Europlanet Science Congress 2022
Palacio de Congresos de Granada, Spain
18 – 23 September 2022
Europlanet Science Congress 2022
Palacio de Congresos de Granada, Spain
18 September – 23 September 2022
EPSC Abstracts
Vol. 16, EPSC2022-130, 2022
https://doi.org/10.5194/epsc2022-130
Europlanet Science Congress 2022
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Solar occultations observed by VIMS-IR: What haze and methane profiles reveal about Titan's atmospheric dynamics and climate.

Pascal Rannou1, Maélie Coutelier2, Sébastien Lebonnois3, Luca Maltagliati4, Emmanuel Rivière1, Michaël Rey1, and Sandrine Vinatier5
Pascal Rannou et al.
  • 1Université de Reims, GSMA, Reims, France (pascal.rannou@univ-reims.fr)
  • 2LATMOS, Université Paris-Saclay, UVSQ, CNRS, 78280 GUYANCOURT, FRANCE
  • 3Laboratoire de Météorologie Dynamique, IPSL, Sorbonne Université, PARIS, FRANCE
  • 4Nature Astronomy, Springer Nature, 4 Crinan Street, N1 9XW LONDON, UNITED KINGDOM
  • 5LESIA, Observatoire de Paris, CNRS, 5 place Jules Janssen, 92195 MEUDON, FRANCE

Titan, the largest satellite of Saturn, has a dense atmosphere mainly composed of nitrogen and methane at a percent level. These two molecules generate a complex prebiotic chemistry, a global haze, most of the cloud cover and the rainfalls which model the landscape. Methane sources are located in liquid reservoirs at and below the surface and it sink is the photodissociation at high altitude. Titan’s present and past climates strongly depend on the connection between the surface sources and the atmosphere upper layers. Despite its importance, very little information is available on this topic. 

 In the last two decades, the observations made by the Cassini orbiter and the Huygens probe have greatly improved our knowledge of Titan’s system. The surface, haze, clouds, and chemical species can be studied and characterised with several instruments simultaneously. On the other hand, some compounds of its climatic cycle remain poorly known. This is clearly the case of the methane cycle, which is, however, a critical component of Titan’s climate and of its evolution. 

We reanalysed four solar occultations by Titan’s atmosphere observed with the infrared part of the Visual Infrared Mapping Spectrometer (VIMS) instrument. These observations were already analysed (Bellucci et al., 2009, Maltagliati et al., 2015), but here we used significantly improved methane spectroscopic data. We retrieved the haze properties (not treated previously) (Figure 1) and the mixing ratios of methane (Figure 2), deuterated methane, and CO in the stratosphere and in the low mesosphere.

Figure 1 : Haze extinction as a function of altitude, retrieved for the four observations, at wavelengths 0.884 μm (channel #97), 1.540 μm (channel #137) and 2.199 μm (channel #177). The extinction profiles retrieved by Seignovert et al. (2021) with Cassini/ISS, at wavelength 338 nm (CL-UV3 filters), are shown with green lines (labelled "S2020"). Those from Vinatier et al. (2010) or Vinatier et al. (2015), scaled at the wavelength 1μm, are shown with black lines ("V2010" or "V2015"). The profiles in cyan ("RP83"), are the extinctions retrieved by Rages & Pollack (1983) at 30◦N in August 1981 (wavelength 0.5 μm). The differences in the detached haze altitudes between VIMS-IR (Ls = 26°), Cassini/ISS (Ls = 14.8°) and Voyager 2/ISS (Ls = 18°) are their dates while the detached is falling down (West et al. (2018); Seignovert et al. (2021)). The grey line shows the haze profile by Doose et al. (2016) with DISR in 2005 at 10°S (labelled "D2016"). 

We find that the methane mixing ratio in the stratosphere is much lower (about 1.1%) than expected from Huygens measurements (about 1.4 to 1.5%). However, this is consistent with previous results obtained with CIRS. Features in the methane vertical profiles clearly demonstrate that there are interactions between the methane distribution and the atmosphere circulation. We find a layer rich in methane at 165 km and at 70°S (mixing ratio 1.45 ± 0.1%) and a dryer background stratosphere (1.1 − 1.2%). In absence of local production, this reveals an intrusion of methane transported into the stratosphere, probably by convective circulation. On the other hand, methane transport through the tropopause at global scale appears quite inhibited. Leaking through the tropopause is an important bottleneck of Titan’s methane cycle at all timescales. As such, it affects the long term evolution of Titan atmosphere and the exchange fluxes with the surface and subsurface reservoirs in a complex way.

Figure 2 : Methane mixing ratio retrieved with the four observation sets, with data between 0.88 and 2 μm (top) and between 2 and 2.8 μm (bottom). We also plot the methane mole fraction retrieved with the GCMS onboard Huygens (Niemann et al. (2010)) and with DIRS (Bézard (2014)) and CISR (Lellouch et al. (2014)). The green dashed profile, in the upper left graph, shows the evaluation made by Rannou et al. (2021). 

 

We also retrieved the haze extinction profiles and the haze spectral behaviour. We find that aerosols are aggregates with a fractal dimension of Df ≃2.3±0.1, rather than Df ≃2 as previously thought. Our analysis also reveals noticeable changes in their size distribution and their morphology with altitude and time. These changes are also clearly connected to the atmosphere circulation and concerns the whole stratosphere and the transition between the main and the detached haze layers. 

We conclude that, to fully understand these results, Global Climate Models accounting for haze and cloud physics, thermodynamical feedbacks and convection are needed. Especially, the humidificaton of the stratosphere, at the present time and its evolution under changing conditions at geological timescale appears as a key process, and our work provide strong constraints to guide studies.

 

References

Bellucci, A., Sicardy, B., Drossart, P., et al. 2009, Icarus, 201, 198 
Bézard, B. 2014, Icarus, 242, 64 
Doose, L. R., Karkoschka, E., Tomasko, M. G., & Anderson, C. M. 2016, Icarus, 270, 355 
Lellouch, E., Bézard, B., Flasar, F. M., et al. 2014, Icarus, 231, 323 
Maltagliati, L., Bézard, B., Vinatier, S., et al. 2015, Icarus, 248, 1 
Niemann, H. B., Atreya, S. K., Demick, J. E., et al. 2010, Journal of Geophysical Research (Planets), 115, E12006 
Rages, K. & Pollack, J. B. 1983, Icarus, 55, 50 
Rannou, P., Coutelier, M., Riviere, E., et al. 2021, Astrophysical Journal, 922
Rey, M., Nikitin, A., Bézard, B., et al. 2018, Icarus, 303, 114 
Seignovert, B., Rannou, P., West, R. A., & Vinatier, S. 2021, The Astrophysical Journal, 907, 36 
Vinatier, S., Bézard, B., de Kok, R., et al. 2010, Icarus, 210, 852 
Vinatier, S., Bézard, B., Lebonnois, S., et al. 2015, Icarus, 250, 95 
West, R. A., Balloch, J., Dumont, P., et al. 2018, Geophysical Research Letters, 38 

 

How to cite: Rannou, P., Coutelier, M., Lebonnois, S., Maltagliati, L., Rivière, E., Rey, M., and Vinatier, S.: Solar occultations observed by VIMS-IR: What haze and methane profiles reveal about Titan's atmospheric dynamics and climate., Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-130, https://doi.org/10.5194/epsc2022-130, 2022.

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