ancient dike emplacement event. Following the initial dike emplacement and collapse of the pit crater, the floor of the pit crater was flooded by the latest‐stage magma. The low rise rate of the magma in the terminal stages of the dike emplacement event favored flooding of the pit crater floor to form a lava lake, and CO gas bubble coalescence initiated a strombolian phase disrupting the cooling lava lake surface. This phase produced a very rough and highly porous (with both vesicularity and macroporosity) lava lake surface as the lake surface cooled. In the terminal stage of the eruption, dike closure with no addition of magma from depth caused the last magma reaching shallow levels to produce viscous magmatic foam due to H2O gas exsolution. This magmatic foam was extruded through cracks in the lava lake crust to produce the bulbous mounds. We interpret all of these activities to have taken place in the terminal stages of the dike emplacement event billions of years ago. We attribute the unusual physical properties of the mounds and floor units (anomalously young ages, unusual morphology, relative immaturity, and blockiness) to be due to the unusual physical properties of the substrate produced during the waning stages of a dike emplacement event in a pit crater. The unique physical properties of the mounds (magmatic foams) and hummocky units (small vesicles and large void space) altered the...

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Constraints on the recent rate of lunar ejecta breakdown and implications for crater ages

Cited by 98 publications

References 18 publications

Temporal Evolution of S‐Band Circular Polarization Ratios of Kilometer‐Scale Craters on the Lunar Maria

Temporal Evolution of S‐Band Circular Polarization Ratios of Kilometer‐Scale Craters on the Lunar Maria

Dating very young planetary surfaces from crater statistics: A review of issues and challenges

The role of substrate characteristics in producing anomalously young crater retention ages in volcanic deposits on the Moon: Morphology, topography, subresolution roughness, and mode of emplacement of the Sosigenes lunar irregular mare patch

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