Photochemical Hazes in Sub-Neptunian Atmospheres with a Focus on GJ 1214b

We study the properties of photochemical hazes in super-Earth/mini-Neptune atmospheres with particular focus on GJ 1214b. We evaluate photochemical haze properties at different metallicities between solar and 10,000× solar. Within the four-order-of-magnitude change in metallicity, we find that the haze precursor mass fluxes change only by a factor of ∼3. This small diversity occurs with a nonmonotonic manner among the different metallicity cases, reflecting the interaction of the main atmospheric gases with the radiation field. Comparison with relative haze yields at different metallicities from laboratory experiments reveals a qualitative similarity to our theoretical calculations and highlights the contributions of different gas precursors. Our haze simulations demonstrate that higher metallicity results in smaller average particle sizes. Metallicities at and above 100× solar with haze formation yields of ∼10% provide enough haze opacity to satisfy transit observations at visible wavelengths and obscure sufficiently the H₂O molecular absorption features between 1.1 and 1.7 μm. However, only the highest-metallicity case considered (10,000× solar) brings the simulated spectra into closer agreement with transit depths at 3.6 and 4.5 μm, indicating a high contribution of CO/CO₂ in GJ 1214b’s atmosphere. We also evaluate the impact of aggregate growth in our simulations, in contrast to spherical growth, and find that the two growth modes provide similar transit signatures (for D _f = 2), but with different particle size distributions. Finally, we conclude that the simulated haze particles should have major implications for the atmospheric thermal structure and for the properties of condensation clouds.