Huygens' Measurements of the Speed of Sound on Titan

Abstract

On 14th January 2005 the Huygens probe descended to Titan’s surface, measuring, amongst other properties, the speed of sound in the moon’s dense, haze filled atmosphere. These measurements were made by the Acoustic Properties Instrument - Velocimeter (API-V). For a binary mixture of gases, with known components, the sound speed, temperature and pressure can be used to determine the mixing ratio. As the Huygens Gas Chromatograph Mass Spectrometer (GCMS) indicated that the only bulk components of Titan’s atmosphere were methane and nitrogen, sound speed has been used as an indicator of methane abundance. To achieve this, flight spare replicas of API-V have been used for calibration purposes and a non-ideal equation of state supplied by the Groupe Europeen de Recherches Gazieres (GERG) has been employed to model the sound speed in the cold dense mixtures of nitrogen and methane found on Titan. The sound speed on Titan was found to decrease from 183.2 m s-1 at 11 km altitude to 194.0 m s-1 at the surface. Use of the GERG equation of state with Huygens temperature and pressure data indicated that the mole fraction of methane at the surface is 0.026 remaining approximately constant up to 7 km altitude, then decreasing slightly to 0.012 at 11 km altitude. The estimated uncertainty in this value is ±0.018. This is lower than the estimate by GCMS of 0.049±0.0025. There is also a possibility of enhanced methane abundance at the surface and suppressed methane abundance at an altitude of approximately 3 km. After impact the sound speed was found to increase by ~2 m s-1. As API-V was in a separate thermal environment to the temperature sensor, this could be due to an increase in temperature of ~2 K or due to methane evaporating from the potentially wet ground increasing the methane mole fraction by 0.08.