Magnetism and geology of the moon

Transcription

1 University of Iowa Iowa Research Online Theses and Dissertations Spring 2017 Magnetism and geology of the moon Staci L. Tiedeken University of Iowa Copyright 2017 Staci L. Tiedeken This thesis is available at Iowa Research Online: Recommended Citation Tiedeken, Staci L.. "Magnetism and geology of the moon." MS (Master of Science) thesis, University of Iowa, Follow this and additional works at: Part of the Astrophysics and Astronomy Commons

2 MAGNETISM AND GEOLOGY OF THE MOON by Staci L. Tiedeken A thesis submitted in partial fulfillment of the requirements for the Master of Science degree in Astronomy in the Graduate College of The University of Iowa May 2017 Thesis Supervisor: Associate Professor Jasper S. Halekas

3 Copyright by STACI L. TIEDEKEN 2017 All Rights Reserved

4 Graduate College The University of Iowa Iowa City, Iowa CERTIFICATE OF APPROVAL This is to certify that the Master's thesis of MASTER'S THESIS Staci L. Tiedeken has been approved by the Examining Committee for the thesis requirement for the Master of Science degree in Astronomy at the May 2017 graduation. Thesis Committee: Jasper S. Halekas, Thesis Supervisor Donald A. Gurnett Steven R. Spangler

5 ACKNOWLEDGEMENTS A special thanks to Dr. Jasper Halekas for providing me the opportunity to delve into lunar research and for his support during the completion of my Master of Science degree. I would also like to thank Dani Law and Stephanie Howard for their feedback and encouragement which have allowed me to successfully complete this research. A final thank you to my parents for their unwavering support throughout my educational journey and to my brother whose coding knowledge has been invaluable. ii

6 ABSTRACT Since different parts of the Moon display varying magnetic field strengths, our goal was to determine whether these differences are due to specific geological characteristics. We found that older materials tend to be more magnetic than younger materials. Additional statistical studies found that the mare regions of the Moon are less magnetic than the plains and terra regions. We did not find a simple relationship between lunar magnetism and crustal thickness, and this is inconsistent with the hypothesis that thicker crust is more magnetic since there is additional material. Thus, it is not just a matter of the amount of magnetic material that determines the magnetic field strength of the lunar crust. Our results demonstrate that magnetism and crustal thickness have a complex relationship, with multiple distinct groups corresponding to various regions of the Moon. The lunar maria formed a particularly distinct group, consisting of low magnetism and thin crust, while the lunar highlands consist of thick crust but typical magnetic field values. The ejecta thickness and magnetic field distributions for specific craters showed that larger impact basins have a thicker and more widespread ejecta blanket than smaller craters. We did not find a consistent pattern of magnetic field enhancements near specific craters, but evidence for these strong magnetic signatures was present for Mare Crisium and Mare Nectaris. These results may support the hypothesis that ejecta materials are carriers of magnetism, and this may be the reason for their tendency to have higher magnetic field strengths. iii

7 PUBLIC ABSTRACT The Moon consists of a wide range of magnetic field strengths. The Moon is also composed of different aged materials and geologic terranes, such as highland regions and lunar maria (the dark patches one sees when looking up at the Moon). Thus, it is plausible that the differences in magnetism may be due to the variations in ages and terranes. Indeed, we found that mare material tends to have the weakest magnetism and that the oldest material is the most strongly magnetic. The crustal thickness of the Moon also varies, and we hypothesized that thicker crust would display higher magnetic field values since there would be additional magnetic material. However, we discovered a complex relationship with multiple distinct magnetism/crustal thickness groups corresponding to various parts of the Moon. These groups outlined specific lunar regions, including the lunar maria and highlands, when they were plotted on a map. When an asteroid or other impactor hits the Moon, ejecta material is spewed outwards from the impact. These ejecta materials can maintain their magnetism, so one might expect to find high magnetic field values around large impact basins. However, we did not find clear evidence for these magnetic enhancements. iv

8 TABLE OF CONTENTS LIST OF TABLES... vi LIST OF FIGURES... vii CHAPTER 1 INTRODUCTION The Lunar Magnetic Field Appearance Origins Relation to Lunar Geology Lunar Crustal Thickness Lunar Elemental Abundances CHAPTER 2 DATA ACQUISITION Lunar Prospector (LP) Magnetic Field Measurements LP Measurement of Elemental Abundances United States Geological Survey (USGS) Gravity Recovery and Interior Laboratory (GRAIL) Spacecraft Overview Measuring Lunar Crustal Thickness Clementine CHAPTER 3 RESULTS Lunar Geology Lunar Crustal Thickness Comparison to Topography Comparison to Magnetism Appearance of Distinct Groups Distinct Groups Comparison by Geologic Terrane and Epoch Comparison by Elemental Abundances Group 6 vs. Group Magnetic Field Strength in Relation to Distance from Specific Impact Craters Ejecta Thickness Magnetic Field Distributions CHAPTER 4 CONCLUSIONS REFERENCES v

9 LIST OF TABLES Table 3.1. Magnetic Field Values for Certain Epochs and Geologic Terranes Table 3.2. The Cutoffs Used for the Partitioning Scheme Table 3.3. Geologic Terrane and Epoch Percentages for Each GRAIL Group Table 3.4. Average Elemental Abundances for Each GRAIL Group Table 3.5. Lunar Impact Basins (after Wilhelms, 1987) vi

10 LIST OF FIGURES Figure 1.1. The global distribution of surface crustal magnetic field intensity is shown on a shaded relief map of the Moon....3 Figure 1.2. The lunar geologic time scale Figure 1.3. The global distribution of lunar crustal thickness as measured by GRAIL gravity and LRO topography Figure 2.1. From Mitchell et al. (2008) illustrating the magnetic mirror effect Figure 2.2. A schematic of the electron reflectometry technique (from Lin et al., 1988) Figure 2.3. A diagram of the electron pitch angle distribution measured at the spacecraft Figure 2.4. The Lunar 5M Geologic Map Renovation Figure 2.5. Geologic terranes of interest from the Lunar 5M Geologic Map Renovation Figure 2.6. A schematic of the GRAIL mission (obtained from NASA/JPL) Figure 3.1. Sample images of different lunar terrane types Figure 3.2. Magnetic field distributions for three of the epochs analyzed Figure 3.3. Magnetic field distributions for three of the terranes analyzed Figure 3.4. Lunar topography as a function of crustal thickness Figure 3.5. A 2D histogram of the magnetic field as a function of the GRAIL lunar crustal thickness Figure 3.6. A 2D histogram of the magnetic field as a function of the GRAIL lunar crustal thickness, along with the resulting distinct groups after the partitioning scheme was implemented Figure 3.7. A map, centered on the lunar nearside, of the distinct GRAIL lunar crustal thickness groups as they appear on the Moon vii

11 Figure 3.8. A map, centered on the lunar nearside, of some of the distinct GRAIL lunar crustal thickness groups as they appear on the Moon Figure 3.9. A map of GRAIL lunar crustal thickness groups 6 and Figure Top: Ejecta thickness models for various lunar maria. Bottom: Ejecta thickness model for the South Pole-Aitken basin Figure Crustal thickness as a function of distance from crater center for Mare Humorum (top) and Mare Crisium (bottom) Figure Crustal thickness as a function of distance from crater center for Mare Serenitatis (top) and Mare Nectaris (bottom) Figure Crustal thickness as a function of distance from crater center for Mare Orientale (top) and the South Pole-Aitken basin (bottom) viii

