Abstract

On the basis of previous ground-based and fly-by information, we knew that Titan's atmosphere was mainly nitrogen, with some methane, but its temperature and pressure profiles were poorly constrained because of uncertainties in the detailed composition. The extent of atmospheric electricity ('lightning') was also hitherto unknown. Here we report the temperature and density profiles, as determined by the Huygens Atmospheric Structure Instrument (HASI), from an altitude of 1,400 km down to the surface. In the upper part of the atmosphere, the temperature and density were both higher than expected. There is a lower ionospheric layer between 140 km and 40 km, with electrical conductivity peaking near 60 km. We may also have seen the signature of lightning. At the surface, the temperature was 93.65 0.25 K, and the pressure was 1,467 1 hPa.

1. LESIA, Observatoire de Paris, 5 Place Janssen, 92195 Meudon, France
2. Université Denis Diderot – Paris 7, UFR de Physique, 2 Place Jussieu, 75006 Paris, France
3. CISAS "G. Colombo", Università di Padova, Via Venezia 15, 35131 Padova, Italy
4. PSSRI, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
5. Department of Geophysics and Planetary Sciences, University of Tel Aviv, 69978 Tel Aviv, Israel
6. NASA/AMES Research Center, MS 244-30, Moffett Field, California 94035, USA
7. ESA Headquarters, Science Directorate, 8-10 rue Mario-Nikis, 75015 Paris, France
8. ESA-ESTEC, European Space Agency, Keplerlaan 1, 2200 AG Noordwijk, The Netherlands
9. Agenzia Spaziale Italiana, Viale Liegi 26, 00198 Roma, Italy
10. CETP-IPSL, 4 Avenue de Neptune, 94107 Saint Maur, France
11. LPCE-CNRS, 3A, Avenue de la Recherche Scientifique, 45071 Orléans cedex 2, France
12. Finnish Meteorological Institute (FMI), Vuorikatu 15 A 00100 Helsinki, Finland
13. Space Research Institute, Austrian Academy of Sciences (IWF), Schmiedlstrasse 6, 8042 Graz, Austria
14. Instituto de Astrofisica de Andalucia (IAA-CSIC), PO Box 3004, 18080 Granada, Spain
15. Applied Electromagnetic Group, Department of Physics, University of Murcia, Murcia 30100, Spain
16. Institut für Geophysik und Meteorologie, Universität zu Köln, Albertus-Magnus-Platz, 50923 Köln, Germany
17. DMFCI, Università di Catania, Viale A. Doria 6, 95125 Catania, Italy
18. Politecnico di Milano, Dipartimento di Meccanica, Piazza Leonardo da Vinci 32, 20133 Milano, Italy
19. Center for Space Physics, Boston University, 725 Commonwealth Avenue, Boston, Massachusetts 02215, USA
20. ‡Deceased

Correspondence to: F. Ferri³ Correspondence and requests for materials should be addressed to F.F. (Email: francesca.ferri@unipd.it).

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