Clouds of Fluffy Aggregates: How They Form in Exoplanetary Atmospheres and Influence Transmission Spectra

Abstract

Transmission spectrum surveys have suggested the ubiquity of high-altitude clouds in exoplanetary atmospheres. Theoretical studies have investigated the formation processes of the high-altitude clouds; however, cloud particles have been commonly approximated as compact spheres, which is not always true for solid mineral particles that likely constitute exoplanetary clouds. Here, we investigate how the porosity of cloud particles evolve in exoplanetary atmospheres and influence the cloud vertical profiles. We first construct a porosity evolution model that takes into account the fractal aggregation and the compression of cloud particle aggregates. Using a cloud microphysical model coupled with the porosity model, we demonstrate that the particle internal density can significantly decrease during the cloud formation. As a result, fluffy-aggregate clouds ascend to altitude much higher than that for compact-sphere clouds assumed so far. We also examine how the fluffyaggregate clouds affect transmission spectra. We find that the clouds largely obscure the molecular features and produce a spectral slope originated by the scattering properties of aggregates. Finally, we compare the synthetic spectra with the observations of GJ1214 b and find that its flat spectrum could be explained if the atmospheric metallicity is sufficiently high (≥100× solar) and the monomer size is sufficiently small (r mon <1 µm). The high-metallicity atmosphere may offer the clues to explore the gas accretion processes onto past GJ1214b.

Aerosols in Exoplanet Atmospheres

“…N. Arney et al, , 2018Lavvas et al, 2019), much like haze particles on Titan (Lavvas et al, 2011b). Several cloud models have also considered aggregates composed of condensates (Ohno et al, 2020a;Samra et al, 2020).…”

Retrieving scattering clouds and disequilibrium chemistry in the atmosphere of HR 8799e

“…Another interesting approach is the recent model by Ormel & Min (2019), which determines the cloud mass fraction and average particle size by solving the steady-state differential equations including cloud settling, mixing, nucleation, condensation, and coagulation as a function of K zz and the nucleation rate. A more general treatment of this setup is presented in Ohno & Okuzumi (2018); Ohno et al (2020) which tracks the time evolution of the cloud until a steady state is reached, and accounts for the porosity of particles, including their fractal growth and compaction. A summary of other models can be found in Helling et al (2008b); Helling & Casewell (2014).…”

SCARLET-1.0: SpheriCal Approximation for viRtuaL aggrEgaTes

“…The final result of the process is a population of large particles with uncorrelated relative velocities that are higher as the decoupling with the flow is more pronounced (Volk et al, 1980). This is, for example, what happens in the case of coarse ash within a turbulent volcanic plume or cloud (Textor and Ernst, 2004) and also for dust grains in protoplanetary disks Okuzumi et al, 2009). The presence of a relative velocity between the core (or the aggregate) and the colliding particle creates a relative kinetic energy that must be dissipated in order to have a successful sticking.…”

“…The formation of aggregates from an initial set of individual monomers is a common topic in science, such as planetary formation, granulation processes, the food industry, meteorology, pollution, and Earth sciences (Brauer et al, 2001;Dominik et al, 2006;Poon et al, 2008;Brown et al, 2012;Cuq et al, 2013;Shi et al, 2015;Dacanal et al, 2016;Pumir and Wilkinson, 2016;Imaeda and Ebisuzaki, 2017;Ohno et al, 2020). In volcanology, aggregation plays a key role in affecting the sedimentation processes of ash in the atmosphere during and after a volcanic eruption, with deep consequences for the accuracy of the dispersal forecasting and hazard assessment (Durant, 2015).…”