Aggregation and charging of mineral cloud particles underhigh-energy irradiation

Abstract

<p>Previous studies have found that high-energy radiation like cosmic rays and stellar energetic particles, can induce the initial nucleation of cloud particles from molecular clusters, but the effect on larger existing particles is still poorly understood.</p><p>This study explores the question “<em>How is the aggregation of mineral cloud particles affected by high-energy radiation and humidity?</em>”. We present experiments conducted in an atmosphere chamber on the charging and aggregation of 50nm SiO<sub>2</sub> particles under varying degrees of gamma radiation and relative humidity. <br>We observe an aggregation of the SiO<sub>2</sub> particles to form larger clusters, and that this aggregation is inhibited by irradiation with gamma radiation. We find that non-irradiation SiO<sub>2</sub> particles are generally more positively charged in comparison to a bipolar charge distribution, and that gamma radiation shifts the particles to a more negative charge. The effect of gamma radiation on the aggregation and charge of the particles is present both at lower (~20%) and higher (~60%) relative humidity. When varying the relative humidity from ~20% to ~80% we find no significant direct effect of relative humidity on the aggregation of the particles. These results are presented and discussed in relation to previous studies of nucleation and condensation.</p><p>In recent years, exoplanet research has focused on how we can interpret atmosphere observations through models, and here cloud formation has proven to be a challenge. Clouds are known to play a role in both the energy balance and chemistry of atmospheres, as well as directly affecting the spectrum observed from a planet. Exoplanet clouds are believed to be very chemically heterogeneous and SiO<sub>2</sub> is one of the species that easily condense, making SiO<sub>2</sub> relevant both as a nucleation seed on Earth-like planets and as a cloud species on Exoplanets. Since cloud formation has been found to be affected not only by the atmospheric properties, but also by high-energy radiation from outside the atmosphere it indicates that the host star and interstellar environment of an exoplanet might affect its clouds.</p>