12 CHAPTER 1 INTRODUCTION 1.1 The Lunar Magnetic Field The Moon has a magnetic field that is quite different from that of the Earth. It is well established that the Earth s magnetic field is generated by a dynamo action which is driven by the convective motion in a fluid core (Elsasser, 1947). However, scientists remain uncertain of whether the Moon ever had a dynamo, and if it did, how long ago it was active. Today, the Moon lacks a global magnetic field and instead has extensive crustal magnetism. The average strength of the Moon s magnetic field is about 8 nt while that of the Earth is roughly μt; thus, the Moon s magnetic field is about 10,000 times weaker. In addition, the solar wind provides an ambient magnetic field strength around 5-10 nt (Lin et al., 1988). Samples returned from the Apollo program have shown that the main ferromagnetic carriers on the Moon consist of microscopic metallic iron particles in the reducing lunar environment; this contrasts with terrestrial magnetization which is typically carried by iron oxides (Hood et al., 2001; Mitchell et al., 2008). The origin of the crustal magnetic anomalies remains debated, but numerous theories have been proposed, many of which focus on the hypothesis that major lunar impact events are important factors in the observed distribution of crustal magnetism. The Lunar Prospector mission had the important tasks of recording gravity, magnetic, and compositional data at high resolution for the entire Moon. Before Lunar Prospector, the Apollo, Luna, and Clementine missions gathered data about various aspects of the Moon, including its major terranes, composition, rock types, and magnetic 1

13 field; however, both the resolution and coverage of the polar regions were poor, and the identities of the crustal magnetic anomaly sources were not well understood (Binder, 1998; Hood et al., 2001). Lunar Prospector was thus designed to improve the measurements made from previous missions, and one of its major accomplishments was the creation of a global lunar crustal magnetic field map Appearance The Moon s magnetic field is not a replica of the Earth s; indeed, it is far from it. Numerous strong magnetic anomalies exist in specific regions of the Moon, as discovered by Lunar Prospector in addition to the Apollo missions. Regions antipodal (diametrically opposite) to four of the largest lunar impact basins - Imbrium, Orientale, Crisium, and Serenitatis - have some of the strongest magnetic field concentrations on the Moon (Lin et al., 1988). Even though the Apollo 15 and 16 subsatellites only covered a range within 30 degrees of the equator, they were still able to find thousands of patches of strong surface magnetic fields dispersed throughout the lunar surface; these patches ranged in size from less than roughly 7 km, which was the resolution limit of the observations, to greater than 500 km (Lin et al., 1988). Figure 1.1 shows the global distribution of lunar crustal magnetic fields, where obvious strong anomalies are present. The most prominent of these is the region corresponding to the antipodes of the Imbrium and Serenitatis impact basins (denoted by 3 and 7, respectively, in Figure 1.1) which have surface fields greater than 40 nt. The high concentration of strong anomalies on the south-central farside of the Moon coincides with the northern rim of the South Pole-Aitken (SPA) basin, which is the oldest and largest distinguishable lunar basin (Hood et al., 2013). The Imbrium basin itself 2

14 Figure 1.1. The global distribution of surface crustal magnetic field intensity is shown on a shaded relief map of the Moon. The white circles denote the main rims of the 15 most recent lunar impact basins while the black circles are antipodal to the white circles (see Table 1 of Mitchell et al., 2008 for the numbering scheme). The magnetic field measurements each have a spatial resolution of 5 km and are binned into 5x5-degree elements (1 deg = 30 km at the lunar equator). Adapted from Mitchell et al., corresponds to the largest concentration of weak surface fields (< 0.2 nt). In addition, there are a few smaller isolated anomalies located near regions of magnetic lows. 3

15 1.1.2 Origins Because the Moon presently has no global magnetic field, but crustal magnetic fields were found by Apollo subsatellites, it is natural to wonder how these fields originated. However, the origin question is difficult to answer because the crustal magnetization that we see today could have formed as a result of impact-related processes or due to a core dynamo. Returned lunar samples display magnetization in the form of natural remanent magnetization (NRM), and it is thought that ancient magnetic fields must have produced this magnetization (Garrick-Bethell et al., 2009). However, lunar rocks are often inept at recording magnetic fields, as most highland samples are brecciated and/or shocked, thus making it difficult to determine whether NRM was acquired instantaneously during shock-magnetization or from long-lived dynamo fields (Garrick-Bethell et al., 2009). Rocks may acquire NRM in a variety of ways, but thermoremanent magnetization (TRM) and shock remanent magnetization (SRM) are the most important for our purposes. If a rock cools in the presence of a steady ambient field, it will pass through a series of critical temperatures at which the magnetic carriers acquire spontaneous magnetization; these critical temperatures are called the Curie points or Curie temperatures and are different for each mineral (Fowler, 2005). Once the rock s temperature is below the blocking temperature, which is typically tens of degrees less than the Curie point, the magnetized grains cannot be reoriented; thus, their magnetic moments will remain aligned with the direction of the ambient field that was present at the time of cooling (Fowler, 2005). This type of permanent residual magnetization is called TRM. 4

16 On the other hand, if an ambient magnetic field is present when a transient shock occurs, then SRM can be generated (Hood & Huang, 1991). The intensity of SRM is proportional to the strength of the ambient field present at the time of shock, and SRM is parallel to this field (Gattacceca et al., 2008). The intensity of SRM increases linearly with the ambient field and for shock pressures up to 45 MPa; this intensity still increases, but at a decreasing rate, for pressures up to one GPa (Gattacceca et al., 2008). Although SRM is not as large as TRM, experimental results demonstrate that SRM may be more efficient, with this efficiency defined by the ratio of SRM intensity to TRM intensity acquired in the same field (Gattacceca et al., 2008). Gattacceca et al. (2008) found that SRM versus TRM acquisition is 17% for basalt samples and is 36% for microdiorite samples; this efficiency depends on the magnetic mineralogy and increases with decreasing coercivity. These relatively high efficiency values demonstrate that a significant remanent magnetization can be acquired by the crust during a hypervelocity impact on the lunar surface in the presence of an ambient field. Note that both TRM and SRM may act to demagnetize in the absence of a strong ambient magnetic field as well. As mentioned earlier, the strongest lunar magnetic fields are found antipodal to large impact basins. These impact basins were formed after meteoroids traveling at high velocities impacted the Moon. One theory pertaining to the formation of these antipodal magnetic anomalies has been developed by Hood & Huang (1991). According to this theory, a hypervelocity (>10 km/s) impact of a silicate meteoroid produces a cloud of partially ionized gas, along with solid and molten ejecta. This plasma cloud then expands around the Moon in a few hundred seconds, compressing and amplifying any ambient magnetic fields at the antipode. The compression lasts for up to roughly a day which is 5

17 not a long enough period of time to allow for the cooling of a large body of rock, so TRM is unlikely. However, large shock pressures sufficient for acquisition of SRM may be produced, and so SRM associated with the focusing of seismic energy at the antipode and with antipodal ejecta deposition may occur (Mitchell et al., 2008). Another theory pertaining to the antipodal anomalies focuses on the deposition of iron-rich ejecta from the SPA impactor as the source. The northern rim of the huge SPA basin corresponds to the antipodes of Imbrium and Serenitatis while the Crisium antipode lies just outside the basin. According to Wieczorek et al. (2012), SPA ejecta is responsible for some of the most prominent grouping of strong magnetic anomalies. They used a series of impact simulations to track the fate of projectile materials from the SPA impact event and found that differentiated projectiles with silicate mantles, iron cores, and impact angles of 45 most easily account for the strong anomalies located near the rim of the SPA basin. Lunar magnetization as acquired by a core dynamo is also a possibility. The Moon has a small partially molten metallic core with a diameter of roughly 330 km, so the presence of a core dynamo could have been possible (Shea et al., 2012). However, the plausibility of a lunar dynamo has been questioned because of the difficulty in sustaining a dynamo for a long time after accretion. In order to identify records of a core dynamo field, one must study slowly cooled samples with high magnetic recording precision that display no signs of shock; unfortunately, there exist few lunar rocks with all these properties (Shea et al., 2012). These constraints are necessary in order to rule out magnetic acquisition by other means such as SRM. However, there is one such lunar rock that meets all these properties, and it is called troctolite sample 76535, the oldest 6

