Thesis (Ph.D.)--Australian National University, 2005.

Thesis (Ph.D.) - University of St Andrews, June 2008.

Thesis (M.S. in environmental science)--Washington State University, August 2009. / Title from PDF title page (viewed on Sept. 22, 2009). "School of Earth and Environmental Sciences." Includes bibliographical references (p. 92-97).

Thesis (Ph. D.)--University of Hawaii at Manoa, 2004. / Includes bibliographical references (leaves 246-257).

Thesis (Ph. D.)--Ohio State University, 2007. / Title from first page of PDF file. Includes bibliographical references (p. 156-159).

Thesis (Ph. D.)--University of Canterbury, 2009. / Typescript (photocopy). Includes bibliographical references (p. 409-433). Also available via the World Web.

After decades of exploration, the martian neutral hydrogen exosphere has remained poorly characterized. The first measurements of this layer by Mariner 6 and 7 revealed it to be optically thick in Lyman α emission, along with a characteristic temperature that was higher than the majority of the collisional atmosphere of Mars. Further exploration revealed that the hydrogen in the martian exosphere was formed from photodissociation of water vapor by solar UV light, and that its escape can be directly linked to the escape of water from Mars. Theoretical analysis of hydrogen transport in the martian atmosphere suggested a steady escape rate limited by diffusion of hydrogen through the martian atmosphere. Subsequent missions to Mars provided a wide range of values for the temperature and density of hydrogen at Mars. It is important to determine the properties of the martian hydrogen exosphere in order to constrain the escape flux, which can then be used to calculate the total amount of water lost by Mars during its evolutionary history. In this dissertation the characteristics of the martian hydrogen exosphere are constrained using data from the Hubble Space Telescope (HST). HST observations of this layer reveal short-term seasonal changes, thereby disproving the theory of constant escape rate for H from Mars. Analysis of these datasets using a radiative transfer model constructed at Boston University revealed a large seasonal variation in the hydrogen escape flux, making it difficult to easily backtrack the martian water loss history. Results also indicate the possible presence of a superthermal population created by non-thermal processes at Mars. Exploration of the latitudinal symmetry of the martian exosphere indicates that it is symmetric above 2.5 martian radii and asymmetric below this altitude, which could be due to temperature differences between the day and night side. Finally, there are large uncertainties in determining the characteristics of the martian exosphere after decades of exploration, due to various assumptions about the intrinsic characteristics of the martian exosphere in the modeling process, degeneracy in the two modeling parameters for hydrogen -i.e. its temperature and density, unaccounted seasonal effects and uncertainties introduced from spacecraft instrumentation and viewing geometry.

We analyzed data from the first year of a survey for Near-Earth Objects (NEOs) that we are carrying out with the Dark Energy Camera (DECam) on the 4 m Blanco telescope at the Cerro Tololo Inter-American Observatory. We implanted synthetic NEOs into the data stream to derive our nightly detection efficiency as a function of magnitude and rate of motion. Using these measured efficiencies and the solar system absolute magnitudes derived by the Minor Planet Center for the 1377. measurements of 235. unique NEOs detected, we directly derive, for the first time from a single observational data set, the NEO size distribution from 1. km down to 10 m. We find that there are 106.6 NEOs larger than 10 m. This result implies a factor of 10 fewer small NEOs than some previous results, though our derived size distribution is in good agreement with several other estimates.

Planetesimals may form from the gravitational collapse of dense particle clumps initiated by the streaming instability. We use simulations of aerodynamically coupled gas-particle mixtures to investigate whether the properties of planetesimals formed in this way depend upon the sizes of the particles that participate in the instability. Based on three high-resolution simulations that span a range of dimensionless stopping times 6 X 10(-3) <= tau <= 2, no statistically significant differences in the initial planetesimal mass function are found. The mass functions are fit by a power law, dN/dM(p) proportional to M-p(-p), with p = 1.5-1.7 and errors of Delta p approximate to 0.1. Comparing the particle density fields prior to collapse, we find that the high-wavenumber power spectra are similarly indistinguishable, though the large-scale geometry of structures induced via the streaming instability is significantly different between all three cases. We interpret the results as evidence for a near-universal slope to the mass function, arising from the small-scale structure of streaming-induced turbulence.

Context. Irregular satellites of Jupiter with their highly eccentric, inclined and distant orbits suggest that their capture took place after the giant planet migration. Aims. We aim to improve our understanding of the surface composition of irregular satellites of Jupiter to gain insight into a narrow time window when our solar system was forming. Methods. We observed three Jovian irregular satellites, Himalia (JVI), Elara (JVII), and Carme (JXI), using a medium-resolution 0.8-5.5 mu m spectrograph, SpeX on the NASA Infrared Telescope Facility (IRTF). Using a linear spectral unmixing model we have constrained the major mineral phases on the surface of these three bodies. Results. Our results confirm that the surface of Himalia (JVI), Elara (JVII), and Carme (JXI) are dominated by opaque materials such as those seen in carbonaceous chondrite meteorites. Our spectral modeling of NIR spectra of Himalia and Elara confirm that their surface composition is the same and magnetite is the dominant mineral. A comparison of the spectral shape of Himalia with the two large main C-type asteroids, Themis (D similar to 176 km) and Europa (D similar to 352 km), suggests surface composition similar to Europa. The NIR spectrum of Carme exhibits blue slope up to 1.5 mu m and is spectrally distinct from those of Himalia and Elara. Our model suggests that it is compositionally similar to amorphous carbon. Conclusions. Himalia and Elara are compositionally similar but di ff er significantly from Carme. These results support the hypotheses that the Jupiter's irregular satellites are captured bodies that were subject to further breakup events and clustered as families based on their similar physical and surface compositions.