Modeling Pluto’s Atmosphere and Volatile Inventory Throughout Time

Johnson, Perianne Elizabeth

Graduate Thesis Or Dissertation

Modeling Pluto’s Atmosphere and Volatile Inventory Throughout Time Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/5d86p185m

Abstract

In this thesis, I explore feedbacks between Pluto’s surface and atmosphere over three distinct timescales. Through volatile transport, Pluto’s atmosphere affects and is affected by a myriad of other systems and processes active on the dwarf planet throughout the age of the solar system.

In Chapter 2, I model Pluto’s atmosphere in the modern day. Constrained by historical ground-based occultations and the New Horizons mission, I predict the annual surface temperature and atmospheric pressure variation for four assumptions about the distribution of nitrogen ice on Pluto’s surface. Pluto’s atmosphere is not likely to collapse at aphelion or winter solstice, and the minimum pressure will remain at or above the 0.1 microbar level for all but extreme ice assumptions. Additionally, I model the atmospheric temperature and pressure over the past 10 million years to encompass several of Pluto’s obliquity cycles and periods of extreme seasons. Even on this longer timescale, haze production is not likely to be interrupted by a reduction in atmospheric pressure, so another mechanism must be responsible for the observed surface heterogeneity.

In Chapter 3, I model the infill of Pluto’s Sputnik Planitia basin via condensation from the atmosphere and the subsequent reorientation of the basin. The timing of the infill is uncertain, but likely occurred around 4 billion years ago. Sputnik Planitia, Pluto’s “heart,” is a 1000-km wide purported impact basin that is filled with a several kilometer thick ice sheet of nitrogen and methane ice and is located very close to the anti-Charon point. This location near the tidal axis is a hallmark of true polar wander, and in fact it has been previously shown that true polar wander is a plausible mechanism for reorienting Sputnik Planitia. My work strengthened this argument by coupling a true polar wander model and a climate model in order to realistically model the growth of the ice sheet subject to insolation patterns. I constrained the total amount of ice in Sputnik Planitia to 1 - 2 km, and the initial location to most likely the 35^◦ - 55^◦N latitude band.

In Chapter 4, I step back even further in time to model the loss of nitrogen from Pluto’s atmosphere during the time period of giant planet migration, 4.5 billion years ago. Pluto and its neighboring Plutinos formed closer to the Sun and were driven outwards in a chaotic process when the orbits of the giant planets of the solar system were unstable, a time period I term the “Wild Years.” In order to know the amount of nitrogen that Pluto started out with, this time period of much hotter and much cooler environmental conditions needs to be modeled. I estimate the loss of nitrogen via three mechanisms: photochemical destruction, atmospheric escape, and impact erosion. While these losses will be significantly underestimated if the present-day rates are used, even accounting for higher rates of loss during the Wild Years results in a small overall loss relative to the amount of nitrogen observed on Pluto today. Thus, I conclude that Pluto’s primordial nitrogen inventory was not significantly different to its present-day inventory

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