a Fractal Aggregate Model of Early Earth Organic Hazes: UV Shielding with Minimal Antigreenhouse Cooling

The Archean Earth (3.8 to 2.5 billion years ago) was probably enshrouded by a Titan-like photochemical haze composed of fractal aggregate hydrocarbon aerosols. In this study a three-dimensional fractal aggregate model of the early Earth photochemical haze is explored and compared with the standard liquid drop haze model used in earlier studies. Fractal aggregate microphysical processes are modeled using the method introduced by Cabane et al. [Planet. Space Sci. 41, 257, 1993]. The optical properties of the aerosols are determined using a mean-field approximation of multiple scattering by fractal aggregates composed of identical spheres [Botet at al. <Applied Optics 36, 8791, 1997]. Early Earth fractal hazes are found to be optically thick in the ultraviolet wavelengths while remaining relatively transparent in the mid-visible wavelengths. At an annual production rate of 1014 grams per year and an average monomer radius of 50 nanometers, the haze has global mean effective optical depths of τuv = 11.2 and τvis = 0.5. Such a haze would provide a strong shield against UV light while causing only minimal antigreenhouse cooling. Protected by a strong UV shield, photolytically unstable greenhouse gases such as CH4 and NH3 may have been able to build up to high concentrations helping warm the young Earth despite the faint young Sun. Our findings reopen the hypothesis of Sagan and Mullen [Science 177, 52, 1972] that the young Earth may have been home to a reducing atmosphere.