Date of Award

Spring 1-1-2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Bruce M. Jakosky

Second Advisor

David Brain

Third Advisor

Shijie Zhong

Fourth Advisor

Brian Toon

Fifth Advisor

Paul Hayne

Abstract

Today, Mars is a cold, dry planet with a thin atmosphere, incapable of sustaining liquid water on the surface. However, there is ample evidence that liquid water once flowed on the surface, which could have been maintained by a warmer and thicker atmosphere. The ancient atmosphere must have either been sequestered into the solid planet or lost to space. Isotopes of major atmospheric species on Mars show an enrichment of heavy isotopes relative to light ones when compared to Earth’s atmosphere. Because molecular diffusion is the dominant mixing process at high altitudes, light isotopes are lost more readily from the top of the atmosphere. This thesis focuses on constraining the total amount of atmosphere that has been removed through an investigation of argon isotopes. Two main pieces make up the foundation of this work – determining the fractionation of Ar isotopes in the upper atmosphere and modeling the evolution of Ar isotopes from ancient Mars to the present-day.

To calculate the fractionation of Ar isotopes in the upper atmosphere, I derive homopause altitudes, exobase altitudes, and scale heights from densities of atmospheric species measured by the NGIMS (Neutral Gas and Ion Mass Spectrometer) instrument on the MAVEN (Mars Atmosphere and Volatile EvolutioN) spacecraft. The homopause is an important concept, and I show that its altitude varies in both time and space. Furthermore, I show that this variation is correlated to changes in gravity wave activity because of the role gravity wave saturation plays in generating turbulence and setting the turbopause level. The derived fractionation of 36Ar and 38Ar is used to estimate total atmospheric loss from Rayleigh distillation.

While Rayleigh distillation provides a useful framework for interpreting Ar isotope measurements, it is unrealistic because it considers removal of Ar from the exobase to be the only process that has altered the isotope ratio over time. By constructing a model for Ar isotope evolution that includes all of the major sources and sinks, I provide a more thorough analysis to understand Mars’ atmospheric history. The processes considered include atmospheric sputtering, outgassing of the interior and crust, and impact supply and erosion. I examine a wide range of parameter space for these time-dependent processes and illustrate how each affects the evolution of the abundances and ratios of 36Ar, 38Ar, and 40Ar. The fractionation derived from NGIMS measurements is used to constrain the model leading to the result that 66% of all the atmospheric 36Ar ever introduced into Mars’ atmosphere has escaped to space. Finally, I discuss in detail what this means for loss of Mars’ most abundant species, carbon dioxide.

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