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The European Space Agency’s Huygens probe, when it landed on the surface of Titan, Saturn’s largest moon, on January 14, 2005, carried the first active acoustic sensors to be deployed on another cosmic body. The Surface Science Package contained a simple sonar (operating at 14.7 kHz) and a speed of sound sensor (operating at 1 MHz). The design and operation of the sensors will be described. Despite being optimized for the case of landing on a surface of liquid hydrocarbons, the landing on a solid surface still produced valuable data. The former sensor obtained echoes from Titan’s surface during the last 90 m of the descent. Interpretation of these data will be discussed in terms of surface properties, such as terrain topography and possible physical nature, around the landing site. Probe landing speed was also derived with high precision. The speed of sound instrument operated in the lower 11 km of the atmosphere. These data have been used to constrain the methane content of Titan’s atmosphere to approximately 2% at 10 km rising to about 3.5% at lower altitudes.

Speed of sound measurements in nitrogen by Younglove and McCarty [J. Chem. Thermodynam. 12, 1121–1128 (1980)] are revisited and an empirical polynomial equation for the speed of sound is derived. The polynomial coefficients differ from those given by Wong and Wu [J. Acoust. Soc. Am. 102, 650–651 (1997)] with the result that discrepancies between predicted and measured values at low temperatures are reduced. The maximal error over the complete temperature and pressure range from 80 to 350 K and 0.031 to 0.709 MPa is reduced from 5.38% to 0.78%.

The Huygens probe descended through Titan's atmosphere in January 2005. On board was the Surface Science Package (SSP), a set of nine sensors, which included a speed-of-sound sensor. We present a detailed description of the SSP speed of sound measurements and report constraints on the methane content in Titan's lower atmosphere based on these measurements. After extensive instrument calibration and subsequent Bayesian analysis of the data, the most likely result derived from our measurements in Titan's lower atmosphere is a methane fraction of approximately 2% at 10 km, increasing to 3.5% at lower altitudes. These estimates are based on a binary composition. Our data show that any large scale variation of methane within the lower 11 km of Titan's atmosphere is unlikely. Within experimental and theoretical uncertainties, our estimates are lower than, but compatible with earlier estimates obtained from the mass spectrometry experiment.