Volumes of Cryptomafic Deposits
on the Western Limb of the Moon:
Implications for Lunar Volcanism



by
Irene Antonenko



B. Sc., University of Toronto, 1989
B. App. Sc., University of Toronto, 1992
M. Sc., Brown University, 1995



Thesis



Submitted in partial fulfillment of the requirements for the
Degree of Doctor of Philosophy
in the Department of Geological Sciences at Brown University



May 1999



©Copyright 1999 by Irene Antonenko





Visitors since February 3, 1999



Table of Contents

Thesis Abstract

Volumes of Cryptomafic Deposits on the Western Limb of the Moon: Implications for Lunar Volcanism. Irene Antonenko, Brown University.

       Originally, the Moon was thought to consist of light, highland regions and dark, mare regions. The identification of dark-haloed craters has shown that some highland regions cover and obscure early volcanic units. Such hidden mafic, or cryptomafic, deposits have important implications for our understanding of lunar history.
       In Chapter 1, techniques are developed for studying the geometry of cryptomafic deposits using dark-haloed craters, which identify hidden mafic material by excavating and emplacing it around the crater. Large and small dark-haloed craters are used to estimate the thickness of these deposits. Areas are determined by their distribution. Cryptomafic deposits on the western limb of the Moon are found to be volumetrically significant (3.4 x 105 km3), contributing notably to the volume of volcanic material.
       However, the technique developed in Chapter 1 is limited by a poor understanding of the dark-halo-forming process. Impact experiments into layered sand targets were conducted to study this process. The results are presented in Chapter 2, where relations are derived for determining improved thickness estimates of cryptomafic deposits.
       The limited resolution of data used in Chapter 1 suggests that smaller dark-haloed craters may not be recognized, affecting thickness estimates. In Chapter 3, Clementine data, with its higher resolution and multispectral capabilities, is used to spectrally identify the presence of basalts on crater walls. Stratigraphy in the Schickard crater region is determined and layer thicknesses are estimated. A geologic history for this region is proposed.
       In the Chapter 4, estimates of volumes, areas, and ages are determined for all known or suspected cryptomafic deposits. Estimates of global mare volumes are updated. This study suggests that cryptomafic deposits may be extremely significant (1.5 x 106 km3), representing an important contribution to lunar volcanism, which may have been more voluminous early on in lunar history than previously suspected. Many additional cryptomafic deposits may still be undiscovered. Efforts should be made to identify and study cryptomafic deposits using the techniques presented in Chapters 1 through 3.

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Acknowledgements

        I find it very difficult to write these acknowledgements, because I don't know what to say. On the one hand, there are many wonderful people that made this dissertation possible; on the other, my graduate experience was a bitter disappointment and a deep source of regret.
        Let me begin with the positive. I have wanted to be a scientist for as long as I can remember. I've always known that I would spend many years in University, because learning was the thing I enjoyed doing most. For me, this meant getting my Ph.D., so that I could continue to work in academia and bask in the hallowed halls of learning for the rest of my life. This goal was never in question, only the path I would take to get there. Through all the problems and pitfalls along the way, my determination carried me, even when it looked like the sensible thing to do was to quit. Some people have another name for my determination and say that I am too stubborn for my own good. In their honour, I would like to acknowledge myself, my stubbornness, my determination, without which this dissertation would not have been possible.
        Of course, I would not be who I am without the love and support I have always received from my family. By their example, my mother Marina Antonenko and my grandmother Nina Petrovna Sekulovich showed me that I could do anything I wanted to. They lovingly accepted and tolerated of all my strange interests, which they often could not understand, but were proud of never-the-less. And it was they, along with my father Alexander Antonenko, who instilled in me this love of learning. They taught me that education was the most important thing in the world, and for this I would like to thank them. Of course, my two older sisters Liz and Annie Antonenko, had much to do with my formative years. Being older, they constantly challenged me to exceed beyond my years. Their continued love and support will be treasured always.
       Despite my stubbornness and the love of learning instilled by my family, I could not have completed this dissertation without the strength of my husband John Prinos. I cannot thank him enough. Through all the hell of grad school, my resolve remained strong because of his love, patience, understanding, friendship, and pride in my achievements. Even across 500 miles of distance, he walked every step of this harrowing journey with me, sometimes propping up my failing strength, sometimes just making me laugh. In the last year of my studies, he supported me financially so that I could focus on my research. I will always treasure his insistence that I finish this thesis, because it shows he understood how much this meant to me. John also did me the great favour of sharing his wonderful family with me. His parents, brother, aunts, uncles, and cousins have all accepted me as one of their own and are as proud of me as my family. I thank them for their support.
       While at Brown, a great many people made my life more tolerable with their unfaltering friendship. An unredeemable debt is owed to Aileen Yingst, who spent many hours trying to help me cope with my troubles, and to Sionnan French, who always knew how to make me laugh at them. Marty Gilmore awed me with her unique insights. Paul Haggerty, Mary Ellen Murphy, Jayne Aubel, Larry Crumpler, Sarah Curtis, Jean Waage, Catherine Hanni, and Bill Fripp could always be depended on for a sympathetic ear. Stefanie Tompkins, Jessica Sunshine, Sue Keddie, and Janice Bishop were not only my friends but my mentors, the only ones I really had at Brown. Cathy Weitz, Victor Zabielski, and all the geo-grads provided much needed distractions. Sasha Basilevsky and Misha Ivanov granted me an invaluable opportunity to practice my Russian language skills. The Graduate Student Council crew expanded my repertoire of friends beyond the geology department, particularly Dan Ostrov, Andy Flescher, Steve Bogusz, and Rich Pina, who made GSC meetings fun. My sincerest thanks go out to each and every one these people. But more than thanks are required for Margo Ballou, Charlotte Manley, Jason Dahl, Carolyn van der Bogert, and Nicole Spaun, who took turns housing me on my periodic visits to Brown in the last year of my study. Without their generous friendship, this thesis would not have been possible.
       A wide range of people has also made a contribution to this thesis in a variety of ways. Peter Neivert, Steve Pratt, Bill Collins, Linda Perotti, Debbie Glavin, and Dorcas Metcalf at Brown and Jim Charters at the University of Toronto, were instrumental in providing various technical and administrative support. Dr. Steve Scott, at the University of Toronto can be credited with introducing me to the idea of planetary geology in the first place, back in 1989. The UofT physics crowd, you know who you are, were my first colleges. We struggled through papers and problem sets together, and in the process you helped me discover how to learn. Dr. Dave McKay, Dr. Carl Allen, Dr. David Black and the LPI Internship program gave me my first taste of real planetary science; I am happy to say I liked it. Dr. Jeff Fawcette, Chair at the University of Toronto Department of Geology, took me in when my funding at Brown was cut. He gave me a TA'ing job, office space, and together with Dr. Ed Spooner made me realize I had something to contribute. The UofT geology grads, particularly my office mate Nawojka Wachowiak, welcomed me in my darkest hour. I am grateful for these contributions.
        At this time, it is customary to thank the thesis advisor, but I cannot. I had no advisor; after mismanaging my potential for over 5 years, my advisor Dr. Jim Head resigned from my committee, 4 months before the defense. I came to graduate school with the hopes of intellectual stimulation. I wanted to learn, to discuss, and to argue finer scientific points. Instead I found myself with a person whose mentoring style clashed terribly with my learning style. Research meetings would often be punctuated with "just do it" or "because I said so" in response to questions such as "help me?" or "why?" I could not work this way, so I suffered terribly. To Jim's credit, he recognized that I was suffering, but his declarations of "I want to help you" came across as insincere, since my opinions on the matter seemed to be ignored. Jim also tended to push me in research directions where he was not qualified to advise me, leaving me totally unsupported. As a result, my graduate experience at Brown turned out to be highly disappointing. Not only were my expectations not met, but my worst nightmares were surpassed. I really regret that Jim could not move beyond his own agenda to realize that my questions and arguments were not personal attacks, but rather my way of learning.
        As a result of Jim's departure, a great burden was laid on the members of my committee, Dr. CarlŽ Pieters, Dr. Marc Parmentier, and Dr. Paul Hess, as well as on Dr. Don Forsyth, the department Chair, who inherited me as a fall out of the ruckus. I would like to extend them my heartfelt thanks for standing by me, and to let them know that I appreciate the amount of time and work that went into seeing me through this unprecedented situation. I would also like to thank the members of my committee for their input into the thesis. Many of the comments were penetrating and insightful, and were truly appreciated. However, they would have been more appreciated if there had been more time to address them, and I regret that there was not more room for comments and discussion before the defense.
       Other people who were not on my committee proved to be very helpful to my research. To them I would also like to extend my thanks. Dr. Jack Mustard was often willing to discuss research, answer questions, or write a letter of recommendation. I regret not making use of his invaluable resources more. Dr. Mark Cintala proved to be a treasure. I truly value the e-mail discussions we had on my Chapter 2 and the edited and annotated drafts we exchanged over e-mail and the internet. These electronic discussions proved so valuable that I often would forget that we were several thousand miles apart. Mark was always available when I needed him and I truly appreciate all his efforts on my behalf.

