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The History of Lunar Impact Bombardm...

Blevins, Austin,

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The History of Lunar Impact Bombardment as Told by Apollo Samples /

Record Type:	Electronic resources : Monograph/item
Title/Author:	The History of Lunar Impact Bombardment as Told by Apollo Samples // Austin Blevins.
Author:	Blevins, Austin,
Description:	1 electronic resource (190 pages)
Notes:	Source: Dissertations Abstracts International, Volume: 87-04, Section: B.
Contained By:	Dissertations Abstracts International87-04B.
Subject:	Astrophysics. -
Online resource:	https://pqdd.sinica.edu.tw/twdaoapp/servlet/advanced?query=32363991
ISBN:	9798297956414

The History of Lunar Impact Bombardment as Told by Apollo Samples /
Blevins, Austin,

The History of Lunar Impact Bombardment as Told by Apollo Samples /Austin Blevins. - 1 electronic resource (190 pages)

Source: Dissertations Abstracts International, Volume: 87-04, Section: B.

The amount of cratering experienced by a surface over time can be described by a production function. A key component of a production function is a chronology function that relates absolute age to relative crater density. The lunar chronology function is anchored by the absolute ages of Apollo samples that have been correlated to surfaces that have known relative crater densities, such as large impact craters known as basins. Absolute ages are measured by radiometric dating (particularly the Ar-Ar method), and relative ages are measured by crater counting.The lunar chronology function is unconstrained from the time of the formation of the Moon until the time of the Imbrium impact ~3.9 billion years ago (Ga). This is because no lunar samples have been successfully correlated to a basin older than Imbrium, which is one of the youngest basins on the Moon. While several constraining points exist for the Imbrian period (3.9-3.1 Ga), the rate of impact during this time is debated between a gradual decline throughout the period and a rapid decline early in the period. In addition, lunar samples may have experienced radiogenic isotope loss, which leads to a young age bias and would further complicate the matter of constraining the lunar chronology function.In this dissertation, I examine each of these topics using an impact bombardment model known as the Cratered Terrain Evolution Model (CTEM). CTEM is a Monte Carlo impact bombardment model that generates random craters throughout a model grid and simulates processes such as ejecta transport and regolith mixing induced by impact gardening. I have added several functionalities to CTEM to support my work in this dissertation. This includes the ability to generate manual craters alongside random ones, the ability to track the source craters of impact melt throughout the surface, and a thermal grid used to model impact heating and subsequent cooling, with the goal of determining fractional argon loss in lunar materials.CTEM was used to investigate the impact rate in the Imbrian period. Model results show that local impacts are the likeliest sources of Imbrian-aged impact melt at the Apollo landing sites, not larger "sub-basin"-sized impacts distal to the region. The age distribution of Apollo impact melts more closely resembles the rapid decline scenario than the gradual decline scenario.CTEM was also used to investigate the source craters of Apollo impact melts in the basin-forming period. It was found that only a few basin-forming impacts are expected to contribute to the Apollo sample collection; of these, only Imbrium is present throughout every Apollo site. With the aid of Bayesian analysis, scenarios were constructed where specific samples were correlated to specific basins with specific ages. It was found that nearly every possible configuration of impact rate is possible with the limited samples to use as constraints; this means that more samples are necessary to further constrain the behavior of lunar impacts during the basin-forming period.Finally, CTEM was also used to investigate the thermal history of lunar impacts, specifically in the Mare Tranquilitatis region. It was found that the impact bombardment of the Mare Tranquilitatis could lead to radiogenic argon loss that may have been present in Apollo 11 mare basalts. The results of the projects presented in this dissertation contribute to a greater understanding of the history of the Moon, with implications for future sampling missions.

English

ISBN: 9798297956414Subjects--Topical Terms:

535904
Astrophysics.
Subjects--Index Terms:

Chronology function

The History of Lunar Impact Bombardment as Told by Apollo Samples /
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520 $a The amount of cratering experienced by a surface over time can be described by a production function. A key component of a production function is a chronology function that relates absolute age to relative crater density. The lunar chronology function is anchored by the absolute ages of Apollo samples that have been correlated to surfaces that have known relative crater densities, such as large impact craters known as basins. Absolute ages are measured by radiometric dating (particularly the Ar-Ar method), and relative ages are measured by crater counting.The lunar chronology function is unconstrained from the time of the formation of the Moon until the time of the Imbrium impact ~3.9 billion years ago (Ga). This is because no lunar samples have been successfully correlated to a basin older than Imbrium, which is one of the youngest basins on the Moon. While several constraining points exist for the Imbrian period (3.9-3.1 Ga), the rate of impact during this time is debated between a gradual decline throughout the period and a rapid decline early in the period. In addition, lunar samples may have experienced radiogenic isotope loss, which leads to a young age bias and would further complicate the matter of constraining the lunar chronology function.In this dissertation, I examine each of these topics using an impact bombardment model known as the Cratered Terrain Evolution Model (CTEM). CTEM is a Monte Carlo impact bombardment model that generates random craters throughout a model grid and simulates processes such as ejecta transport and regolith mixing induced by impact gardening. I have added several functionalities to CTEM to support my work in this dissertation. This includes the ability to generate manual craters alongside random ones, the ability to track the source craters of impact melt throughout the surface, and a thermal grid used to model impact heating and subsequent cooling, with the goal of determining fractional argon loss in lunar materials.CTEM was used to investigate the impact rate in the Imbrian period. Model results show that local impacts are the likeliest sources of Imbrian-aged impact melt at the Apollo landing sites, not larger "sub-basin"-sized impacts distal to the region. The age distribution of Apollo impact melts more closely resembles the rapid decline scenario than the gradual decline scenario.CTEM was also used to investigate the source craters of Apollo impact melts in the basin-forming period. It was found that only a few basin-forming impacts are expected to contribute to the Apollo sample collection; of these, only Imbrium is present throughout every Apollo site. With the aid of Bayesian analysis, scenarios were constructed where specific samples were correlated to specific basins with specific ages. It was found that nearly every possible configuration of impact rate is possible with the limited samples to use as constraints; this means that more samples are necessary to further constrain the behavior of lunar impacts during the basin-forming period.Finally, CTEM was also used to investigate the thermal history of lunar impacts, specifically in the Mare Tranquilitatis region. It was found that the impact bombardment of the Mare Tranquilitatis could lead to radiogenic argon loss that may have been present in Apollo 11 mare basalts. The results of the projects presented in this dissertation contribute to a greater understanding of the history of the Moon, with implications for future sampling missions.
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