Observations from the new horizons (NH) mission provide critical constraints on the composition, thermal structure and escape of pluto's atmosphere that are linked to the evolution and surface…

This federal contract award to the University of Arizona, a public research university and 1862 land grant institution, provides $244,863.89 in funding for a research project focused on characterizing the atmosphere and escape rate of Pluto. The research, which is directly responsive to the NASA New Frontiers Data Analysis Program, will develop a comprehensive thermal structure model to better understand the energy balance and temperature profile of Pluto's atmosphere. This will be coupled with a multi-species escape model of the upper atmosphere to calculate the UV heating efficiencies and escape rates of different atmospheric constituents. The results will provide new insights into the mechanisms controlling the evolution of Pluto's atmosphere over time. This award does not have a set-aside designation.

Observations from the new horizons (NH) mission provide critical constraints on the composition, thermal structure and escape of pluto's atmosphere that are linked to the evolution and surface features of this fascinating dwarf planet. Characterizing the atmosphere and inferring the escape rate are among the most important science objectives of the NH mission. Pluto's atmosphere is supported mainly by the vapor pressure of N2 ICE and IT is the least gravitationally bound in the solar system. Prior to the NH flyby, IT was expected to extend to 5 pluto radii and believed to lose N2 to energy-limited escape at a relatively high rate of 1E27 molecules per second. In contrast, results from ultraviolet (UV) solar occultations obtained by the NH alice instrument (note that alice is a name, not an acronym) indicate that the atmosphere is much cooler and less extended than expected and instead of N2, the atmosphere loses primarily CH4 at a much lower rate of 5E25 molecules per second. Our proposal focuses on the energy balance and temperature profile in the atmosphere that determine the escape rate. We will develop a comprehensive non-LTE (nlte) thermal structure model that accounts for the chemical complexity revealed by the NH observations and use IT to calculate the temperatures in the lower atmosphere (below 300 KM). We will couple this model to a multi-species escape model of the upper atmosphere (above 300 KM) that includes UV heating and the escape of different constituents. We will also calculate the UV heating efficiencies for N2 and CH4 in the upper atmosphere in much greater detail than before. The minor hydrocarbon and nitrile abundances required by these calculations will be obtained from a photochemical model of pluto's atmosphere. The results are constrained by temperatures and density profiles retrieved from the NH radio experiment (REX) occultations, NH alice occultations, and measurements obtained by the atacama large millimeter/submillimeter array (alma). As a part of the proposed work, we will improve the earlier analysis of the alice occultations by extending the retrieval altitude ranges and by introducing new techniques to reduce uncertainties and improve species identification in the data. Once the coupled model is complete and validated with the observations, we will explore changes in the atmosphere caused by different levels of solar irradiation along pluto's orbit. Our results will provide significant new insights to the energy balance on pluto and identify the key mechanisms that control the escape of the atmosphere over time. This proposal is directly responsive to the new frontiers data analysis program in that IT includes new horizons data analysis tasks and modeling tasks that are critical to the interpretation of these data