18 known unshocked lunar rock (Shea et al., 2012). Troctolite was found to have a stable NRM which was formed in a magnetic field with a strength on the order of microteslas (Garrick-Bethell et al., 2009). According to Garrick-Bethell et al. (2009), the magnetic history of this sample, in addition to its slow cooling time scale, suggests that the Moon possessed a dynamo field 4.2 billion years ago. The lifetime of this early lunar dynamo remains uncertain, however Relation to Lunar Geology The Moon is a complex body with vastly differing landscapes, ranging from flat plains regions to the hilly highlands, and distinct regions of the Moon offer varying magnetic field strengths. Even though numerous studies have been conducted, there is a lack of clear relationships between lunar magnetism and lunar geology. This absence of correlation has contributed to the difficulty in determining the origin of the lunar crustal fields. Specific relationships have been discovered, but few overwhelming correlations across multiple regions of the Moon have been found. As mentioned previously, large impact basins such as the Imbrium, Orientale, Serenitatis, and Crisium basins have weak magnetic fields while their antipodal regions have exceptionally strong magnetic fields. Monte Carlo simulations have shown that the antipodal magnetic enhancements of these large impact basins are unlikely to be due to chance; thus, impact events seem to dominate the large scale distribution of surface magnetic fields (Mitchell et al., 2008). However, most basins do not have anomalies located at their antipodes, and the locations of many strong anomalies are not associated with basin antipodes, so we cannot assume that the antipode of an impact basin will have a strong magnetic field. 7

19 The discovery of unusual lunar terranes at the antipodes of major impact basins supports the earlier mentioned hypothesis of Hood & Huang (1991). These modified terranes, which are considered to be furrowed and pitted or grooved and mounded are likely to have formed from the antipodal convergence of ejecta or seismic body waves (Hood et al., 2013). In addition, these unusual terranes tend to be associated with stronger magnetic fields, although this correlation is not very strong (Hood et al., 2013). The magnetic anomalies on the lunar nearside are thought to be due to basin ejecta. The Cayley Formation, lunar light plains in the region south and southeast of the Imbrium basin, is a region of very strong magnetism (Halekas et al., 2001). It has multiple origin stories, but it most likely consists at least partly of Imbrium primary and/or secondary ejecta, mixed with reworked local material (Halekas et al., 2001). Another region associated with a strong magnetic anomaly is that of the Descartes Mountains which are most likely ejecta material associated with either the Nectaris impact basin or the Imbrium event (Hood et al., 2013). This anomaly is also quite near the Apollo 16 landing site (15.3 E, 8.6 S) (Halekas et al., 2001). From these results, one might conclude that ejecta material is always associated with strong magnetic fields. However, the Fra Mauro Formation, which partially consists of Imbrium basin ejecta, lacks any clear correlations with magnetic anomalies (Halekas et al., 2001). Thus, a clear-cut relationship between ejecta material and high magnetism is still lacking, so we can only say that ejecta material tends to be correlated with high magnetism, but that it is not always the case. Three strong anomalies also lie along the southwestern edge of Oceanus Procellarum: the northern one is Reiner Gamma, the southeastern one is Rima Sirsalis, 8

20 and the southwestern one is unnamed (Halekas et al., 2001). The Rima Sirsalis rille is located just northwest of a very strong magnetic anomaly that roughly coincides with the center of an Imbrian-aged smooth plains unit; this plains unit is thought to be a deposit of primary or secondary basin ejecta (Hood et al., 2001). Since there is a tendency for ejecta materials to contain relatively large amounts of microscopic metallic iron (the lunar magnetic carrier), it is hypothesized that the smooth plains unit is the source of the main Rima Sirsalis anomaly and not the rille itself (Hood et al., 2001). Indeed, Halekas et al. (2001) conducted a study of 77 named nearside rilles and were unable to find any other clear associations between rilles and magnetic anomalies. Reiner Gamma is a so-called swirl albedo marking, and two main theories have been proposed to explain the origin of these lunar swirls. Schultz & Srnka (1980) hypothesize that a cometary impact that scoured the lunar surface created the unusual Reiner Gamma albedo marking. On the other hand, Hood & Williams (1989) theorize that the swirl represents an area whose albedo has been preserved against surface darkening. In this theory, the solar wind is deflected by the associated magnetic anomaly which preserves the surface against optical maturation. Since the Reiner Gamma anomalies are oriented radial to the Imbrium impact basin, it is thought that Imbrium ejecta material beneath the surface is the most probable source for the magnetic anomalies (Hood et al., 2001). Trends in age with lunar crustal magnetism have also been discovered. It is not possible to absolutely date impact basins, unless lunar samples originating from specific craters have been collected. However, one can use other means such as superposition relations and crater counting statistics to establish relative ages. For example, Shoemaker 9

21 & Hackman (1962) analyzed the Copernicus region of the lunar surface and found that major craters in this region were surrounded by an overlapping series of deposits. These materials were then grouped into five stratigraphic subdivisions, each corresponding to a different lunar time period. By the law of superposition, the inner deposits had to be older than the deposits overlying them. Numerous studies have since been conducted on the lunar geologic time scale, and an updated version is shown in Figure 1.2. The age dependence of lunar crustal magnetism is more prominent than that of correlations with geologic terranes. Imbrian age and younger impact basins tend to have lower magnetic fields than the surrounding regions (Halekas et al., 2001). Crater materials corresponding to the Imbrian, Eratosthenian, and Copernican epochs all have average magnetic fields of roughly one nanotesla (Halekas et al., 2001). In addition, studies of demagnetization have shown that younger craters are slightly more demagnetized than older craters; thus, some sort of remagnetization process must have occurred in older craters, or magnetized materials may have subsequently been deposited within them (Halekas et al, 2002b). 10

22 Period Imbrium Figure 1.2. The lunar geologic time scale. Also shown are some of the events which occurred during these periods. Adapted from Christiansen & Hamblin (1995). 11

23 1.2 Lunar Crustal Thickness NASA s Gravity Recovery and Interior Laboratory (GRAIL) mission provided high-resolution measurements of the spatial variations in the Moon s gravity field which revealed subsurface density variations (Wieczorek et al., 2013). Constraints on the crustal density and porosity were then made, and a global crustal thickness model was constructed from GRAIL gravity and Lunar Reconnaissance Orbiter (LRO) topography data (Wieczorek et al., 2013). The variations in lunar crustal thickness are dominated by impact basins with diameters ranging in size from 200 to 2000 km, as shown in Figure 1.3 (Wieczorek et al., 2013). The Crisium and Moscoviense impact basins have interior thicknesses close to zero, while numerous others have thicknesses less than 10 km. The South Pole-Aitken basin is represented by the largest region of thin crust on the lunar farside. As one would expect, the largest impact basins have the thinnest crust since these impact events would have excavated through most of the crustal column. In addition, some large impact basins may have actually excavated through the entire crust and into the mantle, and this observation is supported by remote sensing data which show unusual exposures of olivine-rich materials near some impact basins, representing a mixture of crustal and mantle materials (Wieczorek et al., 2013). 12

24 Figure 1.3. The global distribution of lunar crustal thickness as measured by GRAIL gravity and LRO topography. Both the lunar nearside (left) and farside (right) are represented (Wieczorek et al., 2013). The depth to which the lunar crust is magnetized depends on the extent of the magnetizing fields, and the types of sources such as ejecta material (Carley et al., 2012). The magnetization thickness may exist as a thin layer of impact ejecta or mare basalt, or it may exist within the entire thickness of the crust which is 49 ± 16 km on average, as estimated from Lunar Prospector and Clementine gravity and topography data (Carley et al., 2012). Halekas et al. (2002b) found an upper bound to the thickness of the lunar magnetized layer of ~50 km, possibly representing the maximum depth at which magnetic carriers are located. On the other hand, for the Earth, the entire crust extending down to the crust/mantle boundary may be magnetized in the presence of the global magnetic field (Carley et al., 2012). 13

