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More than a decade after the arrival of the Cassini-Huygens mission in Saturn’s system, data returned by the Huygens probe during its descent remain a unique source of in-situ information on the lower atmosphere of Titan. Among the Huygens instrumental suite was the GCMS (Gas Chromatograph Mass Spectrometer) instrument which returned hundreds of mass spectra acquired in the atmosphere below 145 km of altitude. We will present a reanalysis of GCMS data focusing on the methane vertical profile thanks to recent advances in our knowledge of Titan’s atmosphere and in mass spectrometry data treatment [1-3]. We retrieved methane mixing ratio slightly lower than the one reported by the original team[4] and obtained its profile between 145 km and 30 km of altitude with a kilometric vertical resolution, and a sub-kilometric one between 30 km and the surface. Such a vertical resolution unveiled clear oscillations in the methane mixing ratio in Titan’s troposphere and methane vertical concentration diverging from an ideal adiabat. Such features could for example be a sign of small-scale convective zones in the troposphere which could have triggered the gravity waves detected by Huygens in the stratosphere. We hope that the discovery of previously unnoticed features in GCMS data will also enable the reanalysis of data returned by other Huygens instruments such as HASI and DISR on Titan lower atmosphere. [1] Gautier et al. Decomposition of electron ionization mass spectra for space application using a Monte-Carlo approach. Rapid. Com. Mass Spec. 34(8), e8659 (2020) [2] Serigano et al. Compositional measurements of Saturn’s upper atmosphere and rings from Cassini INMS. JGR:Planets, 125 (8), E006427 (2020) [3] Serigano et al. Compositional Measurements of Saturn’s Upper Atmosphere and Rings from Cassini INMS: An extended Analysis of Measurements from Cassini’s Grand Finale Orbits. JGR:Planets, 127, E007238 (2022) [4] Niemann et al. Composition of Titan’s lower atmosphere and simple surface volatiles as measured by the Cassini-Huygens probe gas chromatograph mass spectrometer experiment. JGR 115, E12006, 2010

Titan, the largest moon of Saturn, is characterized by gigantic linear dunes and an active dust cycle. Much like on Earth, these aeolian processes are caused by the wind-driven saltation of surface grains. It is still unclear, however, how saltation on Titan can occur despite the typically weak surface winds and the potentially cohesive surface grains. Here, we explore the hypothesis that saltation on Titan may be sustained at lower wind speeds than previously thought, primarily through granular splash rather than aerodynamic lifting of surface grains. We propose a saltation mass flux parameterization for Titan and use it to quantify sediment transport with a general circulation model. The results suggest that Titan's prevailing circulation can generate highly intermittent yet significant saltation, with mass fluxes of the order of 10^4 kg m^-1 year^-1, and that Titan dunes may be formed primarily by fine grains, approximately 0.1 mm in size.

