ISOTOPIC RATIOS IN TITAN's METHANE: MEASUREMENTS AND MODELING by Nixon, C, Temelso, B, Vinatier, S, Teanby, N, Bezard, B, Achterberg, R, Mandt, K, Sherrill, C, Irwin, P, Jennings, D, Romani, P, Coustenis, A, Flasar, F