25 1.3 Lunar Elemental Abundances In addition to mapping the lunar crustal magnetic fields, Lunar Prospector mapped the lunar surface abundances using both a gamma ray spectrometer and a neutron spectrometer. Uranium, thorium, and potassium are the most abundant elements found in KREEP-rich rocks on the Moon (Binder, 1998). KREEP rocks consist of high amounts of potassium, rare earth elements, and phosphorus, and these rocks are important in understanding crustal evolution since they may signify the last remaining melt after the lunar crust was created (Binder, 1998). In addition, 98% of the mass of all lunar material is composed of oxygen, silicon, magnesium, iron, titanium, aluminum, and calcium, and global distribution maps of these elements can shed light on the origin and development of the Moon (Binder, 1998). From the Lunar Prospector gamma ray data, it is evident that KREEP-rich material is mainly found in the rim areas of Mare Imbrium, the nearside maria and highlands near Imbrium, and the Mare Ingenii South Pole-Aitken basin area on the farside, while there is a relatively low concentration of KREEP material in the highlands (Binder, 1998). These results add support to models which show that the Imbrium impact event excavated KREEP-rich material and dispersed it over the Moon (Binder, 1998). Likewise, the impactor that created the huge South Pole-Aitken basin also exposed KREEP-rich rocks from depth (Binder, 1998). The Lunar Prospector gamma-ray data also found that the maria regions of the Moon, which are large basalt flows, consist of iron, and this result agrees with abundances recorded by Clementine data (Binder, 1998). 14

26 CHAPTER 2 DATA ACQUISITION 2.1 Lunar Prospector (LP) In order to analyze both the lunar crustal magnetic field and lunar elemental abundances, data from the Lunar Prospector (LP) mission was used. LP was launched on January 7, 1998 and commenced its mapping of the Moon on January 16, 1998 (Binder, 1998). It had a 118-minute near-polar orbit and spent the first year of its mission at an altitude of 100 km above the Moon s surface (Binder, 1998; Hood et al., 2001). As the mission progressed into its final six months, the spacecraft s orbit was lowered to an altitude range of 15-to-30 km in order to obtain improved mapping of crustal magnetic and gravity fields (Hood et al., 2001). In addition, most of the crustal magnetic field measurements were made in the Earth s geotail or in the solar wind wake Magnetic Field Measurements It is not possible to directly measure the lunar crustal magnetization distribution itself from orbit, but it can be inferred by measuring the magnetic field distribution. So long as the magnetic coherence scale is not too great, magnetic field measurements allow one to gain useful information about the magnetization distribution. A low coherence scale means that large uniformly magnetized bodies are not present. On scales larger than the magnetic coherence scale, regions of strong magnetic fields also correspond to regions of strong magnetization; this is true for regions of weak magnetic fields as well. Halekas et al. (2002b) found the coherence scale of the lunar magnetic field to be roughly less than 25 km. Thus, since the Moon has a rather small coherence scale, crustal 15

27 magnetic field measurements are suitable approximations to the crustal magnetization distribution. There were two instruments aboard the LP spacecraft with the task of measuring lunar crustal fields: the magnetometer and the electron reflectometer. The magnetometer records the vector field directly at the altitude of the spacecraft by detecting perturbations caused by crustal sources to the ambient field at the spacecraft, and the electron reflectometer indirectly measures the surface field strength by recording the pitch angle α (the angle between the electron velocity vector and the magnetic field) distributions of electrons reflected from crustal magnetic fields (Hood et al., 2001; Mitchell et al., 2008). The magnetometer is able to directly detect polarity information and is more properly suited to mapping strong regions of magnetic fields, while electron reflectometry is able to resolve weaker fields, making it better suited to mapping moderate regions of magnetic fields (Halekas et al., 2001). The electron reflectometry technique can measure surface magnetic field strengths in two ways. First, the ratio of upward going to downward going electron fluxes, called the electron reflection coefficient, can be determined (Halekas et al., 2001). Second, the loss cone angle or cutoff angle can be measured (Mitchell et al, 2008). Crustal magnetic field measurements as obtained from electron reflectometry determination of loss cone angles were used in our analysis. However, electron reflection coefficients obtained by electron reflectometry have also been used to produce maps of crustal anomalies (Halekas et al., 2001). In addition, results obtained by the LP magnetometer have been used in previous studies (see Hood et al., 2001; Hood et al., 2013). 16

28 Electron reflectometry relies on the fact that charged particles have the tendency to be reflected from regions of increased magnetic field strength. In a uniform constant magnetic field, a charged particle will travel in a helical trajectory with a constant velocity parallel to the magnetic field and a circular motion perpendicular to the field (Lin et al., 1988). However, if the magnetic field is not constant and is instead increasing in strength, then the charged particle will experience a force that will tend to decrease its parallel velocity until the particle is eventually reflected from the region of increased magnetic field (Lin et al., 1988). The magnetic field where this occurs is known as the mirror magnetic field, and this process is called the magnetic mirror effect, as shown in Figure 2.1. Figure 2.1. From Mitchell et al. (2008) illustrating the magnetic mirror effect. Instantaneous velocity vectors are represented for a charged particle traveling through a converging (slowly increasing) magnetic field. Solid vectors are pre-reflection and dashed vectors are post-reflection. 17

29 A tenuous plasma with an ambient magnetic field strength around 5-10 nt fills the interplanetary medium (Lin et al., 1988). The presence of the Moon alters the path of the charged particles so that they are guided by the ambient field to the lunar surface. If there are no regions of enhanced surface magnetic fields, then most of the particles are absorbed by the surface; less than around 5% are Coulomb backscattered from surface material (Lin et al., 1988). On the other hand, if surface magnetic fields are present, the total field strength increases as the particles approach the region, and this results in reflection of a portion of the particles, as demonstrated in Figure 2.2 (Lin et al., 1988). The electron reflectometry technique allows one to map the surface magnetic fields with high sensitivity (~0.2 nt) and spatial resolution (~4 km) (Halekas et al., 2001). Figure 2.2. A schematic of the electron reflectometry technique (from Lin et al., 1988). 18

30 Whether the electrons will be reflected depends on the pitch angle α (Halekas et al., 2002a). The upward-going reflected electrons are only seen up to a certain cutoff pitch angle αc; beyond this angle, electrons are no longer reflected and are instead absorbed by the lunar surface (Halekas et al., 2002a). This region devoid of particles is known as the loss cone and is represented in Figure 2.3 (Mitchell et al., 2008). The point at which a charged particle is reflected is dependent on its initial velocity and the magnetic field. If the magnetic field varies spatially and the fractional change in the field is small over the distance traveled by an electron in one gyration, then we can make the assumption that both kinetic energy and magnetic moment are conserved (Mitchell et al., 2008). The cutoff pitch angle αc can then be expressed as sin 2 α c = B sc B surf [2.1] where Bsc is the magnetic field at the spacecraft and Bsurf is the magnetic field at the lunar surface. 19

31 Figure 2.3. A diagram of the electron pitch angle distribution measured at the spacecraft. Electrons that reflect before they reach the lunar surface will travel back up the magnetic field line, while those that would have reflected after hitting the surface will be absorbed (i.e. those with pitch angles beyond the cutoff angle). (Mitchell et al., 2008). Previous studies found it necessary to correct the LP measurements for differential charging of the lunar surface and the LP spacecraft (Halekas et al., 2002a). LP data had shown that electrons are reflected by both electric fields and magnetic fields, so it was necessary to correct for this to ensure that only magnetic field measurements were being recorded. By adding electric fields into the analysis, Halekas et al. (2002a) showed that Equation 2.1, corrected for differential charging, becomes sin 2 α c = B sc (1 e U ) [2.2] B surf E where e is the electron charge, U is the potential difference between the lunar surface and LP (typically -35 V on the lunar night side), and E is the kinetic energy of the particles at LP. Measurements of α c at different energies (220, 340, and 520 ev) were used to determine U. Not all magnetic field measurements needed to be corrected 20

