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We summarize the observations and context for molecular hydrogen (H 2) 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. • Reviews hydrogen sources, sinks and abundance on Titan. • Considers hydrogen as a biosignature and indication of serpentinization. • Demonstrates small solid-state sensor operation in nitrogen-methane atmosphere. • Reviews measurement strategies on a mobile lander. [ABSTRACT FROM AUTHOR]

Abstract: In Titan''s atmosphere consisting of N2 and CH4, large amounts of atomic hydrogen are produced by photochemical reactions during the formation of complex organics. This atomic hydrogen may undergo heterogeneous reactions with organic aerosol in the stratosphere and mesosphere of Titan. In order to investigate both the mechanisms and kinetics of the heterogeneous reactions, atomic deuterium is irradiated onto Titan tholin formed from N2 and CH4 gas mixtures at various surface-temperatures of the tholin ranging from 160 to 310 K. The combined analyses of the gas species and the exposed tholin indicate that the interaction mechanisms of atomic deuterium with the tholin are composed of three reactions; (a) abstraction of hydrogen from tholin resulting in gaseous HD formation (HD recombination), (b) addition of D atom into tholin (hydrogenation), and (c) removal of carbon and/or nitrogen (chemical erosion). The reaction probabilities of HD recombination and hydrogenation are obtained as and , respectively. The chemical erosion process is very inefficient under the conditions of temperature range of Titan''s stratosphere and mesosphere. Under Titan conditions, the rates of hydrogenation > HD recombination ≫ chemical erosion. Our measured HD recombination rate is about 10 times (with an uncertainty of a factor of 3–5) the prediction of previous theoretical model. These results imply that organic aerosol can remove atomic hydrogen efficiently from Titan''s atmosphere through the heterogeneous reactions and that the presence of aerosol may affect the subsequent organic chemistry. [Copyright &y& Elsevier]

We demonstrate the functionalization of single-walled carbon nanotubes (SWNTs) using a glow discharge for generating atomic or molecular radicals. A 30-s exposure to a cold plasma of H[sub 2] results in near-saturation coverage of SWNT with atomic hydrogen. Functionalization of SWNTs with atomic hydrogen is confirmed by an infrared band at 2924 cm[sup -1], characteristic of C-H stretching mode. A corresponding decrease in the ultraviolet absorption is also observed which is due to a loss of some conjugated C-C π bonds in hydrogen covered SWNTs. [ABSTRACT FROM AUTHOR]