abstract: There are more than 20 active missions exploring planets and small bodies beyond Earth in our solar system today. Many more have completed their journeys or will soon begin. Each spacecraft has a suite of instruments and sensors that provide a treasure trove of data that scientists use to advance our understanding of the past, present, and future of the solar system and universe. As more missions come online and the volume of data increases, it becomes more difficult for scientists to analyze these complex data at the desired pace. There is a need for systems that can rapidly and intelligently extract information from planetary instrument datasets and prioritize the most promising, novel, or relevant observations for scientific analysis. Machine learning methods can serve this need in a variety of ways: by uncovering patterns or features of interest in large, complex datasets that are difficult for humans to analyze; by inspiring new hypotheses based on structure and patterns revealed in data; or by automating tedious or time-consuming tasks. In this dissertation, I present machine learning solutions to enhance the tactical planning process for the Mars Science Laboratory Curiosity rover and future tactically-planned missions, as well as the science analysis process for archived and ongoing orbital imaging investigations such as the High Resolution Imaging Science Experiment (HiRISE) at Mars. These include detecting novel geology in multispectral images and active nuclear spectroscopy data, analyzing the intrinsic variability in active nuclear spectroscopy data with respect to elemental geochemistry, automating tedious image review processes, and monitoring changes in surface features such as impact craters in orbital remote sensing images. Collectively, this dissertation shows how machine learning can be a powerful tool for facilitating scientific discovery during active exploration missions and in retrospective analysis of archived data. / Dissertation/Thesis / Doctoral Dissertation Exploration Systems Design 2019

<p>The solar system that we live in today bears only a passing resemblance to the solar system that existed 4.5 billion years ago. As our young star shed the gas nebula from which it was born, a disk of dust and rocky bodies emerged in the space between the Sun and Jupiter. Over the next hundred million years, this planetary disk evolved and gave rise to the terrestrial planets of the inner solar system. Clues left behind during this early stage of evolution can be seen in the orbital architecture of the modern planets, the cratering records of rocky bodies, and the signatures of the solar system's secular modes. </p> <p><br></p> <p>Past works in the fields of terrestrial planet accretion and solar system evolution typically do not include collisional fragmentation. While the mechanics of collisional fragmentation are well studied, the incorporation of this processes into simulations of terrestrial planet formation is computationally expensive via traditional methods. For this reason, many works elect to exclude collisional fragmentation entirely, improving computational performance but neglecting a known process that could have played a significant role in the formation of the solar system. In this dissertation, I develop a collisional fragmentation algorithm, called Fraggle, and incorporate it into the n-body symplectic integrator Swiftest SyMBA. Along with performance enhancements and modern programming practices, Swiftest SyMBA with Fraggle is a powerful tool for simulating the formation and evolution of the inner solar system. </p> <p><br></p> <p>In this dissertation, I use Swiftest SyMBA} with Fraggle to study the effect of collisional fragmentation on the accretion and orbital architecture of the terrestrial planets, as well as the cratering record of early Mars. I show that collisional fragmentation is a significant process in the early solar system that creates a spatially heterogeneous and time-dependent population of collisional debris that fluctuates as the solar system evolves. This ever-changing population results in cratering records that are unique across the inner solar system. The work presented in this dissertation highlights the need for independent cratering chronologies to be established for all rocky bodies in the solar system, as well as the need for future models of solar system accretion to include the effects of collisional fragmentation. </p> <p><br></p> <p>While the cratering records and orbits of the terrestrial planets are two means by which to study the solar system's ancient past, analysis of the evolution of the secular modes of the solar system offers a third method. A secular mode arises due to the precession of the orbit of a planet over time. Each body's orbit precesses at a specific fundamental frequency, or mode, that has the power to shape the orbital architecture of the solar system. I show that jumps in the eccentricity of Mars can trigger short-lived power sharing relationships between secular modes, resulting in periods in which the strength and fundamental frequencies of modes fluctuates. While evidence of these past jumps in Mars' eccentricity would likely not be visible today in the secular modes of the inner solar system, the work presented in this dissertation poses additional questions. In particular, questions related to other possible triggers of power sharing relationships, as well as the effects of power sharing relationships on the stability of small bodies during these periods of fluctuation, are particularly compelling.</p> <p><br></p> <p>The work presented in this dissertation contributes to the fields of numerical modeling, solar system evolution, collisional fragmentation, martian cratering, and secular modes and resonances. As a whole, it explores avenues by which we can understand the very earliest period of our solar system's history and develops a model that will allow for continued research in this field. </p>

