Much speculation on extraterrestrial life has focused on finding environments where water is present. Heating of smaller icy bodies may create and sustain a possible liquid layer below the surface. If liquid water was sustained for geologically significant times (> 108 years) within the ubiquitous small bodies in the outer solar system, the opportunities for development of simple life are much greater. The lifetime of the liquid water layer will depend on several factors, including the rate of rock/water reaction, which will depend on the rate at which water can be segregated from a melting ice/rock core. For the liquid water phase to migrate toward the surface, the denser rock phase must compact. The primary question that this thesis will answer is how fast melt water can segregate from the core of an ice-rich planetesimal. To answer this question we treat the core as two phase flow problem: a compacting viscous “solid” (ice/rock mixture) and a segregating liquid (melt water). The model developed here is based on the approach derived to study a different partially molten solid: in the viscously deforming partially molten upper mantle. We model a planetesimal core that initially a uniform equal mixture of solid ice and rock. We assume chondritic levels of radiogenic heating as the only heat source, and numerically solve for the evolution of solid and melt velocities and the distribution of melt fraction (“porosity”) during the first few million years after accretion. From a suite of numerical models, we have determined that the meltwater is segregated out of the core as fast as it is created, except in the case of very fast melting times (0.75 My vs. 0.62 My), and small ore radius (~25 to 150 km, depending on the viscosity of the ice/rock mixture in the solid core). In these latter cases, segregation is slower than migration and a high water fraction develops in the core. Heat released by water-rock reactions (not included in this model) will tend to drive up melting rates in all cases, which may favor this latter endmember.

Indiana University-Purdue University Indianapolis (IUPUI) / This thesis is a comprehensive investigation into the mechanics of evaporation experienced by planetesimals during accretion, a planet-building process. The evaporation events that these rocky bodies experience govern their chemical evolution, impacting the chemistry of the final body – a planet. Studying these planet-building processes is notoriously difficult (e.g., Sossi et al., 2019). There are still many unknowns surrounding what controls the degree of evaporation these bodies experience, and the resulting chemical signatures. The current study was designed to attempt to define some important parameters that govern silicate melt evaporation. Here, we isolate and evaluate the effects of (1) pressure, (2) oxygen fugacity and (3) the activity coefficient of MgO on evaporating planetesimals through a series of computational models. The model introduced in this study, the ƒO2 Modified KNFCMAS Model, uses a robust stepwise routine for calculating evaporative fluxes from a shrinking sphere. The modelling results are then compared to data from partial evaporation experiments of synthetic chondrite spheres to demonstrate the validity of this model, and to expose unknowns about the physicochemical conditions of high temperature silicate melts experiencing evaporation (in this case, the effective pressure, and the activity coefficient of MgO). Major element-oxide and isotope data from the models yielded two main conclusions concerning planetesimals: (1) the rate of evaporation is controlled by pressure and oxygen fugacity and (2) the chemical composition of the residual melt is controlled by oxygen fugacity and the activity coefficient of MgO. Results from computational modelling and evaporation experiments were used to determine an approximation for the activity coefficient of MgO in a simplified chondritic composition, as well as the effective pressure experienced by the evaporating spheres during the partial evaporation experiments. This study outlines the controls on planetesimal chemistry during evaporation and provides a more accessible means of studying these complex processes.

The Kuiper belt is a population of planetesimals outside the orbit of Neptune. The high inclinations and eccentricities exhibited by many belt members, and its very low mass (M 0.1M) present an enigma to planetesimal accretion scenarios: the high relative encounter velocities (vrei 1 km s-1), and infrequent collisions of the largest members make the growth of Pluto-sized bodies impossible over the age of the Solar system. Accretion in the early stages of planet-building must have been in a more dense environment allowing large objects to grow before growth was halted. The current Kuiper belt population is the left-over relic of accretion, which has undergone collisional re-shaping since the epoch of accretion. The shape of the size distribution can provide constraint on the accretion timescale, the primordial Kuiper belt mass, and the collisional processing the belt has undergone. Thus, a measure of the size distribution provides one of the primary constraint on models which attempt to explain the formation of the Kuiper belt. We have performed a large-scale ecliptic Kuiper belt survey, with an aerial cov¬erage of 3.3 square degrees to a limiting magnitude m(R) 27. From these ob¬servations, we have discovered more than 100 new Kuiper belt objects. Using this survey we have provided the best measurement of the Kuiper belt luminosity function to-date, from which we have inferred the size distribution. We have found that the size distribution is well described by a power-law for large objects with a steep slope q1 = 4.8, that breaks, or rolls over to a shallower power-law with slope q2 = 2 at ob¬ject diameter ~ 60 km. The steep large object slope is indicative of a short accretion phase, lasting no more than a few 100 Myr. The large break diameter demonstrates that the Kuiper belt has undergone substantial collisional processing. We have developed a collisional evolution model which we have used to study the effects of planetesimal bombardment and disruption on the size distribution. We have found that, in the current Kuiper belt, little to no evolution is occurring, or has occurred for the observable Kuiper belt. We conclude that the large break diameter cannot be produced in the current environment over the age of the Solar system. A period of intense collisional evolution in a much more dense, and hence, more massive belt is required. These findings are consistent with accretion models; the typical finding is that growth of the largest Kuiper belt objects over the age of the Solar system requires a much more massive belt than currently observed. These results point to a history in which an initially much more massive Kuiper belt underwent a short period of quiescent accretion producing Pluto size bodies. Some event then occurred, which dynamically excited the planetesimals, producing an erosive environment which effectively halted planet growth and rapidly depleted the majority of the primordial mass. The remnant of this depletion is the Kuiper belt we observe today.

