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Analysis of Small Scale Lunar Gravity Anomalies: Implications for Crater Formation and Crustal History

Citation

Dvorak, John Joseph (1979) Analysis of Small Scale Lunar Gravity Anomalies: Implications for Crater Formation and Crustal History. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/zpdn-2n37. https://resolver.caltech.edu/CaltechTHESIS:06072024-205240041

Abstract

Anomalies in the local gravity field arise from lateral density variations within a planetary body. Hence, a consideration of the nature of these anomalies can be used to infer the subsurface structure and, from this, possibly constrain and characterize the processes which formed these features. Such a consideration is given here to the local gravity anomalies on the Moon.

The lunar free-air gravity data consist of the line-of-sight (i.e., Earth-to-spacecraft) component of the accelerations undergone by a spacecraft in a low lunar orbit. In this analysis the acceleration or gravity profiles across a feature of interest are corrected for the topographic contribution of the lunar surface to the local gravity field to obtain the Bouguer gravity profiles. The availability of lunar features for this analysis is mainly constrained by the nearside areal coverage of low altitude orbiting lunar spacecraft.

A total of twenty-one features are analyzed here of which all but two are of impact origin; these two exceptions are probable sites for intrusive and extrusive igneous activity on the Moon. The impact features have been divided into two major groups on the basis of age with the older group further subdivided into relatively unfilled craters and very shallow craters and also including a small multiringed basin.

The young lunar craters, which range in diameter from 32 to 135 kilometers, have negative Bouguer gravity anomalies (i.e., mass deficiencies or low density regions) associated with them. Presumably, these low density regions represent material brecciated and crushed by the impact events which formed these craters. Calculations reveal a systematic variation in the magnitude of the total mass deficiency with increasing crater size and a comparison of lunar and terrestrial impact features shows that the magnitude of this mass deficiency is proportional to the cube of the crater diameter. Furthermore, studies of terrestrial impact structures indicate that the major contribution to these negative Bouguer anomalies is most likely due to a large lens of low density material extending laterally to approximately the crater rim crest and vertically to a depth of roughly one-third the rim diameter. Also based on studies of many terrestrial impact features, the maximum depth of the transient cavity associated with these lunar craters, as inferred from the apparent vertical extent of brecciation, was roughly one-fourth the crater rim diameter. This depth possibly also corresponded to the maximum depth of excavation of at least a small amount of the lunar crust by these impact events.

In all but one case, the old unfilled lunar craters have distinctly different Bouguer anomalies from the young lunar craters. With the exception of the crater Piccolomini which follows the trend for the younger craters, all of the remaining unfilled old lunar craters have, within the uncertainties of this analysis, zero Bouguer anomalies. The cause for this change in the value of the Bouguer gravity is, of course, expected to be related to the greater age and more modified appearance of these craters. A likely possibility is that the porosity initially produced by the impact events which formed these older craters has been removed by the intrusion of igneous material, roughly contemporaneous with the emplacement on the surface of the lunar mare basalts. A second possibility is that the original porosity has been removed by compaction of the lunar crust during the seismic shaking which accom­panied the formation of the last lunar multiringed basins, however, it is difficult to evaluate this mechanism since the efficiency at this scale is unknown.

The two shallow lunar craters for which it is possible to determine the Bouguer gravity have slight negative anomalies. These particular craters are also filled by light plains units which have been inter­preted to be ejecta from later multiringed basin impacts. These slight negative anomalies are probably due to the higher porosity of the basin ejecta which now almost completely fills these craters.

The Bouguer gravity of a small multiringed basin, Grimaldi, has also been determined. Spacecraft free-air gravity data reveal a mascon associated with the inner ring of this structure. The topo­ graphic correction to the local gravity field indicates a maximum Bouguer anomaly of +90 milligals at an altitude of 70 kilometers. Approximately 20% of this positive Bouguer anomaly can be accounted for by the mare material lying within the inner ring. It is proposed, from a consideration of the Bouguer gravity over large lunar craters comparable in size to Grimaldi, that the remaining positive anomaly is due to a plug of high density lunar mantle material centrally uplifted by the inward collapse of material which resulted in the formation of the concentric outer scarp. In addition, a ring of low density material, possibly ejecta deposited by the formation of Grimaldi, is required to fully reproduce the Bouguer gravity signature across this basin. A similar structure to the one described here for Grimaldi has been previously proposed for the Orientale Basin.

The two non-impact features, Lamont and Marius Hills, have positive Bouguer anomalies. Since these are probable sites for extrusive igneous activity, that is, possible source areas for the surrounding mare material, these positive gravity anomalies are interpreted to be the result of intrusive activity within the lunar crust, much more intensive activity than the intrusion of material proposed beneath the old lunar craters also analyzed here. At Lamont, located in western Tranquillitatis, the mare ridge pattern and local topography suggest a general subsidence of this region. The gravity data indicate that the density anomaly extends beyond the inner circular ring of mare ridges which define the Lamont feature. The occurrence of a magnetic high at Lamont further supports the presence of a major intrusive body probably emplaced early in lunar history. Several other local gravity or magnetic highs in the lunar mare are also interpreted as the result of intrusive activity. At Marius Hills, an isolated plateau in Oceanus Procellarum, the positive Bouguer anomaly is much smaller than at Lamont and is localized in the northeast corner of the plateau.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:(Planetary Science and Geophysics)
Degree Grantor:California Institute of Technology
Division:Geological and Planetary Sciences
Major Option:Planetary Sciences
Minor Option:Geophysics
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Unknown, Unknown
Thesis Committee:
  • Unknown, Unknown
Defense Date:19 February 1979
Funders:
Funding AgencyGrant Number
NASANAS 7-100
Record Number:CaltechTHESIS:06072024-205240041
Persistent URL:https://resolver.caltech.edu/CaltechTHESIS:06072024-205240041
DOI:10.7907/zpdn-2n37
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:16510
Collection:CaltechTHESIS
Deposited By: Tony Diaz
Deposited On:17 Jun 2024 20:52
Last Modified:17 Jun 2024 21:04

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