Mars is a potentially habitable planet given the appropriate planetary engineering efforts. In order to create a habitable environment, the planet must be terraformed, creating quasi-Earth conditions. Benchmarks for minimum acceptable survivable human conditions were set by observing atmospheric pressures and temperatures here on Earth that humans are known to exist in. By observing a positive feedback reaction, it is shown how the sublimation of the volatile southern polar ice cap on Mars can increase global temperatures and pressures to the benchmarks set for minimum acceptable survivable human conditions. Given the degree of uncertainty, utilization of pressure scale heights and the Martin extreme terrain were used to show how less than desirable conditions can still produce results where these benchmarks can be met. Methods for obtaining enough energy to sublimate the southern polar ice cap were reviewed in detail. A new method of using dark, carbonaceous Martian moon material to alter the overall average albedo of the polar ice cap is proposed. Such a method would increase Martian energy efficiency. It is shown that by covering roughly 10% of the Martian polar ice cap with dark carbonaceous material, this required energy can be obtained. Overall contributions include utilization of pressure scale heights at various suggested settlement sites, as well as polar albedo altering as a method of planetary engineering. This project serves as a foundational work for long term solar system exploration and settlement.

The origin of the earliest solids in the solar system, preserved for 4.56 Ga in primitive chondritic meteorites, is poorly understood, in particular because of the lack of detailed chemical data on individual phases within these solids. Because chondrite constituents record the environmental conditions and local chemistry of the protoplanetary disk in which they were formed, examining their chemical composition across chondrite groups enhances our understanding of and provides quantitative constraints on the origin of the earliest solar system bodies, the precursors to our planets. This dissertation examines chondritic meteorites using (1) geochemical analysis of the major and trace element distributions within and among carbonaceous chondrite constituents to address chemical source reservoirs and formation mechanisms, and (2) visible near-infrared (VNIR) spectroscopy of ordinary chondrites under a variety of conditions to improve compositional interpretations of remotely sensed asteroids. Chapter 1 presents a brief introduction to the field of meteoritics via an overview of meteorite types and the various contexts they preserve. Primitive chondritic meteorites and their components fossilize the chemical and physical conditions that existed at the time of their formation in the early solar system, whereas achondritic meteorites provide insight into the structure of planetary interiors. This chapter also reviews fundamentals of mineral condensation in the early solar system environment, and the implications of the presence (or lack) of these minerals in the components that comprise chondrites. In Chapter 2 of this dissertation, I investigate the distribution of trace elements in the components of the carbonaceous Vigarano-type (CV) chondrite group to better reveal the solar system processes that led to the fundamental cosmochemical mechanisms of chondrule formation and chondrite accretion. While the major element and bulk chemical compositions of chondritic meteorites are well established, the distribution of trace elements amongst chondrite components and in the individual minerals within them is not well constrained. The geochemical behavior of trace elements enables them to reveal precursor characteristics, formation conditions, and processing histories of chondrite constituents. In determining the large-scale distribution of trace elements, in particular the rare earth elements (REE), across multiple meteorites in the CV chondrite group, I produced a statistically significant trace element dataset that complements existing major element and isotopic datasets. I observe variable REE patterns in individual mineral phases in chondrite components which combine to produce overall flat bulk REE patterns for each meteorite. This chemical evidence, which is necessary to constrain dynamical accretion mechanisms in astrophysical models of the early solar system, supports the idea of a single reservoir