32 though. For surface fields stronger than 10 nt, the effect of the potential drop was small compared with measurement errors, so the electric field corrections could safely be ignored (Mitchell et al., 2008). The electron reflectometry measurements can then be converted into estimates of crustal field strength by noting that the surface magnetic field Bsurf is the vector sum of the ambient field in which the Moon is immersed (the solar wind field or the Earth s magnetotail field) which can be estimated as the field measured at the spacecraft Bsc, and the crustal field Bc, i.e. Bsurf = Bsc + Bc (Lin, 1979). The location on the lunar surface being measured can be determined by extrapolating the magnetic field measured by the spacecraft magnetometer to where it intersects the surface since electrons follow the magnetic field lines. In order to find Bc, the direction of Bc must be known, but electron reflectometry only allows for scalar measurements. However, we can still determine a minimum value to the strength of the crustal field as given by Equation 2.6 (Mitchell et al., 2008). B c B c B surf B sc [2.3] B sc (1 e U ) B sin 2 α c E sc [2.4] B c B sc ( 1 sin 2 α c 1) B c B sc B sc e U tan 2 α c sin 2 α c E B sc e U sin 2 α c E [2.5] [2.6] In our analysis, we used the magnetic field data obtained from the electron reflectometer, binned at one degree resolution and boxcar-smoothed over 3x3 degrees. The crustal magnetic field values were found using Equation 2.6. This dataset consisted of 21

33 65,160 points, but some of these had NaN magnetic field values, so after excluding all the NaN values, our final magnetic field dataset consisted of 50,288 measurements LP Measurement of Elemental Abundances LP used both a gamma ray spectrometer and a neutron spectrometer to map lunar surface abundances. The resulting gamma ray spectra were analyzed to determine the abundance of major elements (O, Mg, Al, Si, Ca, Ti, and Fe) and radioactive elements (K, Th, and U) within a few tens of centimeters of the surface of the Moon (Prettyman et al., 2006). Results for Fe and Th were easiest to obtain since these elements had intense, wellresolved spectral features (Prettyman et al., 2006). In order to determine the abundance of elements such as Mg, K, and Ti, which lacked well-resolved lunar gamma rays, a spectral unmixing algorithm was developed; this technique allows one to figure out the contribution by gamma rays from each element to the observed spectrum (Prettyman et al., 2006). Lunar elemental abundances acquired by the gamma ray spectrometer aboard the LP spacecraft during the high altitude (~100 km) phase of the mission, as obtained from the Planetary Data System (PDS), were utilized in our study. 2.2 United States Geological Survey (USGS) The geologic data used in this study was obtained from the United States Geological Survey (USGS) Lunar 5M Geologic Map Renovation. In 2013, renovations to a series of six geologic maps that cover the entire Moon at a scale of 1:5,000,000 and which were originally created in the 1970 s were completed. The renovations used new topographic data and image mosaics and made the original work more digital-friendly. Each of the six geologic maps corresponds to a different region of the Moon: the lunar north, south, east, west, near, and far sides. When combining the corresponding maps 22

34 onto one whole map of the Moon, some of the regions overlapped. Thus, in order to avoid duplicate data points, we only kept those that corresponded to the most recent geologic map (i.e. for those overlapping coordinates corresponding to both the near side and the north side, only those from the north side were kept since the original north side map is from 1978 while that of the near side is from 1971). These lunar geologic maps used data from both the Lunar Orbiter (LO) and Lunar Reconnaissance Orbiter (LRO) missions. The LO program, which was active from August , consisted of a series of five unmanned missions tasked with imaging the Moon from orbit (Wilhelms, 1987). The results from regional coverage of Lunar Orbiter IV were particularly useful in the creation of the original geologic maps. In order to ascribe relative ages to different parts of the Moon, crater counting and superposition relationships were utilized. A region consisting of a high number of craters will be older than a region with few craters. This is because younger regions have been resurfaced, resulting in the filling in of impact craters by lunar material. In addition, superposition relationships may be used to date material. For example, if a crater is present on the rim of another crater, then that crater must be younger than the crater on which it rests. LRO was launched from Kennedy Space Center on June 18, 2009 and mapped the lunar topography using the Lunar Orbiter Laser Altimeter (LOLA) instrument, in addition to imaging the Moon with the Lunar Reconnaissance Orbiter Camera (LROC) (Vondrak et al., 2010). Both the LO and LRO missions were instrumental in renovating the original geologic maps, and this renovated map is shown in Figure 2.4. The geologic terranes that we were interested in are highlighted in the map in Figure 2.5. The white areas of this map mainly correspond to different types of crater and basin materials. In particular, the 23

35 white circular region in the western (left) part of the map mostly consists of basin and crater material from the Orientale basin Figure 2.4. The Lunar 5M Geologic Map Renovation. Each color corresponds to a different geologic type (see the U.S. Geological Survey for color codes)

36 25 Figure 2.5. Geologic terranes of interest from the Lunar 5M Geologic Map Renovation. Note that this map shows the entire Moon, including both the North Pole and the South Pole, and is centered on the lunar nearside. The white regions mainly correspond to different crater and basin materials.

37 2.3 Gravity Recovery and Interior Laboratory (GRAIL) Spacecraft Overview The Gravity Recovery and Interior Laboratory (GRAIL) was launched on September 10, 2011 with the goal of mapping the structure of the interior of the Moon by developing a detailed map of the lunar gravity field (Zuber et al., 2013). GRAIL consists of two co-orbiting spacecraft named Ebb and Flow (a nod to the lunar tides) which were placed into a polar orbit on December 31, 2011 and January 1, 2012 (Zuber et al., 2013). Each orbiter contains a Lunar Gravity Ranging System (LGRS) instrument that measures the relative motion between the two spacecraft; this instrument is similar to one used on the Gravity Recovery and Climate Experiment (GRACE) mission which is presently mapping Earth s gravity field (Zuber et al., 2013). The LGRS allows for the precise measurement of changes in range between the two orbiters which is important for determining subsurface density variations. An artist s conception of the GRAIL mission is shown in Figure 2.6. Figure 2.6. A schematic of the GRAIL mission (obtained from NASA/JPL). 26

38 2.3.2 Measuring Lunar Crustal Thickness In order to map the lunar crustal thickness, GRAIL uses the LGRS instrument to measure the change in distance between the two orbiters as they fly above the lunar surface. This distance will vary because the gravity field of the Moon influences the motion of the center-of-mass of each spacecraft. As the lead orbiter passes over surface features such as craters, mass concentrations (mascons), and deep interior structures, its orbit will be perturbed, causing variations in the relative motion between the two orbiters. These perturbations due to gravitational attraction of topography and subsurface mass variations may be isolated and analyzed to create a global map of the lunar crustal thickness (Wieczorek et al., 2013). For our lunar crustal thickness data, we used one of the crustal thickness maps of the Moon derived from GRAIL gravity data as published in Wieczorek et al. (2013). Wieczorek et al. (2013) produced a series of models by using different crustal thickness values near the Apollo 12 and 14 landing sites and by changing the crustal porosity and mantle density. In our analysis, we used Model 1 which assumes a thickness of 29.9 km at the Apollo 12 and 14 landing sites, a crustal porosity of 12%, and a mantle density of 3220 kg m -3 ( This is the same data that was used by Wieczorek et al. (2013) to create Figure Clementine Clementine was launched from Vandenberg Air Force Base on January 25, 1994 and produced the first multispectral global digital map of the Moon, as well as the first global lunar topographic map (Sorensen & Spudis, 2005). The global multispectral 27

39 images of Clementine allowed scientists to analyze the regional distribution of lunar rock types for the first time. The Clementine topography data showed that a wide range of topography is present on the Moon, ranging from deep impact basins to steep mountainous regions (Sorensen & Spudis, 2005). As with our elemental abundances dataset, we used the PDS to gather our topography data which was collected by the Laser Imaging Detection and Ranging (LIDAR) instrument on Clementine. The lunar topography values are relative to a spheroid of radius 1738 km at the equator. In addition, Clementine produced a lunar crustal thickness map. Our initial crustal thickness data involved that obtained from Clementine s Radio Science Subsystem, accessible using the PDS. However, we found the GRAIL crustal thicknesses to be more valuable due to this mission s higher resolution and incredibly precise measurements. 28

