Pluto and Titan: Laboratories for the Production of Organic Molecules

As Kuiper Belt Objects of similar size and albedo, Triton and Pluto were thought to be kindred bodies exhibiting similar geologic histories and features, with possible seasonal volatile transport in their polar regions. During the flyby of Pluto in July 2015, active geological processes were observed (Stern et al., 2015), and a substantial haze layer that was more akin to Titan's was observed (Gladstone et. al., 2016). Multiple haze layers were discovered surrounding the dwarf planet (Cheng et al. 2017). Using a radiative transfer model based on Chandrasekhar's "Planetary Problem" of an atmosphere and a surface of arbitrary single scattering albedo and single particle phase function (Chandrasekhar, 1960; Hillier et al., 1990, 1991; Buratti et al., 2011), we have characterized the optical depth and surface properties of Pluto and Triton. The forward-scattering properties of the haze can also be quantified by this model. Optical imaging data was analyzed for Triton and Pluto. For Titan we made use of published data on Titan (Tomasko and West, 2009) plus new Cassini Visual Infrared Mapping Spectrometer (VIMS) data, which spans the wavelength range between 0.35 and 5.2 microns, and which has several channels in the mid-infrared where both the haze opacity is relatively low and the atmosphere is optically thin. Pluto's atmosphere is more optically thick than Triton's but both are far thinner than Titan's. The composition of Triton's haze layer differs markedly from Titan's. Observations of Pluto's haze reveal a bluish color (Gladstone et al., 2016), but the reddish tint of possible haze deposits on the surface (Stern et al., 2015; Buratti et al., 2017) suggest Pluto's haze composition is Titan-like and rich in carbon-bearing compounds. Our results confirm this latter view, which suggests both Titan and Pluto are "factories" for the production of organic materials in the outer Solar System. These molecules are similar to those that have been transported into the inner Solar System by comets and asteroids, and that may have played a key role in the origins of life on Earth.

Government sponsorship acknowledged.