A physical model for predicting the sound speed and attenuation coefficient in Titan's atmosphere based on Cassini-Huygens data

Abstract

NASA and ESA are discussing plans for a collaborative mission to use montgolfieres to gather long-duration data in Titan's atmosphere. Acoustic sensors can listen for thunder, bolide explosions, wind noise, cryo-volcanoes and many other phenomena. This emphasizes the need for accurate acoustic predictions for Titan. In 2005, during the descent of the Huygens probe on Titan, an active ultrasonic sensor measured the speed of sound over the last 12 kilometers. Using the ambient pressure, density, temperature, and methane concentration measured by Huygens as inputs, as well as temperature-- and pressure--dependent transport parameters extracted from NIST, a theoretical model has been developed to predict the sound speed and attenuation coefficient in Titan's atmosphere. Based upon non-ideal equations of state, the sound speed predictions agree quite well with Huygens measurements in the lower troposphere. The effect of measured zonal winds on tropospheric propagation is presented via ray-tracing, showing quiet zone predictions. The model can be extended to the upper atmospheric layers (since ambient data is available); nevertheless care must be taken to account for altitude dependent processes such as winds, clouds, aerosols, chemistry, gravity waves etc. in order to increase the accuracy.