Tracking Volatile Elements in Protoplanetary Disks and on Planetary Surfaces
<p>The formation of planets begins with collisions of tiny, micron-sized, dust grains. These grains reside in structures known as protoplanetary disks, rotating disks consisting of gas and dust that encircle young protostars as a natural outcome of star formation. Although the processes of pla...
| Summary: | <p>The formation of planets begins with collisions of tiny, micron-sized, dust grains. These grains reside in structures known as protoplanetary disks, rotating disks consisting of gas and dust that encircle young protostars as a natural outcome of star formation. Although the processes of planet formation and evolution take place over millions and billions of years, in our limited view we can only see snapshots of the different stages. Many of the formative processes are difficult, if not impossible, to observe directly. However, evidence of these events exists in the chemical composition of the bulk material and surfaces of planets themselves, the gas and solid components of protoplanetary disks, and planetary debris such as asteroids and comets. This thesis utilizes modeling and observations of the carbon and nitrogen content of protoplanetary disks to shed light on key factors that control the formation and chemical composition of planets. In addition, this thesis advances techniques for the elemental analysis of planetary surfaces facilitating the detection of salts on the surface of Mars.</p>
<p>Chapter 2 estimates the maximum potential destruction of solid, refractory carbon in protoplanetary disks in an effort to explain the lack of carbon found in meteorites and the bulk silicate Earth relative to the interstellar materials that seeded their formation. In a T-Tauri disk assuming uniform turbulence and passive heating from stellar photons destruction of refractory carbon sources via oxidation and UV photolysis is limited to the warm, photochemically-active disk surface layers. Exploration of distinct disk environments, considering non-idealized mass transport or enhanced disk heating due to active stellar mass accretion, is needed to explain the widespread lack of carbon in rocky solar-system bodies.</p>
<p>Chapters 3 and 4 present spectral observations by the Atacama Large Millimeter/submillimeter Array (ALMA) of mature, 5-11 Myr-old, protoplanetary disks in the Upper Scorpius region that indicate diverging behavior of the key carbon and nitrogen species in the disk gas as disks evolve. Selective depletion of CO from the gas may cause disk gas masses to be underestimated if based on CO measurements alone and further investigation of additional gas tracers is warranted.</p>
<p>Depletion of CO from the gas in the outer regions of disks observed by ALMA may be the result of sequestration of carbon into less volatile species such as CO<sub>2</sub> and CH<sub>3</sub>OH. Chapter 5 explores the fate of CO<sub>2</sub> and CH<sub>3</sub>OH ices entering the inner regions of protoplanetary disks. Carbon returns to CO in unshielded transparent regions of the inner disk surface, consistent with infrared observations, but carbon reservoirs in the disk midplane may be distinct depending on the efficiency of mass transport in the disk.</p>
<p>Chapter 6 examines the abilities of the Laser-Induced Breakdown Spectroscopy (LIBS) instrument ChemCam on the Mars rover Curiosity in regards to the detection of salts. LIBS analysis of a set of prepared sample pellets containing decreasing concentrations of salt identifies elemental emission lines of Cl, C, and S that are sensitive to changes in chloride, carbonate, and sulfate salt concentrations, respectively, and provides detection limits for ChemCam measurements of these salts.</p> |
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