The surface brightness of Deimos, groove patterns on Phobos, crustal magnetic anomalies on the Moon and the composition of exoplanet atmospheres represent some of the most interesting and puzzling questions in planetary science. Why is Deimos significantly brighter and smoother than its partner moon Phobos? What is the origin of the crater chain “grooves” on Phobos? Are the magnetic anomalies in the lunar South Pole-Aitken basin a remnant of the basin’s formation, or do they owe their existence to a primordial period of lunar dynamo activity? And finally, as visible wavelength telescopes are designed and tested for space-based exoplanet detections, can we use observed albedo spectra to determine radius, gravity, cloud pressure heights and atmospheric compositions for these planets? I use dynamical modeling, magnetic inversions and Markov Chain Monte Carlo retrievals to address these questions. Major findings include 1) the likelihood of isotropic redistribution of reaccreted ejected material on Deimos, 2) the creation of hemispherical catenae from the creation of primary craters on Phobos, which match the locations and geomorphology of several existing grooves well, 3) the first directional magnetic survey of South Pole-Aitken basin anomalies, and a larger than expected diversity in recovered paleopole directions, and 4) the critical importance of considering the effects of planet phase in exoplanet atmosphere retrievals; changing planet phase, when combined with low signal-to-noise observations, can cause several orders of magnitude of uncertainty in atmospheric methane composition and cloud pressure height, among others.

The goal of this thesis is to investigate the geologic origins of two different lunar magnetic anomalies: Reiner Gamma on the lunar nearside, and the Gerasimovich-area anomalies on the farside. Chapters 2 and 3 are concerned with Reiner Gamma, while Chapter 4 is concerned with the Gerasimovich-area anomalies. Understanding the geologic origins of these lunar magnetic anomalies is key to progressing our understanding of the Moon’s magnetic history, and this work carries out these investigations using data from lunar orbiters.In Chapter 2, I present evidence that the magnetic anomaly Reiner Gamma overlies a relative-negative Bouguer gravity anomaly. This gravity anomaly is likely a buried impact crater, and I determined its age, and thus the age of the Reiner Gamma magnetic source bodies, to be between ∼3.3 Ga and ∼3.9 Ga, which are the approximate temporal bounds of mare volcanism. This range of ages coincides with the putative high-field era of the lunar dynamo (∼3.56–3.9 Ga), thus, the high magnetization of Reiner Gamma could be due to deposition during a time of a high-magnitude ambient field. In Chapter 3, I present observations that a portion of Reiner Gamma appears to have been demagnetized by the emplacement of a dome in the nearby Marius Hills volcanic complex. I created three different models to determine if the observed magnetic anomaly was diminished by thermal demagnetization. First, I created a flexure model, which approximated the dome as a buried laccolith and determined the burial depth of the laccolith based on its surface expression. Second, I created a thermal model, which determined the time-temperature history of areas around the laccolith. Third, I created a dipole model that simulated thermal demagnetization via decreasing the moments of some of the source dipoles. These three models, taken together, suggest that thermal demagnetization did occur at Reiner Gamma, and we use this result to estimate source body burial depths of <∼2 km, age of ∼3.5-3.9 Ga, and a magnetization of ∼3.6 A/m. In Chapter 4, I present the observation that in the region of the Gerasimovich magnetic anomalies, areas of high magnetic field tend to overlie areas of low surface slope. Because the Gerasimovich region is antipodal to the Crisium basin, other work has suggested that the magnetic anomalies could be due to highly magnetic antipodal ejecta (Hood and Artemieva, 2008; Lin et al., 1988; Wakita et al., 2021). My work validates this antipodal ejecta hypothesis and estimates a magnetized layer thickness between ∼0.8-1.2 km and ∼2.4 km and a source body magnetization of ∼3-5 A/m.

In this study we looked to characterize the fundamental shape of the Moon using topography and gravity data and we studied a lunar rock sample in order to measure its dominant magnetic moment. Studying the shape of the Moon would give us a better understanding of the large-scale geophysics that has led to the present state of the Moon and studying lunar rock samples helps us better understand if the observed magnetic fields on the Moon are due to a past global magnetic field. Our findings suggest that the Moon does have a prominent degree-2 shape that is explained by tidal heating and a frozen-in shape from its past tidal stretching. In addition, the rock sample we studied shows signs for a prominent magnetic signature that is likely the result of the rock cooling in an environment that was dominated by a lunar magnetic field.

An enduring mystery since Apollo is that, in spite of the Moon's lack of a global magnetic field, the surface is nevertheless dotted with regional magnetic fields strong enough to be detected from orbit. Did the Moon once have an intrinsic global field that magnetized parts of the crust but has since decayed away? This is a question of fundamental importance to understanding the formation and evolution of solid planetary bodies, and yet it remains unanswered due in part to limitations in our knowledge of these crustal magnetic anomalies. Adding to the puzzle, many of these magnetic anomalies are accompanied by enigmatic optical features, known as swirls, which may hold the key to understanding "space weathering"—a process by which airless bodies change color over time due to exposure to solar wind and micrometeoroids. Here we show both that swirl morphology provides information about the structure of the underlying magnetic sources, and that the color of the lunar surface varies systematically with latitude in a way that allows us to distinguish between the effects of solar wind ion and micrometeoroid bombardment, addressing a decades-old problem in remote sensing, and aiding in the interpretation of the spectra of airless bodies throughout the solar system.