origin for these chondrites, and suggests that some chondrules are in chemical disequilibrium and have inherited CAI-like precursor material. In Chapter 3, I evaluate common standardization techniques used for analysis of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) data and assess the implications for high-precision elemental analyses. LA-ICP-MS has become popular in part due to its ability to measure low trace element abundances in small sample volumes while preserving petrographic context. The capacity for in-situ mineral-scale and sub-mineral scale analyses is particularly useful for diffusion studies or for assessing element partitioning between co-existing solids. Standardization techniques have been developed in order to obtain high-precision concentration data from LA-ICP-MS analyses. Common practice dictates the use of reference material spot sizes similar or equal to the chosen spot sizes of the unknown samples under investigation. However, the effects of using reference material spot sizes for calibration that differ from sample spot sizes are not quantitatively constrained. In this chapter I evaluate the coupled effects of differences in ablation yield and of matching compositions between samples and reference materials (matrix matching), as well as the differences in calculated element abundance resulting from internal standard element choice. I show that element abundances derived from LA-ICP-MS analyses are heavily dependent on the chosen combination of measured element, internal standard element, unknown spot size, and reference spot size. Even varying just one of these parameters does not necessarily yield predictable effects on resulting data. In Chapter 4, I explore the effects of both chemical and physical variables on laboratory infrared spectral analysis of well-characterized meteorite samples with the goal of better quantitatively analyzing asteroid remote sensing data in conjunction with returned extraterrestrial samples. Temperature and grain size are known to each have individual effects on the VNIR spectra of silicate and meteorite powders. Here, I examine the combined effects of physical variables (temperature, particle size) and chemical variables (petrologic type, metal fraction) on VNIR spectra of ordinary chondrite meteorite powders. I prepared six equilibrated (petrologic types 4-6) ordinary chondrite meteorite falls, spanning groups H, L, and LL, at a variety of particle sizes to capture the spectral diversity associated with asteroid regoliths dominated by various grain sizes. VNIR spectra of the ordinary chondrite materials were measured under simulated asteroid surface conditions (~10-6 millibar, -100°C chamber temperature, and low intensity illumination) at a series of temperatures chosen to mimic near-Earth asteroid surfaces. Iused X-ray element maps of meteorite thick sections to calculate the exact mineral abundances for each meteorite, in order to characterize changes in spectral features due to variations in mineralogy. The VNIR spectra show minimal variation in both major orthosilicate absorption bands across the simulated near-Earth asteroid temperature regime. Spectral changes due to particle size are consistent across samples, with the smallest and largest grain sizes having the highest reflectance. Unlike previous spectral investigations of ordinary chondrites, I retained the metal fraction in the meteorite powders instead of analyzing the silicate fraction only. In the measurements, I observe distinct offsets in spectral features when compared to analyses of purely silicate fractions. XRD analysis shows that the largest size fraction of nearly every sample contains relatively more metal, likely due to the retention of metal nuggets in the largest size fraction during sieving. The more petrologically pristine samples (e.g., LL4) from each ordinary chondrite group display relatively shallower band depths than their more petrologically altered counterparts (e.g., LL6). The band depths shift to higher wavelengths as temperature, grain size, and petrologic type increase. Spectral studies of meteorites combined with detailed petrologic analysis of the samples should greatly enhance interpretation of current and future planetary remote sensing data sets. Importantly, understanding the spectral contribution of the metal fraction will aid in upcoming investigations of metal-rich mission targets such as asteroid 16 Psyche.

No description available.

