Oxidation of Photochemical Organic Hazes: A Prebiotic Source of Oxygen-Bearing Organics in Titan's Atmosphere and Early Earth?

Abstract

Photochemical organic hazes such as those found in the atmosphere of Titan, the largest satellite of Saturn, are mainly composed of reduced chemical components. However, in these predominantly reduced atmospheres favorable to the formation of hazes, oxidized species can be produced transiently by one-off events such as surface impacts or via episodic outgassing of water ice particles. Due to their native reduced composition, organic hazes may interact with these oxidative species. This mechanism thus appears as a potential source of oxidized organics in Titan and more generally in hazy planetary atmospheres. To investigate the reactivity of reduced organic photochemical hazes, Titan's aerosol laboratory analogs (or Titan's tholins) have been chosen as a proxy. Two types of tholins were synthetized and systematically compared using gas chromatography coupled with mass spectrometry: a tholin sample exposed to air and an air-preserved tholin sample. This approach allowed us to identify several oxygen-bearing species, mainly urea, oxalic acid, hydroxy-amino-1,3,5-triazine, and carboxylic acids. Mechanisms that explain the formation of these latter oxidized organics were proposed. Possible precursors for urea, oxalic acid, and hydroxy-amino-1,3,5-triazine were identified. These results provide new input in the discussion of the existence of heterogeneous oxidation of photochemical hazes in planetary atmospheres. Two cases are developed here: Titan and Hadean early Earth. In these two contexts, oxidation of photochemical hazes could have provided prebiotic matter as oxidized organics.