GENESIS OF TITAN’S RIAS FROM THE LIGEIA MARE PUGET SINUS REGION USING EARTH ANALOGS FROM NW SPAIN

Ligeia Mare is Titan’s second largest methane sea. It is centered approximately at 78°N latitude and 249°W longitude and has a surface area of approximately 126,000 km². A significant number of large rias present in its almost 2000 km of coastline were recorded by the Cassini spacecraft synthetic-aperture radar (SAR) during several flybys. The genesis of two rias from northwest Spain (Ria de Vigo and Ria de Arousa) was studied to understand the genesis of two rias from the Puget Sinus region (Xanthus Flumen Ria and Unnamed Ria) of Ligeia Mare. These rias present similar lengths, widths and average depths. Ria de Vigo is 35 km long, 7 km wide and 30-35 m deep. Ria de Arousa is 33 km long, 9 km wide and 35-40 m deep. Xanthus Flumen Ria is approximately 50 km long, 15 km wide and 20-25 m deep. The Unnamed Ria is approximately 21 km long, 9 km wide and 30 m deep.

The two rias from NW Spain were created by changes in sea level linked to millennial changes in climate. During the last glacial stage, a lower sea level promoted fluvial erosion of coastal valleys below sea level. These valleys were later flooded during the Flandrian transgression creating the rias. The genesis of the two Titan rias is also explained by changes in sea level, however, it is linked to precipitation and seasonal changes occurring over long periods. Saturn’s orbital period is 29.4 earth years, therefore, the convective precipitation that falls on Titan’s north pole region during its late vernal equinox and summer solstice can last up to 12 earth years. This will drop (by evaporation) and raise (by heavy precipitation) the sea level, slowly generating rias over multiple summers. This process was also observed on Titan’s south pole region in Ontario Lacus Sea which experienced changes in sea level during its summer season. During the fall and winter seasons the polar regions experience less insolation which will drop temperature, inhibiting evaporation and therefore changes in sea level. The Cassini spacecraft surveyed Titan over a 13-year period (during its northern winter solstice and vernal equinox) recording high sea levels in its north pole seas and variable sea levels in its south pole seas suggesting that changes in sea level linked to precipitation and seasonal changes (orbital forcing) are most likely the processes associated to ria genesis on Titan.