Carbon and Hydrogen Isotopic Composition of Methane and C2+ Alkanes in Electrical Spark Discharge: Implications for Identifying Sources of Hydrocarbons in Terrestrial and Extraterrestrial Settings

The low-molecular-weight alkanes-methane, ethane, propane, and butane-are found in a wide range of terrestrial and extraterrestrial settings. The development of robust criteria for distinguishing abiogenic from biogenic alkanes is essential for current investigations of Mars' atmosphere and for future exobiology missions to other planets and moons. Here, we show that alkanes synthesized during gas-phase radical recombination reactions in electrical discharge experiments have values of delta H-2(methane) > delta H-2(ethane) > delta H-2(propane), similar to those of the carbon isotopes. The distribution of hydrogen isotopes in gas-phase radical reactions is likely due to kinetic fractionations either (i) from the preferential incorporation of H-1 into longer-chain alkanes due to the more rapid rate of collisions of the smaller H-1-containing molecules or (ii) by secondary ion effects. Similar delta C-13(C1-C2+) and delta H-2(C1-C2+) patterns may be expected in a range of extraterrestrial environments where gas-phase radical reactions dominate, including interstellar space, the atmosphere and liquid hydrocarbon lakes of Saturn's moon Titan, and the outer atmospheres of Jupiter, Saturn, Neptune, and Uranus. Radical recombination reactions at high temperatures and pressures may provide an explanation for the combined reversed delta C-13(C1-C2+) and delta H-2(C1-C2+) patterns of terrestrial alkanes documented at a number of high-temperature/pressure crustal sites.