Research at our Institute: Interpretation of Huygens Data
Huygens Probe (image credit: ESA)
The Cologne group has been involved in the Huygens Atmospheric Structure Instrument (HASI) onboard the Huygens Probe. The Huygens Probe aimed at investigating the atmosphere and surface of Titan. The probe was separated from the Cassini spacecraft in December 2004 and entered Titan’s atmosphere on 14 January 2005.
During the entry phase in the upper atmosphere the accelerometer onboard HASI measured the variation of the slowdown, from which the atmospheric density and temperature could be derived. A few minutes later a parachute was released in the stratosphere and the other instruments started in situ measurements. The descent to the surface took approximately 2.5 hours. The temperature and pressure of the air was measured continuously by sensors located outside the probe. The electric sensors measured the electric conductivity, electric field and other electric signals and an acoustic sensor registered the sound during the descent. Eventually the probe touched down the surface softly on the surface, where it continued measurements for another hour.
The Cologne group has been instrumental in contributing to the following interpretations of Huygens data:
1) Drizzle
The temperature and pressure profile of HASI and the methane mixing ratio profile of the Gas Chromatograph Mass Spectrometer (GCMS) enabled calculating the vertical profile of relative humidity. It revealed the presence of thin clouds of liquid methane-nitrogen mixture and of solid methane (without nitrogen) one upon the other, although ground-based telescopes could not find any visible clouds during the descent of the Huygens probe. The weak vertical air motion during the landing can rule out that the probe entered a convective cloud. The humidity profile is consistent with the idea that drizzle falling from the lower cloud reached the surface.
Schematic of clouds at the Huygens landing site (from Tokano et al., 2006, Nature 442, 432-435)
2) Planetary Boundary Layer
The temperature and pressure profile near the surface in high precision for the first time provided an unambiguous proof of the existence of a weak planetary boundary layer on Titan. At the time of landing the boundary layer was weakly convective and had a depth of 300 m.
However, it is also possible to interpret that the boundary layer was so weak because the probe landing of during the morning hours. The typical dune spacing of 3 km and the staggered profile of the potential temperature could indicate that the boundary layer further grows in the daytime by convection.
3) Winds
The horizontal winds near the surface are mostly a manifestation of the Hadley circulation and the thermal wind in equilibrium with the weak temperature decrease towards the equator. The measured wind profile does not unambiguously constrain tidal winds caused by Saturn.
The vertical wind could be derived from a precise profile of pressure during the descent. According to this measurement there was a weak upward air motion of a few cm/s, which is consistent with the absence of convective clouds. On the other hand, stronger vertical winds were found in the stratosphere. This could be evidence of a thermally indirect circulation or of atmospheric waves.
Press release of ESA “The way the wind blows on Titan” (1 July 2007)
Publications about these Topics:
Fulchignoni, M., F. Ferri, F. Angrilli, A. J. Ball, A. Bar-Nun, M. A. Barucci, C. Bettanini, G. Bianchini, W. Borucki, G. Colombatti, M. Coradini, A. Coustenis, S. Debei, P. Falkner, G. Fanti, E. Flamini, V. Gaborit, R. Grard, M. Hamelin, A. M. Harri, B. Hathi, I. Jernej, M. R. Leese, A. Lehto, P. F. Leon Stoppato, J. J. López-Moreno, T. Mäkinen, J. A. M. McDonnell, C. P. McKay, G. Molina-Cuberos, F. M. Neubauer, V. Pirronello, R. Rodrigo, B. Saggin, K. Schwingenschuh, A. Seiff, F. Simões, H. Svedhem, T. Tokano, M. C. Towner, R. Trautner, P. Withers, J. C. Zarnecki. In situ measurements of the physical characteristics of Titan’s environment. Nature, 438, 785-791, 2005.
Tokano, T., C. P. McKay, F. M. Neubauer, S. K. Atreya, F. Ferri, M. Fulchignoni, H. B. Niemann. Methane drizzle on Titan. Nature, 442, 432-435, 2006.
Tokano, T., F. Ferri, G. Colombatti, T. Mäkinen, M. Fulchignoni. Titan’s planetary boundary layer structure at the Huygens landing site. J. Geophys. Res., 111, E08007, doi:10.1029/2006JE002704, 2006.
Mäkinen, J. T. T., A.-M. Harri, T. Tokano, H. Savijärvi, T. Siili, F. Ferri. Vertical atmospheric flow on Titan as measured by the HASI instrument. Geophys. Res. Lett., 33, L21803, doi:10.1029/2006GL026982, 2006.
Grard, R., M. Hamelin, J. J. López-Moreno, K. Schwingenschuh, I. Jernej, G. J. Molina-Cuberos, F. Simões, R. Trautner, P. Falkner, F. Ferri, M. Fulchignoni, R. Rodrigo, H. Svedhem, C. Béghin, J.-J. Berthelier, V. J. G. Brown, M. Chabassière, J. M. Jeronimo, L. M. Lara, T. Tokano. Electric properties and related physical characteristics of the atmosphere and surface of Titan. Planet. Space Sci., 54, 1124-1136, 2006.
Simões, F., R. Grard, M. Hamelin, J. J. López-Moreno, K. Schwingenschuh, C. Béghin, J.-J. Berthelier, B: Besser, V. J. G. Brown, M. Chabassière, P. Falkner, F. Ferri, M. Fulchignoni, I. Jernej, J. M. Jeronimo, G. J. Molina-Cuberos, T. Tokano, R. Trautner. A new numerical model for the simulation of ELF wave propagation and the computation of eigenmodes in the atmosphere of Titan: did Huygens observe any Schumann resonance? Planet. Space Sci., 55, 1978-1989, 2007.
Hamelin, M., C. Béghin, R. Grard, J. J. López-Moreno, K. Schwingenschuh, F. Simões, R. Trautner, J. J. Berthelier, V. J. G. Brown, M. Chabassière, P. Falkner, F. Ferri, M. Fulchignoni, I. Jernej, J. M. Jeronimo, G. J. Molina-Cuberos, R. Rodrigo, T. Tokano. Conductivity and electron density profiles of the atmosphere of Titan from the Huygens PWA-HASI instrument: Mutual Impedance probe measurements. Planet. Space Sci., 55, 1964-1977, 2007.
Béghin, C., F. Simões, V. Krasnoselskikh, K. Schwingenschuh, J. J. Berthelier, B. Besser, C. Bettanini, R. Grard, M. Hamelin, J. J. López-Moreno, G. J. Molina-Cuberos, T. Tokano. A Schumann-like resonance on Titan driven by Saturn’s magnetosphere possibly revealed by the Huygens probe. Icarus, 191, 251-266, 2007.
Tokano, T. Near-surface winds at the Huygens site on Titan: interpretation by means of a general circulation model. Planet. Space Sci., 55, 1990-2009, 2007.
Lorenz, R. D., P. Claudin, B. Andreotti, J. Radebaugh, T. Tokano. A 3 km atmospheric boundary layer on Titan indicated by dune spacing and Huygens data. Icarus, 205, 719-721, 2010.