40 CHAPTER 3 RESULTS Since each set of data was obtained from a different lunar mission, in order to properly compare the same locations on the Moon, we had to match the (longitude, latitude) coordinates for the datasets being compared. The number of data points we ended up with depended on what datasets were being matched. For example, because GRAIL involved high resolution lunar crustal thickness data, this matching process was quite simple, as there was an exact match in coordinates for both magnetism and crustal thickness. Thus, each of our lunar magnetism coordinates had a matching crustal thickness value, resulting in a total of 50,288 points. On the other hand, the Clementine topography coordinates did not exactly match up with the magnetism coordinates, so we instead had to calculate which magnetic field coordinate was closest to each topography coordinate and then assign that topography coordinate the value of the closest magnetic field coordinate. In doing this, the coordinates do not overlap exactly; however, we found that the biggest discrepancy corresponds to only 0.5 which corresponds to ~15 km at the lunar equator. The matching procedure for the Clementine topography to lunar magnetism resulted in 50,140 points. Similar matching procedures were also followed for the elemental abundance and geology datasets. 3.1 Lunar Geology We first compared our lunar magnetism data to the geology data obtained from the USGS. As mentioned above, we had to match the coordinates for the geology data with the magnetism coordinates, and this process was more tedious than for the other 29

41 datasets since Global Information System (GIS) software was required to analyze this digital set of data. For our purposes, we ended up using both ArcGIS and QGIS. From previous lunar missions, each location on the Moon has been assigned a specific set of geologic attributes including geologic terrane (i.e. mare material, terra material) and epoch (i.e. Copernican, Nectarian), and these attributes are assigned different colors so one may distinguish amongst them. Thus, the digital dataset consisted of many different colored regions, called polygons, that each corresponded to a different location on the Moon. Obviously, each polygon covers more than just one (longitude, latitude) coordinate, so we had to figure out how to take the attributes of each polygon and ascribe them to all the magnetic field coordinates that fell within that polygon. It turns out that ArcGIS has a built-in function that makes this incredibly simple to do. Note that since some small areas of the Moon lacked geology data, we ended up discarding these coordinates so that each magnetic field value had a corresponding set of geologic attributes. After doing so, our matched lunar magnetism and geology data resulted in 50,114 data points. With this set of data, we then looked for any relationships between lunar magnetism and geology to see how the magnetic field varies with age and geologic terrane. As noted in the introduction section, no clear correlations have been found yet, but we thought it important to verify those that have displayed themselves and search for any that remain hidden. For our statistical studies, various terranes were grouped into broad categories such as mare material or plains material. In addition, those unusual terranes corresponding to furrowed and pitted material, hilly and furrowed material, 30

42 or material of grooves and mounds were grouped into one class called unusual terranes. Examples of the appearance of these different terranes is shown in Figure 3.1. a) b) c) d) Figure 3.1. Sample images of different lunar terrane types. a) Hilly and furrowed terrain (hf), where the arrow (I) is radial to the Imbrium basin. b) Example of terra materials. c) & d) The flat regions filling topographic lows in these two images are light plains deposits. (Obtained from NASA). In addition, we only looked at single epochs and excluded those with a mixture of more than one age. The resulting distributions for the various epochs are shown in Figure 3.2 while Figure 3.3 displays the distributions for some of the terranes. Note that a 31

43 magnetic field of 0.2 nt corresponds to the lowest resolvable value, so the large spikes seen at 0.2 nt correspond to all the points which are below our threshold. These are the magnetic field values that are equal to zero within our precision. The median and average magnetic fields for the various ages and terranes that were analyzed, in addition to the average lunar magnetic field, are provided in Table 3.1. From Figure 3.2, there is an apparent bimodal magnetic field distribution for the older epochs which is not present in the younger epochs. We hypothesized that this bimodal appearance might be due to the presence of two distinct groups of crustal thicknesses on the Moon, one with a higher magnetic field and one with a lower magnetic field. After creating two plots of magnetic field strength versus crustal thickness, corresponding to the Nectarian and pre-nectarian epochs, we found that both plots contained two different magnetic field peaks but that the peaks corresponded to roughly the same crustal thickness. Thus, the apparent bimodal magnetic field distribution for the older epochs is unlikely to be due to variations in crustal thickness. Further investigations are needed to determine the cause of this apparent bimodality, but it is interesting to note that the younger epochs lack this bimodal magnetic field distribution. 32

44 Figure 3.2. Magnetic field distributions for three of the epochs analyzed. Figure 3.3. Magnetic field distributions for three of the terranes analyzed. 33

45 Table 3.1. Magnetic Field Values for Certain Epochs and Geologic Terranes. Mean (nt) Median (nt) Entire Moon Copernican Eratosthenian Imbrian pre-imbrian Nectarian pre-nectarian Fra Mauro Formation Mare material Plains material Terra material Unusual material We used the Kolmogorov-Smirnov (K-S) test, which determines whether two independent samples are drawn from the same parent distribution, to determine whether two magnetic field distributions are statistically different. If the resulting p-value is above some significance threshold, we cannot reject the hypothesis that the distributions of the two samples are the same. In our analysis, all of our statistically significant results had very small p-values (our greatest p-value was on the order of 10-8 ). Thus, because the p-value is so small, there is a low probability that the two samples were drawn from the same distribution. So, in these cases, the two corresponding magnetic field distributions are considered statistically different. We found that the magnetic properties of the following epochs were statistically quite significantly different: Copernican and Imbrian; Copernican and Nectarian; Copernican and pre-nectarian; Imbrian and Nectarian; Imbrian and pre-nectarian, and Eratosthenian and pre-nectarian. We also found that the 34

46 magnetic properties of the following terranes were statistically different: Mare material and terra material; mare material and plains material; mare material and unusual material; and terra material and plains material. From our statistical analysis, we found that the magnetic field tends to increase with the age of the material. We also found that the unusual material had an abnormally high magnetism compared to the other terranes, and this agrees with what was mentioned in Section In addition, mare material tends to have the lowest magnetic field values than any other terrane. This agrees with Halekas et al. (2001) who found similar results using electron reflection coefficients. They also found that light plains and terra materials on the lunar nearside had higher average magnetic fields than the maria. Similarly, for the entire Moon, we found that terra and plains materials had stronger magnetic fields than mare material. Our results are consistent with the fact that lunar soils and breccias contain about 10 times the metallic iron content of mare basalts (Mitchell et al., 2008). In our study, ejecta material corresponding to the plains regions tends to have higher magnetic fields than the entire Moon. The Fra Mauro Formation, which consists of Imbrium ejecta material, possibly mixed with secondary impact ejecta, is relatively weakly magnetic, but the plains material is strongly magnetic. Thus, one cannot just conclude that all ejecta materials have strong magnetic fields; this conclusion may only be made with respect to specific regions of the Moon. In addition, the large difference between the mean and median magnetic fields for some of the epochs and terranes suggests that there are some strong magnetic outliers in the data. The magnetic field values for the entire Moon are also highly dependent on the region one is looking at since 35

47 the farside tends to have a higher average magnetic field than the nearside (due to strong antipodal anomalies). 3.2 Lunar Crustal Thickness Comparison to Topography As a first look at the GRAIL lunar crustal thickness dataset, we compared these values to that of lunar topography. Because the GRAIL dataset contained over one million data points while the topography dataset consisted of 64,800 points, we matched the GRAIL crustal thickness values to the corresponding topography coordinates to ensure an efficient matching process. Looking at Figure 3.4, one notices a strong trend between the two variables. Indeed, a statistical analysis results in a Spearman correlation coefficient of 0.81 with a p-value of nearly zero. Since the Spearman correlation is a nonparametric measure of the monotonicity of the relationship between two datasets, having a correlation coefficient value near one means that the two datasets are positively correlated. Thus, as the crustal thickness increases, so too does the topography which is evident from Figure 3.4. This is logical since thicker crust would tend to correspond to higher altitudes. The South Pole-Aitken (SPA) basin is the most prominent topographic feature on the Moon, with a diameter of 2500 km and a maximum depth of 8.2 km below the reference ellipsoid (Zuber et al., 1994). Topographic results from the Clementine mission showed that this structure was the largest and deepest impact basin in the solar system (Zuber et al., 1994). Figure 1.3 also demonstrates that the SPA basin is a region of thin crust. In contrast, the region of highest topography (and the thickest crust) corresponds to the lunar far side in the vicinity of the Korolev Basin (Zuber et al., 1994). 36

