In situ measurements of the physical characteristics of Titan's environment


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.

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Figure 1: The atmospheric density profile of Titan as measured by HASI.
Figure 2: The atmospheric temperature profile.
Figure 3: The temperature lapse rate.
Figure 4: Pressure profile of the lower atmosphere as measured by the Pressure Profile Instrument (PPI)28.
Figure 5: Temperature profile of the lower atmosphere as measured by the temperature sensors, TEM 27 (expanded from Fig. 2).
Figure 6: The temperature lapse rate for the low atmosphere (expanded from Fig. 3).
Figure 7: A synopsis of PWA data: the signature of the ionosphere.
Figure 8: A synopsis of PWA data: electric field, acoustic pressure and radar measurements.
Figure 9: The HASI signature of the impact trace, at 200 samples per second.


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We thank the following people for their contributions to the realization of the HASI experiment: A. Buccheri, R. DeVidi, and M. Cosi of Galileo Avionica, A. Aboudan, S. Bastianello and M. Fabris of CISAS, M. Chabassière of LPCE, V. Brown, J.M. Jeronimo and L.M. Lara of IAA, R. Hofe of IWF, A. Smit, L. Smit and J. Van der Hooke from RSSD-ESTEC, H. Jolly from the UK, R. Pellinen, G. Leppelmeier, T. Siili, P. Salminen from FMI, and at the Aerodynamics Laboratory of Helsinki University of Technology T. Siikonen and B. Fagerström. HASI has been realised and operated by CISAS under a contract with the Italian Space Agency (ASI), with the participation of RSSD, FMI, IAA, IWF, LPCE and PSSRI sponsored by the respective agencies: ESA, TEKES, CSIC, BM:BWK, CNES and PPARC. We also acknowledge the long years of work by some hundreds of people in the development and design of the Huygens probe. The Huygens probe is part of the Cassini-Huygens mission, a joint endeavour of the National Aeronautics and Space Administration (NASA), the European Space Agency (ESA) and the Italian Space Agency (ASI).

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Fulchignoni, M., Ferri, F., Angrilli, F. et al. In situ measurements of the physical characteristics of Titan's environment. Nature 438, 785–791 (2005).

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