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

Abstract

Saturn’s moon Titan is the only satellite in our solar system with a dense atmosphere and hydrocarbon seas. The proposed 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, their influence on the development of a cryogenic depth sounder, and on an approach to infer the transducer’s response, sensitivity and performance when unable to perform in-situ calibration measurements or to replicate key environmental conditions. This effort was conducted under the auspices of the Civilian Space Independent Research and Development program from the Johns Hopkins University Applied Physics Laboratory.

Composition, seasonal change, and bathymetry of Ligeia Mare, Titan, derived from its microwave thermal emission

“…There has been considerable recent interest in future in situ exploration of Titan's seas from floating capsules [e.g., Stofan et al ., ; Mitri et al ., ]: by drifting in currents and winds [ Lorenz , ], such capsules could traverse Ligeia Mare and measure a profile of the depth and bottom characteristics with sonar at much higher resolution than Cassini. The Huygens probe carried an echo sounder, and detailed sonar modeling in Titan liquids was performed in support of the Titan Mare Explorer (TiME) mission proposal [ Arvelo and Lorenz , ]: such measurements can usefully complement microwave observations from Cassini. The analysis here, suggesting a compacted seabed on Ligeia, indicates that the acoustic reflectivity of the seafloor will yield strong sonar echoes.…”

Voyage across Ligeia Mare: Mechanics of sailing on the hydrocarbon seas of Saturn׳s Moon, Titan

“…However, Since a wind-driven traverse is of interest for public appeal as well as scientific reasons (e.g. to make a bottom profile with a depth sounder (Arvelo and Lorenz, 2014), to inspect the shoreline, etc. ), and for mission safety splashdown would be aimed at the central, deepest part of Ligiea, the nominal plan would be to sail without an anchor.…”

“…The turbidity and methane humidity measurements were implemented with novel fiber-coupled instrumentation, allowing the optoelectronic components to be retained inside the comfortably warm capsule interior. One other measurement system was a down-looking echo sounder (Arvelo and Lorenz, 2014), able to measure the depth of the sea as well as potentially detect layering or suspended sediments.…”

Silence on Shangri-La: Attenuation of Huygens acoustic signals suggests surface volatiles

“…Of these studies, the Titan Mare Explorer (TiME) was the most advanced, with detailed study of sonar operation (e.g. Arvelo and Lorenz, 2013) and cryogenic testing of prototype acoustic transducers . Given this recurrent interest we therefore reexamine here the surface operation of the Huygens acoustic instrumentation, whose results were not interpreted previously.…”