48 However, there are some odd features at the lower crustal thickness end of the plot which are most likely the cause for the deviation from a perfect correlation. Figure 3.4. Lunar topography as a function of crustal thickness. The off-trend region with topographies between -2.5 km and -3.5 km and crustal thicknesses between 5 km and 15 km corresponds to the nearside maria (Mare Imbrium, Serenitatis, Nectaris, Humorum, and Orientale in particular). The off-trend region just below this grouping, with topographies between -3.5 km and 5 km and crustal thicknesses between 1 km and 10 km also corresponds to the nearside maria (Mare Crisium, Smythii, and Humboldtianum in particular). Thus, the mare regions tend to have higher topography values than what one would expect based on the distribution of 37

49 Figure 3.4. This means that the lunar maria have higher elevations than expected relative to the reference spheroid of radius 1738 km at the equator. Large impacts excavate the lunar surface, and this causes thinning of the crust beneath the basin cavity. The heating and weakening of the crust after the impact allows denser mantle material to seep upwards, and this results in the concentration of excess mass near the central part of the basin (Konopliv et al., 1998). Uplift of the crust-mantle boundary is a result of this mantle rebound; thus, even though the impact itself causes a thinning of the crust and thus a decrease in the local topography, the mantle rebound is likely behind the higher than expected topography that we see for the nearside maria. In addition, isostatic uplift since the original formation of the crater is supported by the low depth-to-diameter ratio of the maria (Kaula, 1969). If no rebound or uplift was present, one would expect a higher depth-to-diameter ratio of the maria and thus a lower topographic depression. However, the presence of the off-trend group mentioned above suggests that the mare regions of the Moon have undergone mantle rebound, resulting in the higher than expected topography values Comparison to Magnetism After analyzing the relationship between lunar crustal thickness and topography, we turned our attention to lunar magnetism. To this end, we wanted to see how lunar magnetism varies with lunar crustal thickness, with the hypothesis that thicker crust might have higher magnetization since there would be more magnetized material. Our resulting plot is shown in Figure 3.5, a self-titled flame plot. It is evident that different regions of this plot can be made into groups of specific magnetic field and crustal thickness ranges. 38

50 Figure 3.5. A 2D histogram of the magnetic field as a function of the GRAIL lunar crustal thickness. The frequency of each magnetic field/crustal thickness pair is shown in the color bar at the right Appearance of Distinct Groups We partitioned distinct regions of Figure 3.5 by utilizing boxes representing different magnetic field and crustal thickness ranges in order to see if these various boxes signified specific locations or features on the Moon. Figure 3.6 demonstrates this partitioning scheme, and Table 3.2 denotes the cutoffs used for each box. These boxes enable one to see the distinct groups more clearly. Once these boxes were drawn, we used the lunar coordinates of each point within the boxes to plot them on a geographic map. Our results are shown in Figure 3.7, where it is evident that the different groups outline various features on the Moon. 39

51 Figure 3.6. A 2D histogram of the magnetic field as a function of the GRAIL lunar crustal thickness, along with the resulting distinct groups after the partitioning scheme was implemented. Table 3.2. The Cutoffs Used for the Partitioning Scheme. Group # Magnetic Field Range (nt) Crustal Thickness Range (km) B t B t B t B t B t B t B t B t

52 41 Figure 3.7. A map, centered on the lunar nearside, of the distinct GRAIL lunar crustal thickness groups as they appear on the Moon. Note that the white spaces correspond to the outer areas of Figure 3.6 that do not fall within any of the boxes. The solid black shapes outline the nearside maria.

53 From Figure 3.7, one can see that the different boxes of Figure 3.6 bring out various regions of the Moon quite nicely. Groups 1 (dark pink) and 2 (green) consist of the lowest crustal thicknesses and low magnetic field values. These regions correspond to the mare regions of the Moon. Groups 3 (light pink) and 4 (orange) have thinner crustal thickness values but anomalously high magnetic field strengths. From the map, we see that these groups mainly correspond to the South Pole-Aitken basin on the lunar farside. Group 5 (aqua-green) has a crustal thickness range slightly below average and low magnetism and corresponds to a larger swath of mare materials, mainly consisting of the Oceanus Procellarum region of the Moon. Groups 6 (yellow) and 7 (blue) appear to make up typical lunar crust as they correspond to medium lunar crustal thickness values. However, these two groups represent much different magnetic field values, with group 6 consisting of higher magnetism than group 7. This is a curious result, one which we will discuss in further detail in Section Finally, group 8 (purple) consists of the highest crustal thickness values and medium magnetic field strengths and corresponds to the rims of craters and regions just outside impact basins. Figure 3.8 provides a map of specific GRAIL groups, along with labels for various maria. 42

54 Orientale Humorum Imbrium Serenitatis Nectaris Crisium Humboldtianum Smythii Figure 3.8. A map, centered on the lunar nearside, of some of the distinct GRAIL lunar crustal thickness groups as they appear on the Moon. Specific mare regions are also denoted. Note: Groups 6 and 7 were assigned the same color in an effort to make the plot easier to read. 43

55 3.3 Distinct Groups Comparison by Geologic Terrane and Epoch In an effort to determine what distinguishes the different GRAIL groups, we calculated the percentage of specific geologic terranes and epochs that compose each group. In doing so, we were able to easily compare the groups by use of Table 3.3. From these results, we note that groups 1 and 2 have very little old material which is reasonable since these groups correspond to the young mare regions on the nearside of the Moon. The fact that groups 1 and 2 consist of mainly mare material is evident in Table 3.3. Group 5 is also composed of mainly mare material, and since these three groups are made up of a significant amount of mare material, one would expect them to have low magnetic fields due to demagnetization processes. Indeed, groups 1, 2, and 5 are the least magnetic of the GRAIL groups. We also note that group 8 has very little young material but quite a significant amount of older material. Recall that this group has the thickest crust and tends to correspond to the lunar highlands on the farside of the Moon. In addition, there is quite a large amount of Nectarian and pre-nectarian material in groups 3 and 4 which correspond to the old South Pole-Aitken impact basin. It is interesting to note that none of the groups with high magnetism (groups 3, 4, and 6) are dominated by unusual material which is associated with high magnetic field strengths. However, these groups are composed of a significant amount of terra and plains materials which tend to be more magnetic than other terranes. It is curious that the unusual material is most prevalent in group 1; this group consists of the fewest amount of data points though, so this result is likely not significant. Since the Fra Mauro Formation is interpreted as the ejecta blanket of the Imbrium basin, 44

56 one would expect to find groups near this basin to consist of Fra Mauro material. This is indeed the case, as the five groups in Table 3.3 that contain Fra Mauro material are near Mare Imbrium. Imbrian-aged material tends to dominate, no matter the group, which indicates that a majority of lunar material is at least partially composed of material from the Imbrian epoch. Table 3.3. Geologic Terrane and Epoch Percentages for Each GRAIL Group. Geologic Terrane (%) Fra Mauro Mare Material Plains Material Terra Material Unusual Material Total # Epoch (%) Copernican Eratosthenian Imbrian pre-imbrian Nectarian pre-nectarian Total #

57 3.3.2 Comparison by Elemental Abundances In addition to computing the percentage of geologic terranes and epochs corresponding to each GRAIL group, we calculated the average elemental abundances for each group, along with the abundances for the Moon as a whole, as displayed in Table 3.4. Mare basalts have relatively high Fe abundance, so the high FeO percentage for the mare regions (groups 1, 2, and 5) is reassuring. The South Pole-Aitken basin also has a higher than average FeO abundance, as verified by groups 3 and 4 in Table 3.4. In addition, mare basalts have been classified by using variations in TiO2 content, so maria are expected to have high concentrations of TiO2 (Prettyman et al., 2006). This is indeed what we see in Table 3.4. On the other hand, basalts have low Al2O3 abundance, whereas feldspar-rich rocks have high Al2O3 abundance (Prettyman et al., 2006). Thus, group 8, which corresponds to the feldspathic highlands terrane, is seen to have the highest Al2O3 abundance while the mare regions have the lowest Al2O3 abundance. Groups 2 and 5 correspond to the Procellarum KREEP Terrane and thus have high K abundances. In addition, the region around the Imbrium basin consists of the highest Th abundances on the Moon (Lawrence et al., 2000). This mainly corresponds to group 5 which indeed has a high Th abundance. 46

