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Streamers, thin, ionized plasma channels, form the early stages of lightningdischarges. Here we approach the study of extraterrestrial lightning bystudying the formation and propagation of streamer discharges in variousnitrogen-methane and nitrogen-oxygen mixtures with levels of nitrogen from 20%to 98.4%. We present the friction force and breakdown fields Ek in variousN2:O2 (Earth-like) and N2:CH4 (Titan-like) mixtures. The strength of thefriction force is larger in N2:CH4 mixtures whereas the breakdown field inmixtures with methane is half as large as in mixtures with oxygen. We use a 2.5dimensional Monte Carlo particle-in-cell code with cylindrical symmetry tosimulate the development of electron avalanches from an initial electron-ionpatch in ambient electric fields between 1.5Ek and 3Ek. We compare the electrondensity, the electric field, the front velocities as well as the occurrence ofavalanche-to-streamer transition between mixtures with methane and with oxygen.Whereas we observe the formation of streamers in oxygen in all consideredcases, we observe streamer inceptions in methane for small percentages ofnitrogen or for large electric fields only. For large percentages of nitrogenor for small fields, ionization is not efficient enough to form a streamerchannel within the length of the simulation domain. In oxygen, positive andnegative streamers move faster for small percentages of nitrogen. In mixtureswith methane, electron or streamer fronts move 10-100 times slower than inmixtures with oxygen; the higher the percentage of methane, the faster thefronts move.

Streamer propagation in the atmosphere of Titan and other N2:CH4 mixtures compared to N2:O2 mixtures