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Preface

        Ever since human beings first looked up at the sky and saw the Moon, we have been aware that the lunar surface is composed of bright regions and dark regions. Galileo, the first person to look at the Moon through a telescope, recognized that the dark regions were smooth and relatively uncratered, while the bright regions were very rugged. He named these two types of regions maria, the Latin word for seas, and terrea, the Latin word for land. While the term terrea is rarely used in common scientific literature today, replaced by the term highlands, the term mare has persisted. However, it is now known, from the analysis of Apollo samples that were returned from the Moon, that the maria are not seas, but in fact represent massive deposits of flood basalts [i.e., Hieken, 1991]. Their presence on the lunar surface has profound implications for the total heat budget and thermal history of the Moon [Solomon and Head, 1979, 1980].
       Originally, it was believed that the surface of the Moon was very simple to understand. All the bright areas were thought to be highland crustal regions, and it was believed that all volcanic materials were represented by the dark mare regions [Head, 1975]. Only in the last 20 years has it been recognized that the lunar surface is considerably more complex. Studies of small impact craters have revealed that some highland regions cover and obscure early volcanic units [Schultz and Spudis, 1979, 1983], which have been called cryptomaria [Head and Wilson, 1992]. However, the mare nature of these units cannot be confirmed by remote sensing methods. Therefore, the term cryptomafic deposits may be more suitable to describe these hidden volcanic units.
        Cryptomafic deposits form when impacts into the highland crust excavate large quantities of bright highland material and emplace it on top of pre-existing volcanic units [i.e., Antonenko et al., 1995]. These highland ejecta deposits can be very thick, especially when formed by basin-sized impacts, and so tend to obscure the low albedo of volcanic deposits, making them difficult to recognize. The identification and study of cryptomafic deposits, however, is very important. Cryptomafic deposits are significant because they change our understanding of the flux and timing of lunar volcanism [i.e., Hess and Parmentier, 1995]. This has implications for our understanding of the thermal history of the Moon [Solomon and Head, 1979, 1980]. Therefore, it is imperative that these cryptomafic deposits be studied, their geometries understood, and their volumes determined.
       Generally, the assumption is made that these hidden mafic deposits were extruded onto the lunar surface before being obscured [i.e., Head and Wilson, 1992]. However, the extrusive or intrusive nature of these volcanic materials cannot be confirmed by remote sensing methods. Cryptomafic deposits may, therefore, represent intrusive volcanic materials.
        In Chapter 1, which has been submitted for publication in the Journal of Geophysical Research - Planets and is currently in the review process, the problem of determining the volume of cryptomafic deposits is approached. We develop techniques for determining the areas, thicknesses, and volumes of cryptomafic deposits, using impact-derived dark-haloed craters. Dark-haloed craters form when an impactor penetrates through the obscuring layer and excavates volcanic material [Schultz and Spudis, 1979, 1983; Antonenko et al., 1995]. The ejected volcanic material is emplaced around the rim of the newly formed crater and, over time, matures [Adams and McCord, 1973] to produce a dark-haloed crater. The presence of such dark-haloed craters can be used to determine the areal extent of cryptomafic deposits, providing estimates of the cryptomafic area. These craters can also be used to determine the thickness of cryptomafic deposits and the units that overlie them. In any given area, the excavation depth of the smallest dark-haloed craters that are observed can be used to estimate the depth to the top of the cryptomafic unit, or the thickness of the overlying layer. Similarly, the excavation depth of the largest dark-haloed craters that are observed in a given area can be used to estimate the depth to the bottom of the cryptomafic unit. The thickness of cryptomafic material can be determined from the difference between the depths to the top and bottom of the cryptomafic unit. Volumes can be determined by multiplying the thickness estimates by the area estimates.
       This technique is applied to a study area on the western limb of the Moon, where the presence of cryptomafic deposits in the Schiller-Schickard area, the area west of Mare Humorum, and the area west of Oceanus Procellarum is assessed. Using high sun-angle telescopic photos of the Moon, a survey of dark-haloed craters is conducted for this region. Craters are catalogued and their diameters measured. Depths of excavation are determined from the crater diameters and used to estimate the thicknesses and volumes of the cryptomafic units. Cryptomafic deposits on the western limb of the Moon are found to be volumetrically significant, adding a notable contribution to the volume of known volcanic deposits.
       However, the technique outlined above has several limitations. Most importantly, the halo-forming process is poorly understood, therefore, thickness estimates obtained from excavation depths of dark-haloed craters may be inaccurate. For example, some quantity of volcanic material must be excavated before a dark halo is formed. Therefore, the depth of excavation not only measure the depth to the top of the cryptomafic unit, but also includes the thickness of volcanic material that corresponds to the amount in the dark halo. To measure the thickness of the overlying layer accurately, the amount of penetration into the volcanic layer that is required for a dark halo to form must be known. Similarly, the amount of penetration into the underlying highland substrate that is required for a dark halo to be obscured, must be known in order to measure the bottom of the cryptomafic deposit accurately.