Life on Earth likely began as an RNA world, where cell-free or compartmentalized ribonucleic acid (RNA) molecules dominated as the replicating and evolving lifeforms prior to the emergence of DNA- and protein-based life. The focus of this thesis is on when and how this RNA world emerged. We use astrophysical and geophysical studies to constrain when the Earth was habitable, and biosignature studies to constrain when the Earth was inhabited. From this we obtain a time interval for the emergence of life. Considering all these constraints, we find that the Earth was habitable as early as 4.5 Ga, or as late as 3.9 Ga, depending on whether the early influx of asteroids inhibited life from emerging. The time that the Earth was inhabited is more precisely constrained to 3.7 Ga. This suggests life emerged within 800 Myr, and possibly in < 200 Myr. Between 4.5–3.7 Ga, the continental crust was slowly rising up from the global ocean, providing dry land on which warm little ponds could form. We develop the theory for the emergence of RNA polymers in these pond environments, whose wet-dry cycles promote polymerization. RNA is comprised of chains of nucleotides, and the latter is made up of ribose, phosphate, and a characteristic nucleobase. We numerically model the survival and evolution of nucleobases in warm little ponds from meteorite and interplanetary dust sources. The wet-dry cycles of our ponds are controlled by precipitation, evaporation, and seepage. The nucleobase sinks include photodissociation, seepage, and hydrolysis. Nucleobase and nucleotide seepage is efficient, therefore nucleotides and RNA molecules must have emerged rapidly (< a few years) in order to avoid falling through pores at the base of the pond. We find that meteorites, not interplanetary dust particles, are the dominant source of nucleobases used for RNA synthesis. Finally, under these conditions, we find that first RNA polymers likely emerged before 4.17 Ga. / Thesis / Master of Science (MSc)

<p dir="ltr">Here we present two studies of different aspects of meteoritic impacts. The first study is about the behavior of ejecta plumes after a hypervelocity impact onto a body with an atmosphere. The second study looks at the effect vaporization has on meteoroids as they descend through Earth's atmosphere, specifically the effect permeability and meteor size have on the vaporization during their explosive fragmentation.</p><p dir="ltr">Atmospheres play an important role in ejecta deposition after an impact event. Many impact experiments and simulations neglect the effect of atmospheres. In the first study, we simulate ejecta plumes created by craters with transient diameters of 2 km and 20 km on Mars and Earth to show the difference atmospheric density and crater size have on the strength of the interaction. The interaction of ejecta with an atmosphere is explored in this study using a two-fluid hydrocode that simultaneously simulates ejecta and atmospheres as coupled, continuum fields to correctly capture the transfer of mass, energy, and momentum between the two. Here we study the effect of vaporization of plume material as well as the effect of the bow shock. We find that only the fastest ejecta is vaporized with a peak vaporized mass of 2.5x10⁵ kg, 3.5 s after the impact in our 2 km diameter Terrestrial crater. Terrestrial meteorites are preferentially formed from the fastest ejecta. However, that fastest ejecta is mostly vaporized in our simulations, so to form a Terrestrial meteorite there must be a sufficiently large impact for solid material to be ejected and not vaporize. Thus, we place a lower limit of 33 km on the size of crater needed to generate terrestrial meteorites, but the crater size needed could be substantially larger. The bow shocks in our simulations result in lofting of ejecta, especially vaporized material, in the wake of the impactor. We find that Mars' thin atmosphere slows the ejecta but does not significantly change the trajectory of the plume. Earth's atmosphere can stop and entrain ejecta particles to suspend heated material long after the majority of material has already been deposited, resulting in 4x10¹⁰ kg of material being suspended in the atmosphere 100 seconds after the impact for a 2 km diameter crater. For larger craters, we find that Earth's atmosphere has a more limited effect and ejecta more closely follows a ballistic trajectory.</p><p dir="ltr">The 1908 Tunguska bolide event and the 2013 Chelyabinsk bolide event underscore the potential damage posed by relatively small meteoroids as compared to the dinosaur-killing Chicxulub meteoroid. In this study, we model Tunguska- and Chelyabinsk-sized bolide events, extending the work of Tabetah and Melosh (2018) by exploring a larger parameter space and introducing the novel feature of material vaporization. Building upon their findings that the porosity and permeability of a meteoroid significantly influence fragmentation, we investigate additional factors such as meteoroid size, entry speed, and entry angle. Furthermore, we demonstrate that vaporization plays a crucial role, lowering the fragmentation height by extracting energy through latent heat. We find that a larger meteoroid size or higher entry speed increases the amount of vaporization that occurs while lowering the altitude of disruption of the meteoroid, and that a shallower entry angle decreases the amount of vaporization and increases the altitude of disruption. Our study not only refines the understanding of bolide events but also introduces a novel perspective with potential implications for planetary science and impact risk assessment.</p>

