Quantitative 3D model of light transmittance through translucent rocks applied to the hypolithic microbial community

In extreme desert environments, photosynthetic microorganisms often live on the buried undersides of translucent rocks. Computing the light level reaching these locations requires 3D modeling of a finite rock. We report on Monte Carlo calculations of skylight and sunlight transmission through a partially buried flat cylindrical rock using one billion photons per simulation. Transmitted light level drops inversely with increasing rock opacity, as expected for purely scattering media. For a half-buried rock with an extinction coefficient of 0.1 cm(-1) (opacity of 0.2), transmission at the bottom is 64 % for sunlight at a solar zenith angle of 60° and 82 % for skylight. Transmitted light level increases slowly with increasing scattering asymmetry factor of the rock independent of illumination or depth buried. Transmitted sunlight at zenith through a thick half-buried rock (opacity of 0.6) is six times brighter at the bottom than the subsurface sides. Skylight transmits equally to the subsurface sides and bottom. When the sun is not straight overhead, the sunward side of the rock is brighter than the underside of the rock. Compared to the sunlight transmitted to the bottom, transmitted sunlight inclined at 60° is 24 times brighter at the subsurface side towards the sun and 14 times brighter at the subsurface side 70° away from the sun. Transmitted sunlight emitted from zenith and skylight is uniformly bright at the bottom regardless of how deeply the rock is buried. Sunlight not at zenith transmits preferentially to the sunward bottom edge depending on the depth the rock is buried.