Clouds of Fluffy Mineral Particles: a Possible Explanation for the Flat Transmission Spectra of Super-Earths
Recent observations of transmission spectra have revealed the ubiquity of featureless spectra of warm super-Earths. One of the interpretations of such observations is the presence of opaque clouds formed via condensation of vaporized minerals. Because the vertical distributions of clouds drastically affect the observed spectra, it is crucial to understand the formation mechanism of mineral clouds. Most previous studies assume that cloud particles are compact spheres. However, this is not always true for solid condensates, as we know from snowflakes whose internal density is much lower than the material density.
In this study, we have investigated the impacts of a particle porosity on the vertical structures of mineral clouds using a one-dimensional microphysical model that takes into account the growth, porosity evolution, and vertical transport of cloud particles in a self-consistent manner. We demonstrate that the vertical extent of mineral clouds becomes much larger than that for compact sphere when the grains that constitute the aggregates are sufficiently small. We apply our model to GJ1214 b, a Super-Earth known to exhibit a featureless spectrum, and find that mineral clouds composed of porous particles can reach the high altitude suggested by observations, whereas that composed of compact spheres cannot. Our results suggest that particle microstructure might be a key to interpret observed spectra correctly.