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An intense thermospheric jet on Titan

Nature Astronomy volume 3, pages 614–619 (2019)Cite this article

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Abstract

The presence of winds in Titan’s lower and middle atmosphere has been determined by a variety of techniques, including direct measurements from the Huygens Probe1 over 0–150 km; Doppler shifts of molecular spectral lines in the optical, thermal infrared and millimetre ranges2,3,4, which together have probed the ~100–450 km altitude range; inferences from the thermal field over 10–0.001 mbar (that is, ~100–500 km)5,6; and inferences from central flashes in stellar occultation curves7,8,9. These measurements predominantly indicated strong prograde winds, reaching maximum speeds of ~150–200 m s−1 in the upper stratosphere, with important latitudinal and seasonal variations. However, these observations provided incomplete atmospheric sounding; in particular, the wind regime in Titan’s upper mesosphere and thermosphere (500–1,200 km) has remained unconstrained so far. Here we report direct wind measurements based on Doppler shifts of six molecular species observed with the Atacama Large Millimeter/submillimeter Array (ALMA). We show that contrary to expectations, strong prograde winds extend up to the thermosphere, with the circulation progressively turning into an equatorial jet regime as the altitude increases, reaching ~340 m s−1 at 1,000 km. We suggest that these winds may represent the dynamical response of forcing by waves launched at upper stratospheric/mesospheric levels and/or of magnetospheric–ionospheric interaction. We also demonstrate that the distribution of the hydrogen isocyanide (HNC) molecule is restricted to Titan’s thermosphere above ~870 km altitude.

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Fig. 1: Evidence for zonal winds.
Fig. 2: Doppler wind maps.
Fig. 3: Molecular abundance profiles and probed altitudes.

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Data availability

The data that support the plots within this paper and other findings of this study are available from the ALMA archive (http://almascience.nrao.edu/aq/; Cycle 3 programme 2015.1.01023.S (PI M. Gurwell)) and from the corresponding author upon reasonable request.

Code availability

Novel methods developed for this research and not available from previous studies are available from the corresponding author upon reasonable request.

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Acknowledgements

E.L., R.M. and S.V. acknowledge support from the Programme National de Planétologie (PNP-INSU). This paper makes use of the following ALMA data: ADS/JAO.ALMA#2015.1.1023.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. The paper is dedicated to the memory of Daniel Lellouch, deceased on 13 February 2019.

Author information

Authors and Affiliations

  1. LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université Paris Diderot, Sorbonne Paris Cité, Meudon, France

    E. Lellouch, R. Moreno & S. Vinatier

  2. Center for Astrophysics | Harvard & Smithsonian, Cambridge, MA, USA

    M. A. Gurwell

  3. Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD, USA

    D. F. Strobel

  4. Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD, USA

    D. F. Strobel

  5. SOFIA/USRA, NASA Ames Research Center, Moffett Field, CA, USA

    A. Moullet

  6. National Radio Astronomical Observatory, Socorro, NM, USA

    B. Butler

  7. Instituto de Astrofísica de Andalucia — CSIC, Glorieta de la Astronomia, Granada, Spain

    L. Lara

  8. Bandung Institute of Technology, Bandung, West Java, Indonesia

    T. Hidayat

  9. Joint ALMA Observatory, Alonso de Cordova, Vitacura, Santiago, Chile

    E. Villard

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  1. E. Lellouch
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Contributions

M.A.G. wrote the ALMA proposal. M.A.G. and R.M. reduced the ALMA data. E.L. analysed and modelled the data (with contribution from R.M.) and wrote most of the manuscript. S.V. provided initial thermal profiles as well as insight in the general science context. D.F.S. led the interpretative part. All authors discussed the manuscript.

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Correspondence to E. Lellouch.

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The authors declare no competing interests.

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Journal peer review information: Nature Astronomy thanks Agustín Sanchez-Lavega and Jan-Erik Wahlund for their contribution to the peer review of this work.

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Supplementary Information

Supplementary text, Supplementary Table 1, Supplementary Figures 1–7.

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Lellouch, E., Gurwell, M.A., Moreno, R. et al. An intense thermospheric jet on Titan. Nat Astron 3, 614–619 (2019). https://doi.org/10.1038/s41550-019-0749-4

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