The Lunar Polar Regions: Illumination Environment, Seasonal Water Cycle and Exospheric Volatile Delivery

Permanently shadowed regions (PSRs) at the lunar poles are of unique interest for science and exploration due to their low surface temperatures and potential for volatile sequestration. Spacecraft have indicated the presence of water ice and other volatiles compounds within these locations, yet questions remain as to their stability, abundance and distribution; broadly, this thesis will seek to advance understanding in all three of these areas. First, we examine the spatial, temporal and spectral variability of the diffuse sources of light that reach PSRs. Through development of a comprehensive scattering model, we show that scattered sunlight is the dominant PSR illumination source at visible and IR wavelengths, and that it contributes a considerable supply (up to 35 W m2) of far ultraviolet photons as well. The latter finding is significant because the enhanced flux of high energy scattered solar photons to the PSRs, which has previously not been considered, suggests a higher rate of photodesorption and lower adsorption residence times for water molecules than previously proposed. In the second half of this thesis, we apply numerical techniques to explore the seasonal variations in the surface volatile densities and infall into PSR craters. Here, we show that the growth and decay of seasonal shadow throughout the year enforce distinct patterns in the pole-ward migration of water as well as a cyclical variation in the polar surface hydration. As well, we find that northern hemisphere PSRs accumulate more water per unit area than southern hemisphere PSRs. Due to its relatively long lifetime against photolysis, CO2 is shown to have the largest infall rate of the species examined, where surface concentrations near the poles exceed 2000 the strength of the source. Using meteoritic supply rates and estimates of thermal and non-thermal loss rates, we show that net deposition of CO2 and H2O may be ongoing process within some of the coldest PSR locations. Assuming the meteoritic supply rate to be constant over time, we calculate that up to 1.510^10 g of meteoritic CO2 would be trapped over a 2 billion year timescale, most of which would reside at the lunar south pole.