Analysis of EUV Dayglow Spectra of Triton, Titan and Earth

We have constructed a coupled ion and neutral chemical model of the ionosphere and thermosphere of Titan. Along with density profiles of ions and minor neutrals, we have computed the heating rates and heating efficiencies for the neutrals and chemical heating rates and efficiencies for the ions. We find that the neutral heating efficiency in our standard model varies from about 30% near 800 km to 22% near 2000 km. The most important heating processes are neutral-neutral reactions and photodissociation. The ion chemical heating rates maximize at about 10 eV cm(sup -3) s(sup -1), and the corresponding ion heating efficiencies peak near 1000 km at about 0.6%

Thermally-Diven Atmospheric Escape

Accurately determining escape rates from a planet's atmosphere is critical for determining its evolution. Escape can be driven by upward thermal conduction of energy deposited well below the exobase, as well as by non-thermal processes produced by energy deposited in the exobase region. Recent applications of a model for escape driven by upward thermal conduction, called the slow hydrodynamic escape model, have resulted in surprisingly large loss rates for the thick atmosphere of Titan, Saturn's largest moon. Based on a molecular kinetic simulation of the exobase region, these rates appear to be orders of magnitude too large. Because of the large amount of Cassini data already available for Titan's upper atmosphere and the wealth of data expected within the next decade for the atmospheres of Pluto, Mars, and extrasolar planets, accurately determining present escape rates is critical for understanding their evolution. Therefore, the slow hydrodynamic model is evaluated here. It is shown that such a model cannot give a reliable description of the atmospheric temperature profile unless it is coupled to a molecular kinetic description of the exobase region. Therefore, the present escape rates for Titan and Pluto must be re-evaluated using atmospheric models described in this paper

Mars' Energetic Plume Ion Escape Channel

Mars is losing its atmosphere. The planet’s small size results in relatively low energy requirements for atmospheric particles to escape into deep space, and its lack of a planetary magnetic field allows the solar wind to directly interact with the upper atmosphere, providing an additional source from which particles may obtain this requisite energy. The escape of particles from Mar’s atmosphere over the course of billions of years is not only a story of atmospheric evolution; it is a story of the evolution of a global climate. It is now thought that oceans worth of liquid water may have existed on a warmer ancient Mars, and atmospheric escape of hydrogen and oxygen is one explanation of how such an ocean may have vanished. The research presented here revolves around the examination of one particular "loss channel" for oxygen (and other "heavy" ions) from Mars. This loss channel, known as the "energetic plume," consists of pickup ions, electrically charged planetary particles that, finding themselves in the solar wind flow past Mars, are accelerated in the direction of the solar wind's convective electric field (ESW). In the spatially zoomed out view, the acceleration in this direction is just the initial part of the first gyration of an ESW-cross-B drift in the direction of solar wind flow. Zoomed in closer to Mars, where ion-observing satellites have orbited, a result of the huge gyroradius of these pickup ions is that, in addition to having high energies, energetic plume particles have flight directions distinct from other escaping particles and are observed at locations not reached by other escaping particles. This dissertation introduces the Mars space environment and the problem of atmospheric escape generally before presenting the search for this distinct phase space signature of the energetic plume in ion data from the Mars Express satellite. It was found that despite the presence of obstacles to observing the energetic plume using the Ion Mass Analyzer (IMA) onboard Mars Express, it is possible to both identify unambiguous instances of energetic plume observations in IMA data and to see signatures of the energetic plume in statistical maps of the Mars space environment made using IMA observations. Furthermore, it was found that accounting for “weathervaning” – the subsolarward bending of magnetic field lines draped around the ionosphere – can be used to improve estimates of the direction of ESW. The resulting more accurate estimate for the direction of ESW improves statistical representations of the energetic plume in IMA data, and significant quantities of energetic plume type ions are observed by IMA ~ 60% more frequently in the newly estimated direction of ESW than in the previously estimated direction of ESW. We conclude that the improved method of estimating the direction of ESW should be used in place of previously existing proxies in studies concerning the variation of energetic plume fluxes for different solar conditions during the time period between Jan. 2004 and Oct. 2006.PHDAtmospheric, Oceanic & Space ScienceUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/143899/1/blakecjo_1.pd

Atmospheric Escape Processes and Planetary Atmospheric Evolution

The habitability of the surface of any planet is determined by a complex evolution of its interior, surface, and atmosphere. The electromagnetic and particle radiation of stars drive thermal, chemical and physical alteration of planetary atmospheres, including escape. Many known extrasolar planets experience vastly different stellar environments than those in our Solar system: it is crucial to understand the broad range of processes that lead to atmospheric escape and evolution under a wide range of conditions if we are to assess the habitability of worlds around other stars. One problem encountered between the planetary and the astrophysics communities is a lack of common language for describing escape processes. Each community has customary approximations that may be questioned by the other, such as the hypothesis of H-dominated thermosphere for astrophysicists, or the Sun-like nature of the stars for planetary scientists. Since exoplanets are becoming one of the main targets for the detection of life, a common set of definitions and hypotheses are required. We review the different escape mechanisms proposed for the evolution of planetary and exoplanetary atmospheres. We propose a common definition for the different escape mechanisms, and we show the important parameters to take into account when evaluating the escape at a planet in time. We show that the paradigm of the magnetic field as an atmospheric shield should be changed and that recent work on the history of Xenon in Earth's atmosphere gives an elegant explanation to its enrichment in heavier isotopes: the so-called Xenon paradox

The Reactions of Dications with Neutral Species: Understanding Planetary Ionospheres

Doubly charged cations (dications) of molecular and atomic species are predicted to be influential in high-energy environments such as the interstellar medium, the ionospheres of planets and satellites, and plasmas. However, definitive detection of dications in these environments are not yet available and the presence of these ions is often overlooked. Early investigations of dication-neutral collisions, often at high collision energies, only resulted in the observation of electron-transfer reactivity. Modern experiments, using lower collision energies, have revealed a range of exotic chemistry such as bond-formation with rare gas elements. This chemistry, coupled with the significant abundance of dications predicted in ionospheres, suggests that these ions could play important roles in atmospheric processes. For example, dications could be involved in the chemistry of complex molecule assembly. The study of dications and their reactions is clearly important to understanding ionospheric processes in planets and satellites including the prebiotic Earth. This thesis explores the bimolecular reactivity of various dications with neutral species in order to better understand the processes occurring in the ionospheres of planets and satellites. The position-sensitive coincidence mass spectrometry technique employed in this work utilises coincident, position-sensitive, detection of ions to reveal comprehensive information concerning the dynamics and energetics of the consequences of dication-neutral reactions. Specifically, the reactions following collisions of Ar2+, S2+ and CH2CN2+ with atoms and small molecules have been investigated. These dication-neutral collision systems exhibit intriguing reactivity clearly demonstrating the diversity of dication chemistry. For example, many of the electron-transfer reactions observed show evidence of proceeding via collision complexes, contrary to the orthodox (direct) mechanism. Of the bond-forming reactions detected, those generating molecular species containing a rare gas, such as ArO+ and ArN+, are the most notable. Despite the observation of the involvement of collision complexes in electron-transfer, many of the bond-forming reactions described in this thesis have been shown to occur via direct mechanisms. The observation of bond-forming reactions and the involvement of collision complexes clearly shows the facility of dications to form associations despite their often-high potential energies