58 Table 3.4. Average Elemental Abundances for Each GRAIL Group. Elemental Abundance Entire Moon Al2O3 (%g/g) CaO (%g/g) FeO (%g/g) MgO (% g/g) SiO2 (%g/g) TiO2 (%g/g) K (ppm) Th (ppm) U (ppm) Group 6 vs. Group 7 Recall that group 6 has higher magnetic field values than group 7 but that both of them have the same crustal thickness range. We endeavored to discover what is causing such a difference in magnetism given the fact that these two groups have the same crustal thickness range. From Table 3.3, we see that group 6 has a slightly higher trend toward older epochs than group 7. Since group 6 has higher magnetic field values, one might expect this group to consist of materials from older epochs when compared to group 7 since older materials tend to have stronger magnetic fields (see Section 3.1). Group 6 also consists of material that is more magnetic (terra, plains, and unusual materials) than group 7 but then also contains more mare material than group 7. However, for both the geologic terrane and epoch comparison, the differences are quite small. Thus, it doesn t appear as though the terrane or age of the materials in groups 6 and 7 are significantly 47

59 different, so there must be some other factor that is causing the stark difference in magnetism between these two groups. Turning to Table 3.4, we immediately notice that both groups 6 and 7 contain similar percentages for all elemental abundances, so composition must not be the cause of the difference in magnetism either. A map of the locations of groups 6 and 7 on the Moon is shown in Figure 3.9. Group 6 (yellow) appears to dominate the lunar east side more so than group 7 (blue). However, the cause of the difference in magnetism does not appear to be due to geologic terrane, age, or composition, so some other factor must be acting on these areas of the eastern part of the Moon to cause such a difference in magnetism for the same crustal thickness range. Figure 3.9. A map of GRAIL lunar crustal thickness groups 6 and 7. 48

60 3.4 Magnetic Field Strength in Relation to Distance from Specific Impact Craters Since distinct lunar maria are evident in Figure 3.8, we decided to look at how the ejecta thickness varied as a function of radial distance from the impact crater for a few of these denoted basins. We also analyzed the distribution of magnetic field for various impact basins, taking into account both the lunar crustal thickness and the distance from the crater. Seven well-known lunar basins, along with their diameters and the coordinates of their centers, are listed in order of increasing diameter in Table 3.5. We used these values in the following calculations, along with a value of km for the radius of the Moon. Table 3.5. Lunar Impact Basins (after Wilhelms, 1987). Basin Long (deg E) Lat (deg N) Diameter (km) Humorum Crisium Serenitatis Nectaris Orientale Imbrium South Pole-Aitken Ejecta Thickness To model the variation of ejecta thickness with increasing radial distance from a specific lunar crater, we used an expression from McGetchen et al. (1973) which is given by Equation 3.1, where t is the ejecta thickness starting at the crater rim, r is the range from the center of the crater, and R is the crater radius, all in meters. The thickness 49

61 estimates derived from this equation are average values which assume a symmetrical distribution of ejecta around the crater and which do not take into account the existence of rays or the influence of topography. The thickness of ejecta deposits due to large lunar nearside impact basins at each of the Apollo sites have been estimated using Equation 3.1 (McGetchen et al., 1973). t = 0.14 R 0.74 ( r R ) 3.0 [3.1] The range from the center of the crater was calculated using the haversine formula, given by Equation 3.2, which allows one to compute the distance between two points on a sphere. r = 2R Moon sin 1 ( (sin ( λ 2 λ 1 2 )) 2 + cos λ 1 cos λ 2 (sin ( φ 2 φ 1 )) 2 ) [3.2] 2 where R Moon is the radius of the Moon, λ 2 is the latitude of the point of interest, λ 1 is the latitude of the crater center, φ 2 is the longitude of the point of interest, and φ 1 is the longitude of the crater center. The resulting plots for some lunar impact basins are shown in Figure

62 Figure Top: Ejecta thickness models for various lunar maria. Bottom: Ejecta thickness model for the South Pole-Aitken basin. Note: All curves begin at the rim of the corresponding crater. 51

63 One would expect larger impact basins such as South Pole-Aitken to have corresponding ejecta found farther from the crater center since a significant amount of energy is released when such a large impactor crashes into the Moon. This is indeed what we see in Figure As the size of the crater increases, the thickness of the ejecta blanket also increases, and as one moves away from the crater, the ejecta thickness decreases Magnetic Field Distributions In addition to computing the expected ejecta thickness as a function of distance from the crater center, we looked at the crustal thickness versus distance from crater center, with the magnetic field strength as a third dimension. In Figures , we extended the crater distance out to the opposite side of the Moon (corresponding to a distance of ~5500 km since the circumference of the Moon is ~11000 km). In doing so, we were able to see how the crustal thickness and magnetism vary across the entire Moon, beginning at the center of a specific impact crater. There is a widespread distribution of crustal thickness for the same distance from the crater center since we are dealing with a radial distance from the crater (not just in one direction). In addition, there are different magnetic field values and crustal thicknesses as one moves farther away from the crater center due to overlapping craters and other lunar features. In all of the following plots, various dips are present. These dips represent other impact craters that are present at those specific distances from the original impact crater. Most of these dips consist of low magnetic fields which is expected from the mare material corresponding to those craters. As one nears the antipode of some of the impact basins, a large spike in the magnetic field strength occurs; this is especially evident in the 52

64 plots corresponding to Mare Serenitatis and Mare Orientale. This is another representation of the antipodal magnetic anomalies that have previously been studied. Near the center of the impact basin, we also see a large dip in the lunar crustal thickness which is due to the impactor carving out the lunar crust. As one nears the crater rim, which is marked by the solid line in each of the plots, the thickness increases dramatically before leveling out to some extent. Since the SPA basin is quite large, this increase in crustal thickness is much less dramatic. In these plots, we were expecting to find evidence of magnetic field enhancements near impact basins which may be due to thicker crust and ejecta. However, there does not appear to be any clear magnetic field enhancements near the impact craters, except possibly for Mare Crisium and Mare Nectaris. Looking at Figures 3.7 and 3.8, it is evident that these two regions are surrounded by areas of higher magnetism. Since ejecta material is associated with higher magnetism, one might expect to see thicker ejecta regions near these areas. However, when looking at crustal thickness as a whole, the regions around Mare Crisium and Mare Nectaris do not appear to have thicker crust than the other maria shown in Figures For some of these impact basins, high magnetic fields are present in both the thickest and thinnest terrane near the crater rim. This is logical for thick crust, as there is more magnetic material and thus higher magnetism, but it is inconsistent for thin crust. To determine whether this was an artifact of the plotting process, we plotted the colored points in a different order. However, after doing so, we still found high magnetic field values for both thick and thin crust near the crater rim of Mare Nectaris. 53

65 Antipode Antipode Figure Crustal thickness as a function of distance from crater center for Mare Humorum (top) and Mare Crisium (bottom). The magnetic field strength is given by the color bar, and the solid vertical lines denote a distance of one crater radius. The antipode is located at the far-right end of each plot. 54

66 Antipode Antipode Figure Crustal thickness as a function of distance from crater center for Mare Serenitatis (top) and Mare Nectaris (bottom). The magnetic field strength is given by the color bar, and the solid vertical lines denote a distance of one crater radius. The antipode is located at the far-right end of each plot. 55

67 Antipode Antipode Figure Crustal thickness as a function of distance from crater center for Mare Orientale (top) and the South Pole-Aitken basin (bottom). The magnetic field strength is given by the color bar, and the solid vertical lines denote a distance of one crater radius. The antipode is located at the far-right end of each plot. 56