This thesis finds that vortex instabilities are not necessarily a barrier to their potential as sites for planetesimal formation. It is challenging to build planetesimals from dust within the lifetime of a protoplanetary disc and before such bodies spiral into the central star. Collecting matter in vortices is a promising mechanism for planetesimal growth, but little is known about their stability under these conditions. We therefore aim to produce a more complete understanding of the stability of these objects. Previous work primarily focusses on 2D vortices with elliptical streamlines, which we generalise. We investigate how non?constant vorticity and density power law profiles affect stability by applying linear perturbations to equilibrium solutions. We find that non?elliptical streamlines are associated with a shearing flow inside the vortex. A ?saddle point instability? is seen for elliptical?streamline vortices with small aspect ratios and we also find that this is true in general. However, only higher aspect ratio vortices act as dust traps. For constant?density vortices with a concentrated vorticity source we find parametric instability bands at these aspect ratios. Models with a density excess show many narrow bands, though with less strongly growing modes than the constant?density solutions. This implies that dust particles attracted to a vortex core may well encounter parametric instabilities, but this does not necessarily prevent dust?trapping. We also study the stability and lifetime of vortex models with a 2D flow in three dimensions. Producing nearly?incompressible 3D models of columnar vortices, we find that weaker vortices persist for longer times in both stratified and unstratified shearing boxes, and stratification is destabilising. The long survival time for weak, elongated vortices makes it easier for processes to create and maintain the vortex. This means that vortices with a large enough aspect ratio have a good chance of surviving and trapping dust for sufficient time in order to build planetesimals.

abstract: This study explores how bulk composition and oxygen fugacity (fO2) affect the partitioning of sulfur between the molten mantle and core of an early planetesimal. The model can be used to determine the range of potential sulfur concentrations in the asteroid (16) Psyche, which is the target of the National Aeronautics and Space Administration/Arizona State University Psyche Mission. This mission will be our visit to an M-type asteroid, thought to be dominantly metallic. The model looks at how oxygen fugacity (fO2), bulk composition, temperature, and pressure affect sulfur partitioning in planetesimals using experimentally derived equations from previous studies. In this model, the bulk chemistry and oxygen fugacity of the parent body is controlled by changing the starting material, using ordinary chondrites (H, L, LL) and carbonaceous chondrites (CM, CI, CO, CK, CV). The temperature of the planetesimal is changed from 1523 K to 1873 K, the silicate mobilization and total melting temperatures, respectively; and pressure from 0.1 to 20 GPa, the core mantle boundary pressures of Vesta and Mars, respectively. The final sulfur content of a differentiated planetesimal core is strongly dependent on the bulk composition of the original parent body. In all modeled cores, the sulfur content is above 5 weight percent sulfur; this is the point at which the least amount of other light elements is needed to form an immiscible sulfide liquid in a molten core. Early planetesimal cores likely formed an immiscible sulfide liquid, a eutectic sulfide liquid, or potentially were composed of mostly troilite, FeS. / Dissertation/Thesis / Masters Thesis Geological Sciences 2020

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Orientador: Ernesto Vieira Neto / Resumo: O estudo da formação e evolução do Sistema Solar é uma fonte de informação para entender sob quais condições a vida poderia surgir e evoluir. Nós apresentamos, nesta Tese de doutorado, um estudo numérico da fase final de acresção dos planetas gigantes do Sistema Solar durante e após a fase do disco de gás protoplanetário. Em nossas simulações, utilizamos um modelo recente e confiável para a formação de Urano e Netuno para esculpir as propriedades do disco trans-Netuniano original (Izidoro et al. , 2015a). Nós fizemos este estudo de uma maneira autoconsistente considerando os efeitos do gás e da evolução dos embriões planetários que formam Urano e Netuno por colisões gigantescas. Consideramos diferentes histórias de migração de Júpiter, devido a incerteza de como Júpiter migrou, durante a fase de gás. As nossas simulações permitiram obter pela primeira vez as propriedades orbitais do disco trans-Netuniano original. Então, calculamos o tempo de instabilidade dos planetas gigantes a partir de sistemas planetários que formam similares Urano e Netuno. Nossos resultados indicam fortemente que a instabilidade dos planetas gigantes acontecem cedo em até 500 milhões de anos e mais provável ainda ter acontecido em 136 milhões de anos após a dissipação do gás. Nós também realizamos simulações para discutir alguns efeitos dinâmicos que acontecem na região do cinturão de Kuiper. Estes efeitos acontecem quando Netuno esteve em alta excentricidade durante a instabilidade planetária. Para es... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: A study of the formation and evolution of the Solar System is a source of information for an understanding of what conditions life could arise and evolve. We present a numerical study of the final stage of accretion of the giant planets of the Solar System during and after the protoplanetary gas disc phase. In our simulations, we use a recent and reliable model for the formation of Uranus and Neptune to sculpt the properties of the original trans-Neptunian disk (Izidoro et al. , 2015a). We have done this study in a self-consistent way considering the effects of gas and the evolution of planetary embryos which form Uranus and Neptune by mutual giant collisions. We considered different Jupiter migration stories due to the uncertainty of how Jupiter’s migration was during the gas phase. Our simulations provide for the first time to obtain the orbital properties of the original trans-Neptunian disk. We then calculate the instability time of the giant planets from planetary systems which form similar Uranus and Neptune. Our results strongly indicate that the instability of the giant planets occurs early within 500 million years and even more likely to happen at 136 million years after gas dissipation. We also perform simulations to discuss some dynamical effects that happen in the Kuiper belt region. These effects happen when Neptune was in high eccentricity during planetary instability. For this problem, we use the simulations performed by Gomes et al. (2018) who investigated the... (Complete abstract click electronic access below) / Doutor