Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1992. / Includes bibliographical references (leaves 175-182) and index. / by Robert A. Boldi. / Ph.D.

Thesis: Ph. D. in Planetary Science, Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2016. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 299-311). / The Kepler space observatory and other surveys have revealed thousands of planets and planetary systems that look significantly different from our own. In particular, the preponderance of super-Earths and mini-Neptunes (planets with radii smaller than Neptune's but larger than Earth's) at short orbital radii has challenged planet formation theories developed in the context of our own Solar System. How and where these planets form remains an outstanding question. Given the large frequency with which such planets occur around other stars, understanding the formation pathways of these planets has the potential to reveal dominant mechanisms for planet formation and evolution and to place our own Solar System within a broader context. The purpose of this thesis is to address and develop frameworks by which these questions can be answered. The thesis is comprised of two parts. In Part I, we consider the question of how and where close-in exoplanets formed. We do this in two ways. First, we use that fact that many close-in super-Earths and mini-Neptunes possess gaseous envelopes that comprise several percent or more of the total planet mass in order to construct a self-consistent planet formation history that accounts for core accretion, thermal evolution, and dynamical interactions during the core assembly process via giant impacts. We find that envelope masses accreted from the gas disc by planetary embryos are typically much smaller than those inferred for many exoplanets, and that the envelope mass fraction is further reduced substantially during the assembly phase when embryos merge. Fully assembled planets can accrete observed envelope masses from the residual disc only if energy exchange between the envelope and the underlying core is totally inhibited. It is therefore very unlikely that such planets formed at their observed semimajor axes, but instead formed further out and migrated inwards. Second, we consider the late-stage evolution of exoplanets after gas disc dissipation in order to explain the bulk structural diversity of observed super-Earths and mini-Neptunes. Whereas naive application of core accretion models suggests a narrow mass-radius relationship for these exoplanets, the population possesses a great deal of diversity in mean density. While photoevaporative mass loss from the host star is the most-commonly invoked explanation for this diversity, we use the fact that many exoplanetary systems are in tightly packed orbital configurations to propose instead that late-stage collisions are at least in part responsible for the observed diversity. We infer envelope mass fractions for planets in the literature whose masses and radii have been measured, and on the basis of this, we identify multiplanet systems whose bulk structural diversity favors late-stage impacts as opposed to photoevaporative devolatilization. In Part II, we turn our attention to the formation and evolution of our own Solar System. We do this within the context of NASA's OSIRIS-REx asteroid sample return mission. OSIRIS-REx, which launches in September 2016, will arrive at the near-Earth asteroid 101955 Bennu in 2019 with the objective of constraining its composition, orbit, and other bulk properties. In order to better understand the composition of Bennu, an instrument designed to measure its elemental abundances via X-ray fluorescence spectroscopy called REXIS was developed and built at MIT. In this part of the thesis, we investigate the ability of REXIS to constrain the composition of Bennu via fluorescence spectroscopy, as well as its potential to place Bennu within an analogue meteorite class. We carry out our analysis by modeling Solar X-ray activity and the X-ray fluorescence from Bennu, as well as by simulating data product and analysis from the instrument in order to predict REXIS's ability to carry out its goal of contextualizing Bennu within the asteroid and meteorite population. / by Niraj K. Inamdar. / Ph. D. in Planetary Science

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2009. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 35-36). / The Aegean region has undergone several episodes of extensional deformation from early Miocene to present time. Among the structures that accommodate extension are faults that bound and cut sediments within young fault-controlled sedimentary basins. The objective of this study is to add to constraints on the history of within the upper plate of the Hellenic subduction zone. In particular, this study is aimed at mapping and, eventually, dating sediments and related normal faults in the Limni-Istiea basin of northern Evia. Field mapping in the southern portion of this basin reveals eight sedimentary units and suggests several periods during which steep relief was formed within and adjacent to the basin, interspersed with periods of deposition in fluvial and shallow lacustrine environment. The three sets of faults identified in the mapped area are consistent with the orientation of structures observed at the western end of the North Aegean trough system and within the Central Hellenic shear zone. The oldest faults are low-angle, north-south trending and east-dipping; younger structures are high angle, west-east or southwest-northeast trending and generally south-dipping; the youngest faults are high-angle, northwest-southeast trending and dipping to the southwest. The paleomagnetic analyses results show clockwise rotation of the Limni- Istiea basin by 18° during or after the last stages of extension within the basin. / by Aleksandra M. Hosa. / S.M.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1985. / Microfiche copy available in Archives and Science. / Bibliography: leaves 266-279. / by Jane Elizabeth Selverstone. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1991. / Includes bibliographical references. / by Niall Charles Slowey. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1994. / Includes bibliographical references (leaves 216-221). / by Xiaomin Zhao. / Ph.D.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric and Planetary Sciences, 1986. / Microfiche copy available in Archives and Science. / Bibliography: leaves 81-82. / by Carolyn Denise Ruppel. / M.S.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2011. / Cataloged from PDF version of thesis. Vita. / Includes bibliographical references (p. 151-165). / I have investigated the effect of composition, especially ferric iron and aluminum, on the equations of state and phase stability of perovskite and post-perovskite. The presence of trivalent cations decreases the bulk modulus of perovskite at pressures corresponding to the upper lower mantle. Ferric iron in perovskite undergoes a spin-pairing transition from the high spin state to low spin in the octahedral site. Ferric iron in the dodecahedral site remains high spin. In the absence of aluminum, the spin transition is gradual between 0 and 55 GPa, and bulk modulus increases at the completion of the spin transition. In the presence of aluminum, there is an abrupt increase in the amount of low spin ferric iron near 70 GPa, likely the result of site mixing. The high compressibility of the structure below 70 GPa results in the volume nearing that of magnesium endmember, MgSiO₃ , perovskite. Concurrent with the spin transition in aluminum-bearing perovskite, the structure stiffens. The increase in density and bulk modulus at -70 GPa results in an increase in bulk sound speed that may be related to heterogeneities in bulk sound speed observed seismically at 1200-2000 km depth in the Earth. The effect of composition on the perovskite to postperovskite phase transition was also investigated. No change in the spin state of ferric iron was found at the perovskite to post-perovskite phase transition: ferric iron is low spin in the octahedral site and high spin in the dodecahedral site. At the phase transition, ferric iron only slightly broadens the perovskite plus post-perovskite mixed phase region while ferrous iron and aluminum were each found to significantly broaden the mixed phase region to hundreds of kilometers thick. The effect of background mineral phases was assessed for a basaltic system, rich in aluminum. The coexisting minerals were found to significantly reduce the effect of the aluminum, producing a boundary that is potentially sharp enough for seismic detection in silicon-rich systems, such as basalt. / by Krystle Carina Catalli. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2002. / Includes bibliographical references (p. 119-136). / The goal of this work is a quantitative description of the morphology of the surface of Mars, in order to constrain the nature of processes acting during the ancient past through today. Emphasis is placed on linking geometric properties to physical mechanisms. Surface smoothness on Mars is distinctive in the vast northern hemisphere plains. Amazonis Planitia is remarkable in its smoothness, exhibiting an rms variation in topography of < 2 m over a 100-km baseline, that is most comparable to planetary surfaces that are depositional in origin. The region of concentrated hematite mineralization in Sinus Meridiani is also relatively smooth, but neither region forms a closed basin. Mars' slope distribution is longer tailed than those of Earth and Venus, indicating a lower efficiency of planation processes relative to relief-building tectonics and volcanics. The shallower long-wavelength portion of the lowlands' topographic power spectrum relative to the highlands' can be accounted for by a simple model of sedimentation such as might be expected at an ocean's floor, but the addition of another process such as cratering is necessary to explain the spectral slope in short wavelengths. Large drainage systems on Mars have geomorphic characteristics that are inconsistent with prolonged erosion by surface runoff. We find the topography has not evolved to an expected equilibrium terrain form, even in areas where runoff incision has been previously interpreted. / (cont.) We demonstrate that features known as slope streaks form exclusively in regions of low thermal inertia, steep slopes, and incredibly, only where daily peak temperatures exceed 275 K during the martian year. The results suggest that at least small amounts of water may be currently present and undergo phase transitions. We detect subtle changes of the polar surface height during the course of seasonal cycles. Using altimetric crossover residuals, we show that while zonally averaged data captures the global behavior of CO2 exchange, there is a strong dependence of the pattern on longitude. Decomposition of the signal into harmonics in time shows the amplitudes are correlated with the polar cap deposits. / by Oded Aharonson. / Ph.D.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric and Planetary Sciences, 1984. / Microfiche copy available in Archives and Science. / Vita. / Bibliography: leaves 153-156. / by Stephen Keith Park. / Ph.D.