       In an effort to improve the accuracy of our thickness calculations, a series of experiments was begun to study dark halo formation. The results of these experiments are presented in Chapter 2, which is in preparation for submission. The experiments consist of impacts into layered targets of sand, which are constructed to simulate the light-dark-light layering of a cryptomafic deposit on the Moon. These experiments are conducted in three phases. The first phase, the simplest, is designed to find the depth of penetration into the dark layer that is required to produce a dark-haloed crater. In the second phase, an attempt is made to find the depth of penetration into the bottom light layer that is required to obscure a dark halo. In the third phase, attempts are made to improve the analogy between these experiments and the Moon by replacing the top light layer with a layer consisting of a mixture of light and dark sand. On the Moon, the obscuring layer often incorporates some volcanic material into the deposit during emplacement [Oberbeck, 1975]. In phase 3, therefore, a mixed top layer is simulated in order to study how this affects dark halo formation.
       On the basis of these experimental studies, equations are developed that provide more accurate estimates for the thickness of overlying layers, thus allowing thickness estimates of cryptomafic deposits to be improved. However, in using these equations, it is important to consider the differences between the experimental setting and conditions on the Moon. While an attempt is made to simulate lunar conditions as closely as possible, some factors cannot be accurately represented. The most important difference between these experiments and the Moon is scale. The lunar craters from the study in Chapter 1 range in size from 2 - 22 km, while the experimental craters of Chapter 2 range in size from 10 - 16 cm. The problems involved in scaling experimental craters to planetary sizes have long been recognized [i.e.Melosh, 1989]. However, the absence of other alternatives for cratering studies requires that these uncertainties must be accommodated. Scaling factors will have a bearing on the applicability of these experimental results to the Moon, and so these limitations should always be considered when applying the equations developed in Chapter 2 to lunar dark-haloed craters.
       Another limitation of the study from Chapter 1 is the resolution of the data sets that were used. Because of the relatively low resolution of this data, a concerned is raised that the smallest possible dark-haloed craters are not being recognized, thus affecting the thickness estimates of the cryptomafic deposits. In Chapter 3, therefore, attention is turned to the Clementine data set that, with its higher resolution and multispectral capabilities [Nozette et al., 1994], allows cryptomafic deposits to be studied in greater detail. Using the Clementine UVVIS camera data, we find it is possible to identify the presence of immature basalts in crater ejecta. Also, Clementine multispectral data can be used to provide spectral determinations of the composition along crater walls. This technique can, therefore, be used to determine the layering of the pre-impact surface.
        Using the Clementine data set, the stratigraphy of the Schickard crater region is determined and presented in Chapter 3. Crater diameters are measured and, with the aid of dark-haloed crater calculations, thicknesses of the various stratigraphic layers are estimated. The presence of two distinct cryptomafic layers is postulated, separated by a speculative unit of highland materials. On the basis of this stratigraphic model, a geologic history for the Schickard crater area of the Moon is proposed.
        In the final chapter, Chapter 4, a lunar survey of all known or suspected cryptomafic deposits is conducted, in order to summarize our understanding of global lunar volcanism. Estimates of volumes and areas are determined for each known deposit, using a variety of data sets. The age of each cryptomafic deposit is also roughly calculated. In addition, estimates of global mare volumes are recalculated, giving improved, updated values. The results of this study suggest that cryptomafic deposits on the Moon are extremely significant, representing an important contribution to known lunar volcanic areas and volumes. Furthermore, the average volcanic flux of earlier deposits may be comparable to fluxes in the late Imbrian period [Head and Wilson, 1992], implying that volcanism may have been more voluminous in the early period of lunar history than previously believed. Such findings would suggest a hotter lunar interior [Solomon and Head, 1979, 1980]and earlier melting of volcanic source regions [Hess and Parmentier, 1995], placing important constraints on models of lunar evolution.
       Doubtlessly, many cryptomafic deposits may still be undiscovered, thus the results of the Chapter 4 study should not be considered as comprehensive. Much work remains to be done to identify and catalogue all volcanic deposits on the Moon. However, the results of Chapter 4 do lay a strong foundation for future investigations of lunar cryptomafic deposits. Furthermore, the techniques presented in Chapters 1 through 3 of this thesis set a framework for how such cryptomafic deposits can be studied.


References

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Antonenko, I., J.W. Head, J.F. Mustard, and B.R. Hawke, Criteria for the detection of lunar cryptomaria, Earth, Moon, Planets, 69, 141-172, 1995.

Head, J.W., Lunar mare deposits: Areas, volumes, sequence, and implication for melting in source areas, in Origins of Mare Basalts and their Implications for Lunar Evolution, Lunar Science Institute, Houston, TX, 66-69, 1975.

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