Hydrogen sensing in Titan's atmosphere: Motivations and techniques

We summarize the observations and context for molecular hydrogen (H2) in Titan's atmosphere where it is the third most abundant gas. Hydrogen escapes to space but is replenished by methane photochemistry. An open question is whether there are sources and/or sinks in the surface and subsurface: sources might include serpentinization reactions in the deep interior while sinks might involve reactions with acetylene mediated by chemical or even biological catalysts. Cassini data provide weak evidence of a surface sink, and also point to variations with latitude of the tropospheric hydrogen abundance, so further measurements would be of value. We demonstrate that a simple solid-state sensor can provide the required measurement precision in an oxygen-free atmosphere, and consider how measurements on a mobile platform may inform the question of sources and sinks. We underscore the importance of simultaneous methane and hydrogen measurements: whereas the stratosphere is a hydrogen source and methane sink, serpentinization could be a subsurface source of both gasses.