The remarkable diversity of the outer solar system's satellites provides important clues about the formation and evolution of the solar system. Many of the satellites have surprisingly young surfaces, owing in some cases to on-going geologic activity. Moreover, the existence of subsurface oceans within some of the satellites raises the intriguing possibility of extant habitable environments in the outer solar system. Determining the properties of their ice shells and the structures of their deep interiors places fundamental constraints on how the icy satellites formed and evolved, and on what governs their behavior today. Using gravity and topography data from Cassini, we develop analytical models showing that Titan's ice shell may be very rigid and therefore unlikely to be geologically active. In contrast, we also model the internal structure of the tiny, but highly geologically active moon Enceladus, and find that a subsurface liquid ocean is likely.

This dissertation thesis is a combination of three projects on magnetism and gravity studies of the Moon and a magnetism study of Mars, each with a heavy focus on uncertainty estimation. The goal of each chapter is to elucidate some portion of the thermal history of the Moon and Mars. Analysis of crustal magnetic fields can explain past dynamo behavior, which is tied to the amount of heat within a planet and how long it retains that heat. Elastic thickness, determined from correlations between gravity and topography, indicates the heat flux at the time of load emplacement and we can use the elastic thickness of a region to determine its formation age. The results from this thesis place constraints on the ancient dynamo behavior of the Moon and Mars (magnetism studies) and on the formation ages of portions of the farside of the Moon (gravity studies).Chapter 1 focuses on lunar crustal magnetic anomalies. The Moon no longer has an active global magnetic field, but evidence of an ancient field can be found in portions of the crust, which have been magnetized in the presence of intense fields thought to be generated by an extant dynamo. Quantifying the magnetization directions of these anomalies elucidates the behavior of the paleo-magnetic field by determining the magnetic paleopole (i.e., the orientation of the dipolar axis). Previously, distinguishing between paleopole locations was impossible because of their large uncertainties. Without distinguishing between paleopole locations, determining the history of the lunar dynamo is impossible. I propose an alternative method of estimating uncertainty using a Monte Carlo method to add synthetic noise to the best-fit modeled fields, which allows us to determine how easily perturbed the magnetization direction is in the presence of noise (i.e., uncorrelated anomalies). The new method more accurately describes the uncertainty of the inversion method and allows for better discernment of paleopole locations. I determined that the dipolar axis of the lunar dynamo must have been misaligned with the spin-axis at some point in lunar history, or that there were significant multipolar components to the magnetic field. Chapter 2 focuses on gravity and topography studies of the Moon. I use admittance analysis to determine the lunar elastic thickness and how it varies across the Moon. Elastic thickness allows us to determine the heat flux at the time of load emplacement, which in turn elucidates the thermal history of the Moon. Regions of low elastic thickness indicate high heating at the time of loading, and we can infer these locations formed earlier in lunar history than areas with higher elastic thicknesses. However, as in Chapter 1, variations in elastic thickness are meaningless without a clear estimate of uncertainty to distinguish between values. In this chapter, I describe how to determine elastic thickness using the spectral domain and the Markov chain Monte Carlo (MCMC) technique. Careful consideration is given to where these techniques are valid, including an analysis of the uncertainties from the MCMC technique using synthetic testing. I find that there are several locations on the Moon with low elastic thickness, implying these regions formed very early (<200 Myr) after the lunar magma ocean solidification. I also find one region with higher elastic thickness, which may be recording loading events as late as 3.5 Ga. Chapter 3 focuses on martian crustal magnetic anomalies. Like the Moon, Mars no longer has a global magnetic field, but there is evidence of a dynamo-generated field in the presence of widespread crustal magnetism. The issue of martian paleopole locations has plagued the magnetism community for two decades, many of the inferred paleopole locations do not correlate with each other, and several paleopole locations do not correlate with inferred paleo-spin axes calculated by other geophysical means. Additionally, there has not been a thorough paleopole analysis of crustal magnetic anomalies since the release of new magnetometer data from the NASA Mars Atmosphere and Volatile Evolution (MAVEN) mission. I use the methods described in Chapter 1 to determine the magnetic paleopole locations and uncertainties of ten martian crustal magnetic anomalies. I then use the Maximum Angular Deviation (MAD) technique to quantify the degree of clustering of our paleopole uncertainty ellipses and find significant clustering around a low latitude (45°N), which corresponds with a paleo-spin axis found through paleo-shoreline analysis (Perron et al., 2007). I show that the wide spread of other paleopole locations cannot be explained by a multipolar field or a hemispherical dynamo.