Plumbing the depths of Ligeia: Considerations for depth sounding in Titan's hydrocarbon seas

Abstract

Saturn's moon Titan is the only satellite in this solar system with a dense atmosphere and hydrocarbon seas. The Titan Mare Explorer (TiME) mission would splashdown a capsule to float for 3 months on Ligeia Mare, a several-hundred-kilometer wide sea near Titan's north pole. Among TiME's scientific goals is the determination of the depth of Ligeia, to be achieved with an acoustic depth sounder. Since Titan's surface temperature is known to vary around 92 K, all instruments must be ruggedized to operate at cryogenic temperatures. This paper's contributions include an approach to infer key acoustic properties of this remote environment and the extraterrestrial environment's influence on the development of a cryogenic depth sounder. Additionally, an approach is formulated to infer the transducer's response, sensitivity, and performance when in situ calibration is impossible or when replicating key environmental conditions is too costly.

How speed-of-sound measurements could bring constraints on the composition of Titan's seas

“…In this work, I shown that the use of a realistic model, based on PC-SAFT, prevents the appearance of unphysical situations, like a speed of sound discontinuity, already noticed in previous published papers (Hagermann et al 2005;Arvelo & Lorenz 2013). However, even such sophisticated models need to be constrained by empirical data.…”

“…However, they did not give the full derivation of the computed speed of sound. In the context of depths sounding, Arvelo & Lorenz (2013) made computations based on the speed of sound in cryogenic liquids. Their work also involves the NIST14 database.…”

“…Their work also involves the NIST14 database. Unfortunately, the obtained velocities exhibit discontinuities which are certainly not physical (see figure 3 of Arvelo & Lorenz 2013). In addition, these authors assumed that their adopted linear fit can be extrapolated to temperatures higher than 92.5 K. For all these reasons, I decided to explore the question of speed of sound in cryogenic liquids and to revisit its sensitivity to chemical composition.…”

Deep Ocean Passive Acoustic Technologies for Exploration of Ocean and Surface Sea Worlds in the Outer Solar System

“…These sources should be similar to those common on Earth and include fracturing of cryo-and/or lithic-crust (quakes), explosive volcanism, debris flows/landslides, meteor impacts, and buoyant hydrothermal fluid plumes. Moreover, as is the case on Titan where there are open fluid lakes or seas, meteorological sound sources from precipitation, surface wave breaking, waterfalls, and wind-wave interaction can also be expected (Arvelo and Lorenz, 2013). Thus, the unique scientific contribution achieved by using hydrophones and hydroacoustic signal detection techniques on Earth is likely to be the same for other ocean worlds.…”

“…Note that the vertical extent of a 1 km hydrocarbon sea is small enough compared with Titan's radius (2,575 km) that gravity g can be considered constant with depth, in contrast to the deeper (~100 km) liquid water oceans of icy satellites like Europa, where g varies appreciably with depth (Leighton et al, 2013). Arvelo and Lorenz (2013) estimate acoustic signal loss due to sound absorption α in Titan's methane seas is expected to be very low (~0.035 dB km -1 measured at 20 kHz) and likely to have little effect on sound propagation.…”