Aggregate Hazes in Exoplanet Atmospheres

Abstract

Photochemical hazes have been frequently used to interpret exoplanet transmission spectra that show an upward slope towards shorter wavelengths and weak molecular features. While previous studies have only considered spherical haze particles, photochemical hazes composed of hydrocarbon aggregate particles are common throughout the solar system. We use an aerosol microphysics model to investigate the effect of aggregate photochemical haze particles on transmission spectra of warm exoplanets. We find that the wavelength dependence of the optical depth of aggregate particle hazes is flatter than for spheres since aggregates grow to larger radii. As a result, while spherical haze opacity displays a scattering slope towards shorter wavelengths, aggregate haze opacity can be gray in the optical and NIR, similar to those assumed for condensate cloud decks. We further find that haze opacity increases with increasing production rate, decreasing eddy diffusivity, and increasing monomer size, though the magnitude of the latter effect is dependent on production rate and the atmospheric pressure levels probed. We generate synthetic exoplanet transmission spectra to investigate the effect of these hazes on spectral features. For high haze opacity cases, aggregate hazes lead to flat, nearly featureless spectra, while spherical hazes produce sloped spectra with clear spectral features at long wavelengths. Finally, we generate synthetic transmission spectra of GJ 1214b for aggregate and spherical hazes and compare them to space-based observations. We find that aggregate hazes can reproduce the data significantly better than spherical hazes, assuming a production rate limited by delivery of methane to the upper atmosphere. ModelWe calculate the equilibrium haze particle size distribution using the 1-D Community Aerosol and Radiation Model for Atmospheres (CARMA;Turco et al., 1979;Toon et al., 1988; Jacobson and Turco, 1994;Ackerman et al. 1995;Bardeen et al., 2008;Wolf and Toon, 2010). CARMA solves the continuity equation of aerosol particles that experience production via particle nucleation, growth via condensation and coagulation, loss via evaporation, and transport. We follow the methodology outlined in Gao et al. (2017a) for Pluto hazes to simulate haze

Deflating Super-puffs: Impact of Photochemical Hazes on the Observed Mass–Radius Relationship of Low-mass Planets

“…However, the increase in opacity is dependent on the porosity of the aggregates, with higher porosity leading to decreased opacity. Ohno et al (2019) showed that compression forces in exoplanet atmospheres are unlikely to strongly impact aggregates, and that they will maintain a fractal dimension of ∼2, rather than 2.4 as assumed in Adams et al (2019). This leads to a wavelengthdependence of the opacity more similar to that of the individual monomers, which are small enough to create spectral slopes (Lavvas et al 2019).…”

“…As the haze composition is uncertain, we consider the refractive indices of both the Titan haze analog, tholins (Khare et al 1984), as well as soots (Morley et al 2015;Lavvas & Koskinen 2017). As soots are more absorbing than tholins in our wavelengths of interest (∼0.3-5 µm; Adams et al 2019), rather than assigning their refractive indices to their compositions, we treat tholins as representing scattering hazes, and soots as representing absorbing hazes.…”

“…In our work we consider only spherical haze particles. However, Adams et al (2019) showed that haze particles composed of fluffy aggregates of small monomers can form in exoplanet atmospheres, and that they can significantly increase haze opacity and reduce the wavelength dependence of haze opacity compared to a case with the same haze production rate of spherical particles. However, the increase in opacity is dependent on the porosity of the aggregates, with higher porosity leading to decreased opacity.…”

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

“…Number of Monomers Figure 11. Comparison of our porosity model with that used in Adams et al (2019). The vertical and horizontal axes show the fractal dimension D f and number of monomers N mon .…”

“…The vertical and horizontal axes show the fractal dimension D f and number of monomers N mon . Different colored lines exhibit the evolution track of D f for different monomer size, and the gray line shows the track assumed in Adams et al (2019). We assume P = 0.01 mbar to evaluate the collision velocity.…”