Hydrocarbon and nitrile species in Titans upper atmosphere

Hydrocarbon and nitrile species in Titan′s upper atmosphere from multiple occultations Siteng Fan 1, Linfeng Wan 1, Donald E. Shemansky 2, Mao-Chang Liang 3, Yuk L. Yung 1 1: California Institute of Technology 2: Space Environment Technologies 3: Academia Sinica 1

Titan • Radius: 2575 km • Dense atmosphere: • ~1000 km • Gravity: 1. 35 m/s 2 • 1. 5 bar on the surface • Liquid body on surface • 98. 4% N 2, 1. 4% CH 4 2

ESA 3

Photochemistry Atreya et al. (2006) 4

Observations by Cassini Horst (2017) 5

) S O L ( t h g i S e of Lin Cassini Titan 850 km Cassini Stellar occultation by Cassini/UVIS 6

Stellar occultations • 7

Previous work • Only 3 flyby cases (TB, T 41 i, T 53) were used out of tens of stellar occultation observations. Shemansky et al. (2005) 8

Pointing problem Attitude control: 0. 5 mrad NO τ=I /I ! pixel: Spectral Spectrum: Iλ λ 0 0. 25 mrad Esposito et al. (2000) 9

Method • New method fits photon counts (intensity) spectra instead of extinction. • Tens of observations during 127 Titan flybys. • Forward model has 14 parameters, including one for pointing value. • We use MCMC to search the parameter space. • A flat prior is used to set the frames. 10

Result: T 35 at 668 km • 11

Result: CH 4 • 12

Result: C 2 Species • 13

Result: Peaks of C 2 species • West et al. (2011) 14

Summary • Pointing problem is solved by fitting photon counts spectra. • CH 4: Slightly lower towards equinox. • C 2 species: two peaks with altitudes coinciding with detached haze layers during southern summer. 15