The spectacularly successful new horizons (NH) encounter and recent modeling efforts raise important questions about pluto's atmospheric thermal structure and energy balance, global circulation,…

This federal award is a $435,251 grant from the National Aeronautics and Space Administration (NASA) to Wellesley College, a private women's liberal arts college located in Wellesley, Massachusetts. The grant was awarded on April 10, 2019 for research to explore the atmospheric dynamics and thermal structure of Pluto. The research will aim to reconcile discrepancies between recent observations from the New Horizons spacecraft and decades of previous stellar occultation data, in order to better understand Pluto's atmospheric circulation, waves, and energy balance. The grant does not have a set-aside designation, and is scheduled for completion by April 15, 2025. As a private academic institution, Wellesley College regularly receives federal research grants from agencies such as NASA, NSF, NIH, and DOE to support its faculty's work in astrophysics, marine biology, mathematics, and other scientific fields.

The spectacularly successful new horizons (NH) encounter and recent modeling efforts raise important questions about pluto's atmospheric thermal structure and energy balance, global circulation, wind regimes, and the propagation of inertia-gravity waves. We propose to apply a suite of realistic global atmospheric and radiative transfer modeling tools to explore the underlying physical processes at work in pluto's atmosphere and to identify their observable consequences. We will predict the dynamical response of the atmosphere to a set of hypothesized and modeled forcings and surface conditions, and compare these predictions with new horizons observations and with a substantial catalog of high signal-to-noise ratio (SNR) earth-based stellar occultations that provide our best record of decadal changes in pluto's atmosphere. Our proposed investigation will focus on two important and answerable questions: Q1. What are the dynamical and temporal implications of pluto's apparently cooler thermal structure? New horizons observations of pluto's atmospheric thermal profile revealed upper atmospheric temperatures to be nearly isothermal at about 60 K, or about 30-60 K cooler than those derived from decades of previous stellar occultations, as well as a less stable lower atmosphere (L. Young, 2018). What does this imply for changes in the dynamics and flows of pluto's atmosphere, and what does this tell US about the temporal evolution of pluto's atmosphere? How can we reconcile the discrepancy between the NH results and stellar occultation thermal profiles? Are recent models invoking haze as pluto's atmospheric coolant (zhang ET AL. 2017) consistent with stellar occultation observations? Q2. What is the source and effect of pluto's atmospheric waves? There is abundant evidence for wave activity in pluto's atmosphere: scintillations ("spikes") in steldlar occultation lightcurves (toigo ET AL. 2010; french ET AL. 2015; pasachoff ET AL. 2005, 2015), and wave-like features in the vertical thermal profiles derived from high-SNR occultations, indicate that the waves are regionally and/or temporally variable. Previously, we have suggested that pluto's atmospheric waves are driven by vertical flow at the surface resulting from a diurnal cycle in the sublimation and condensation of surface ices (toigo ET AL. 2010; french ET AL. 2015). However, new horizons observations provide clear evidence of KM-scale topographic relief (moore ET AL. 2016), raising the possibility that pluto's observed inertia-gravity waves may instead (or additionally) be orographic in origin. Are waves in pluto's atmosphere driven more by sublimation or by topography? How do the characteristics of these waves compare to observations (gladstone ET AL. 2016; cheng ET AL. 2017) of thin layers of hazes in the atmosphere? Our work directly addresses the prime facie contradiction between atmospheric temperatures inferred from years of earth-based occultations and the new horizons observations. At the conclusion of this investigation, we will have a much deeper understanding of two possible mechanisms for cooling the atmosphere of pluto: atmospheric water vapor or photochemical haze, and how those factors influence the global atmospheric circulation and structure. This is critical to understanding how pluto's atmosphere maintains thermal equilibrium. Additionally, we will understand how atmospheric waves are generated on pluto: by diurnally-driven frost sublimation and condensation and/or by horizontal flow over topographic relief. We will determine the predicted wavelengths, strengths, vertical extent, and variability (both regional and temporal) of the modeled waves in each case, and compare their ability to account for the observed variability of wave characteristics in a rich set of high-SNR stellar occultation observations, as well as haze observations from new horizons.