Six Days on Ligeia Mare : An Assessment of Meteorological and Oceanographic Phenomena on Titan's Seas

Titan's hydrocarbon seas are a scientifically appealing target for Cassini observations and for future exploration. The proposed Titan Mare Explorer ('TiME') Discovery mission will expand the temporal span of in-situ measurements at Titan's surface by 3 orders of magnitude from the (fortuitous) ~1 hr surface operation of the Huygens probe to TiME's nominal mission of 6 Titan days (96 Earth days) on Ligeia Mare, enabled by its radioisotope power source. This talk reviews Cassini observations and considers what time-varying features might be observed over this interval. Global Circulation Models (GCMs) suggest northern polar surface winds during the Cassini prime mission have been weak, consistent with radar backscatter and infrared specular reflection observations that indicate an absence of waves. These models also predict that winds are presently freshening, to a maximum of ~1.5m/s in midsummer (2017, the end of the Cassini Solstice Mission). Windspeeds should then decline, with expected speeds during the TiME mission epoch (2023) of ~1m/s, producing only small waves (~0.2m in height). Diurnal pressure changes of ~0.5 mbar are expected, while air and surface temperatures are expected to vary by ~1K or less. GCMs suggest that the wind vector on Ligeia may rotate substantially over the course of a Titan day, such that the drift of a floating capsule may follow an epicyclic trajectory with a net drift rather slower than its instantaneous drift rate, although nonetheless covering >100km over several Titan days. A bottom profile measured by an acoustic depth sounder (sonar) may therefore have several crossovers. At Ligeia's western and eastern margins, assuming a rigid crust, a tidal range of ~1m is encountered and may be detectable (on the other hand, a high Love number may reduce tidal effects). Evidence suggests Ligeia may have a central depth of ~300m or more, which means tidal currents will be ~1cm or less. The sun seen from Ligeia in 2023 remains up but low in the sky (peak elevation 20 degrees) for most of the day, dipping only briefly below the horizon. The changing illumination conditions over the long observing period will permit study of Titan's haze scattering and may allow imaging of rainbows,halos, mirages or other optical phenomena. A review of telescopic observations of cloud systems, and numerical models of precipitating cloud systems, suggests that during polar summer there is a ~0.1-2.5% chance of being rained on during the TiME mission. Such a precipitation event might last ~2 hours, providing a guiding timescale for meteorological measurements. In addition to meteorological, optical and acoustic signatures of such an event, the TiME capsule may detect a transient perturbation in near-surface liquid composition. Observations of these effects will not only bring a new era in Titan studies, but promise to open a new dimension in oceanography via study of marine processes in an exotic planetary environment.