The Sulfur Conundrum on Titan: Cryovolcanism, Outgassing, and Chemical Impact on the Atmosphere

Titan’s atmosphere is a unique laboratory for complex organic gas and solid phase chemistry. Many exogenic and endogenic chemical and physical mechanisms were revealed by the Cassini mission which studied the Saturnian system for nearly 13 years. The N₂ and CH₄-dominated atmosphere produces a plethora of N-bearing and hydrocarbon volatiles from photo-dissociative processes in the ionosphere (R > 1.3 R_Titan), which may be the dominant origin of Titan’s 1.5 bar atmosphere. However, the near-surface atmosphere (R < 1.3 R_Titan) may be driven by outgassing. In the present study, we investigate how cryovolcanism, explosive events driven by clathrate-hydrates, and an array of energetic geologic processes may have impacted Titan’s near-surface chemistry. In particular, it is surprising that sulfur-bearing species have never been detected. Only H₂S upper limits of 330 ppb high in the stratosphere (R > 1.1 R_Titan) have been suggested (Nixon et al., 2013), and observations involving anything other than CHNO-based compounds in photochemical models are scarce (e.g. Hickson et al., 2014; Vuitton et al., 2019). The stability of clathrate-hydrates on the surface remains an important element to the outgassing narrative on Titan (Fortes et al. 2007, Grindrod et al. 2008 e.g.). Using knowledge of abundances in Titan’s interior based on rocky chondritic materials indicates there should be sulfur in the interior in the form of ammonium sulfate (40 wt %). Furthermore, the pressure-dependent sulfides and sulfates duality means that ice and ammonium sulfate surface deposition should happen and is also a factor when studying the hydration of rocky material, mineral transport in the ocean, and resurfacing on Titan (e.g. Fortes et al. 2007; Castillo-Rogez & Lunine 2010). Here, we use a semi-analytic modeling (DISHOOM; Oza et al., 2019) while synergistically addressing the interior, geologic and gas phase quantum chemistry implications presumed to be involved in the outgassing processes supplying sulfur-bearing compounds to the surface and atmosphere. Based on our tidally-driven mass loss model, we obtain a rate corresponding to ~0.3% of the SO₂ outgassing rate at Io.

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