This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Thesis: Ph. D., Massachusetts Institute of Technology, Department of Earth and Planetary Science, 1977 / Vita. / Bibliography : leaves 135-137. / By Robert Wayne Siegfried, II. / Ph. D. / Ph. D. Massachusetts Institute of Technology, Department of Earth and Planetary Science

<p dir="ltr">We present a model describing an impact jetting origin for the formation of chondrules, the mm– scale, igneous components of chondritic meteorites which originated during the first few million years of solar system history. The ubiquity of chondrules in both non-carbonaceous and carbonaceous chondrites suggests their formation persisted throughout the protoplanetary disk, but their formation mechanism is debated and largely unexplored in the outer disk.<b> </b>Using the iSALE2D shock physics code, we generate models of the process of impact jetting during mixed material (dunite and water ice) impacts that mimic accretionary impacts that form giant planet cores. We show that the process of impact jetting provides the conditions necessary to satisfy critical first-order constraints on chondrule characteristics (size, shape, thermal history). We then explore the implications of chondrule formation by impact jetting during the formation of giant planet cores by combining the original results with simulations of giant planet core accretion generated using a Lagrangian Integrator for Planetary Accretion and Dynamics (LIPAD) code.</p><p dir="ltr">The second closest Galilean satellite to Jupiter is Europa, an ocean world with an outer ice shell and subsurface water ocean encapsulating its rocky core. The surface of Europa is covered in double ridges. These features are defined by two topographic highs about 100 meters tall, with a central trough between them, which extend for hundreds of kilometers over the surface of the moon. Accurate models for the formation of features as prominent as double ridges will help to further constrain the interior structure and dynamics of the interior of the body. We use analytical and numerical finite element models to show that the incremental growth of an ice wedge within the ice shell can cause deformation matching the observed size and shape of observed double ridges on Europa. These models indicate that the total height and width of the ridges correspond to the depth of the wedge, so that deeper wedges create shorter and broader ridges. We consider different sources for the wedge material and ultimately argue in favor of local sources of liquid water within the ice shell.</p>

<p dir="ltr">The climate and habitable potential of a planet is controlled in part by its orbital configuration, including its obliquity, eccentricity, rotation period, and separation from the host star. Recent studies have suggested the exoplanets with higher eccentricity or obliquity than Earth might be able to produce larger biospheres, potentially leading to "super-habitable" worlds. However, high-obliquity and high-eccentricity planets have also been shown to be susceptible to increased water loss, which would decrease the habitable lifetime.</p><p dir="ltr">I use ExoPlaSim, a 3D General Climate Model, to investigate the habitable lifetimes of a diverse range of possible orbital configurations by varying the planetary obliquity (0-90^o), eccentricity (0-0.4), rotation period (6-96 hr), and stellar constant (1350-1650 W/m²). I study each orbital parameter independently while also co-varying obliquity with eccentricity and rotation period for the entire range of stellar constants. I find that stellar constant is the primary control on atmospheric water vapor, but also that the planetary obliquity, eccentricity and rotation period can determine the escape regime. Increasing the obliquity or eccentricity can push the climate into the significant escape regime at lower stellar constants relative to low-obliquity or low-eccentricity planets. Increasing the rotation period at high obliquities maximizes the habitable lifetime of an exoplanet.</p>