Introduction More than a decade after its descent through Titan’s atmosphere, the data acquired by the instruments on the Huygens Probe remain the only in situ measurements in Titan’s deep atmosphere. Among these, the Gas Chromatograph Mass Spectrometer (GCMS) experiment aboard Huygens measured hundreds of mass spectra below 150 km to investigate the atmospheric composition [1,2]. However, there has not yet been a comprehensive attempt to decompose the mass spectra acquired by Huygens in order to extract the identity and abundances of many minor components and refine the retrieved mixing ratios of major atmospheric components (N2, CH4, Ar) Actually, at least 60% of the signal in the GCMS spectra has not been identified yet. As an example, while predicted ethane is strikingly absent from GCMS measurements during its descent [3-5]. The absence of ethane in data near the surface can be explained by its condensation, but its non-detection above 100 km re-mains one of the major discrepancies in our understanding of GCMS measurements. In addition to ethane, many organics predicted both by models and measurements at higher altitude remain unquantified in GCMS. In order to fill this gap, we have developed numerical tools to decompose flight mass spectrometry data, and applied it to conduct an in-depth reanalysis of the GCMS data collected during Huygens’ descent, leveraging recent advances in both knowledge of Titan’s atmosphere and mass spectral deconvolution for organic species. Methods We decompose the mass spectra acquired by GCMS below 146 km using the fragmentation pattern of the molecules detected by the instrument. This approach has been previously applied successfully to INMS data of Titan’s higher atmosphere during Cassini flybys [6]. However, up to now, this decomposition method required extensive knowledge of the instrument calibration data, which was the case for INMS. Such calibration data was unfortunately unavailable for the GCMS instrument. We thus developed a Monte-Carlo approach, simulating the diversity of possible fragmentation for a given molecule in the spectrometer ionization chamber, to handle degeneracy in the mass spectra decomposition. Results We analyzed all GCMS acquired mass spectra, from 135km altitude down to the surface. Between m/z 5 and 50. We thus retrieved the relative abundances of most volatiles species in Titan’s lower atmosphere. We were able to clearly identify and relatively quantify not only CH4, N2 and Ar as in the original GCMS paper [2], but we also identify 8 new species in this mass spectrum, including ethane. By decomposing the mass spectra at every altitude during Huygens descent, we obtained the vertical profile of major volatile species in Titan’s lower atmosphere Figure 1: Preliminary retrieved methane vertical profile above Huygens landing site. Our work is in red, to be compared with the original retrieval (dark blue). Discrepancies appear at altitudes above 80 km which might be able to reconcile GCMS measurements with Cassini CIRS and VIMS observations. Retrieved mixing ratio at high altitude (120 – 135 km) were compared with mixing ratios determined from CIRS or ground based observations and showed an excellent agreement between the methods. This allowed us to carry on at lower altitude and to retrieve volatiles mixing ratios throughout Huygens descent. We will present the retrieved mixing ratios for several minor volatiles species in Titan’s atmosphere, together with their associated probability density function and vertical profiles. Acknowledgements T.G. was supported by the Programme National de Planétologie (PNP) of CNRS/INSU, cofunded by CNES. This work was originally supported by the National Aeronautics and Space Administration under16-CDAP16_2-0087, initially issued through the Cassini Data Analysis Program, and NASA's Planetary Science Division Internal Scientist Funding Program through the Fundamental Laboratory Research (FLaRe) work package to M.G.T. References [1] Owen, T., Planetary science: Huygens rediscovers Titan, Nature 438(7069), pp. 756-757, 2005 [2] Niemann, H. B., S. K. Atreya, S. J. Bauer, G. R. Carignan, J. E. Demick, R. L. Frost, D. Gautier, J. A. Haberman, D. N. Harpold, D. M. Hunten, G. Israel, J. I. Lunine, W. T. Kasprzak, T. C. Owen, M. Paulkovich, F. Raulin, E. Raaen and S. H. Way: The abundances of constituents of Titan's atmosphere from the GCMS instrument on the Huygens probe, Nature 438(7069), pp. 779-784, 2005 [3] Bézard, B.; Yelle, R.; Nixon, C. A.: The Composition Of Titan's Atmosphere, In Titan: Surface, Atmosphere and Magnetosphere, Eds: I. Muller-Wodarg, C. Griffith, E. Lellouch and T. Cravens, Chap. 6, Cambridge University Press, 2014. [4] Vinatier S., B. Bézard, T. Fouchet, N. A. Teanby, R. de Kok, P.G.J. Irwin, B. J. Conrath, C. A. Nixon, P. N. Romani, F. M. Flasar, A. Coustenis: Vertical abundance profiles of hydrocarbons in Titan's atmosphere at 15° S and 80° N retrieved from Cassini/CIRS spectra, Icarus 188 (1), pp. 120-138, 2007 [5] Lavvas, P., C. A. Griffith and R. V. Yelle: Condensation in Titan’s atmosphere at the Huygens landing site, Icarus 215(2), pp. 732-750, 2011 [6] Gautier, T., Serigano, J., Bourgalais, J., Hörst, S.M., Trainer, M.G., 2020. Decomposition of electron ionization mass spectra for space application using a Monte‐Carlo approach. Rapid Commun. Mass Spectrom. 34. https://doi.org/10.1002/rcm.8684 [7] Serigano, J., Hörst, S.M., He, C., Gautier, T., R. V. Yelle, T.T. Koskinen, M. Trainer, 2020 Compositional Measurements of Saturn’s Upper Atmosphere and Rings from Cassini INM. Journal of Geophysical Research: Planets. [8] Cui, J., R.V. Yelle, V. Vuitton, J.H. Waite, W.T. Kasprzak, D.A. Gell, H.B. Niemann, I.C.F. Muller-Wodarg, N. Borggren, G.G. Fletcher, E. L. Patrick, E. Raaen, B.A. Magee: Analysis of Titan’s neutral upper atmosphere from Cassini Ion Neutral Mass Spectrometer Measurements, Icarus 200, pp. 581-615, 2009

International audience; Using a novel approach, we reanalyzed data acquired by the GCMS instrument aboard Huygens during its descent to Titan. We were able to retrieve concentration of several key volatile species in Titan's troposphere and their vertical profiles.