The Huygens probe Surface Science Package acoustic instrument: Results at Titan

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.