Presenter: Kazumasa OHNO
Abstract:
Observational efforts have suggested the ubiquity of clouds in exoplanetary atmospheres. One of the remarkable features is that some of them are placed at an extremely high altitude. Recent studies with cloud microphysical models investigate the nature of such high-altitude clouds, but the formation process is still under debate. Conventionally, cloud microphysical models have approximated cloud particles as compact spheres: however, it may not a good assumption for exoplanetary clouds that consist of solid mineral particles, as we know from nonspherical snowflakes on Earth.
Here, we investigate the impacts of particle porosity on cloud vertical profiles and observable spectra. Using a microphysical model taking into account the porosity evolution of cloud particles, we demonstrate that the porosity significantly extends the cloud vertical extent, which results in the absence of molecular signatures in transmission spectra. The fluffy-aggregate clouds also produce a spectral slope originated by the aggregate scattering properties, which could be an observational signature of the aggregates. Finally, we show that the flat spectrum of GJ1214b, a super-Earth possesses the high-altitude clouds, could be explained by the fluffy-aggregate clouds if the atmospheric metallicity is sufficiently high. Our results suggest the importance of particle microstructure in future investigations.