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Measurement of Nanoparticle Surface Energies with Application to Nucleation and Condensation in Exoplanet Atmospheres

Presentation #223.12 in the session Exoplanet Atmospheres (Poster + Lightning Talk)

Published onOct 23, 2023
Measurement of Nanoparticle Surface Energies with Application to Nucleation and Condensation in Exoplanet Atmospheres

Aerosols are a ubiquitous feature of exoplanet atmospheres, sometimes obscuring the spectral determination of atmospheric gas composition. Aerosol composition is usually determined from models of nucleation rates for condensable species in an atmosphere of a given composition. For hot Jupiters, condensable species include various silicates, sulfides, and other high temperature condensates. For lower temperature objects, such as warm Neptunes and super Earths, photochemical production of aerosol particles is predicted to be dominant. Because it is not yet possible to definitively determine aerosol composition through astronomical observations, it is important to model aerosol production as accurately as possible. A key factor in the determination of nucleation rate is the surface energy of the nucleating material. High surface energy materials, such as forsterite, will nucleate much more slowly compared to lower surface energy materials, such as sulfides. Here, we measure surface energies for several important and likely exoplanet condensates including ZnS, enstatite, forsterite, and spinel with planned measurements for MnS and Na2S (sodium sulfide). Surface energies are determined by synthesizing nanoparticles and micron-sized particles of the compound of interest, and then using calorimetric techniques to measure the excess enthalpy as a function of particle surface area. From these data, an accurate surface energy is determined for both hydrous and anhydrous phases. The synthesis and calorimetry is carried out in the Navrotsky laboratory at ASU. This new surface energy data, together with the existing surface energy data that has up to now not been utilized by the exoplanet community, will be included in nucleation rate calculations for hot Jupiters. These surface energies will be used with the Community Aerosol and Radiative Model for Atmospheres (CARMA) code to calculate particle size distribution and radiative transfer properties of nucleating aerosols for representative hot Jupiter atmospheres. This research is of significant importance to the interpretation of observations of exoplanets. In particular, our research provides laboratory data of high relevance to a broad range of exoplanet and brown dwarf atmospheres.

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