The Physical and Chemical Life Cycle of Acetylene on Titan’s Surface

Acetylene (C₂H₂) is the most abundant solid product of methane photochemistry in Titan’s stratosphere. Unlike ethane (C₂H₆), it does not form a liquid under surface conditions. It should be widespread across Titan’s surface, and yet it hardly, if at all, appears in VIMS spectroscopic data. Where is the acetylene? Both chemical and physical processes are at play on the surface. Preliminary results from a production-loss-transport model of acetylene showing substantial depletion of pure acetylene on the equatorial and mid-latitude regions of Titan are presented. Production is by atmospheric photochemistry and sedimentation in aerosols to the surface. Solid acetylene is metastable and so physical disturbances (see below) can induce cyclization to benzene (C₆H₆) or polymerization to various forms of polyacetylene (with interesting physical properties).  Reaction with surface HCN (produced by atmospheric chemistry in somewhat lesser amounts) or other nitriles or imines at first glance would be thermally inhibited, but recent calculations of quantum tunneling under Titan conditions suggests acceleration of reaction rates by many orders of magnitude, and so that chemistry is included here. Physical transport is latitudinal, by sublimation/condensation (acetylene is volatile enough to be moved from equator to pole on timescales of 10⁵ years), and from highlands to lowlands by mechanical transport during the intense methane rainfall events observed by Cassini. Aeolian processes, including particle growth (to the sand-sized material in the dunes) and triboelectric charging are included. Static discharge and mechanical disturbance due to aeolian and fluvial processes provide the disturbances to include cyclization or polymerization.   The variation of sunlight through axial precession is relevant on the timescale of sublimation.  Surface gardening by micrometeoroids is unimportant because of the thick atmosphere. Localized processes around lakes and seas, such as dissolution and co-crystal formation, will have a small effect on the overall global budget of acetylene. The predicted abundances and geographic distribution of the acetylene and its products will inform JWST spectroscopic observations and the in-situ investigations by Dragonfly.

Part of this work was performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA.