Taking Titan Temperature : seasonal variations of boreal sea temperatures

Titan’s surface thermal emission at 2.2-cm has been recorded for almost 13 years by the RADAR onboard the Cassini spacecraft operated as a radiometer (passive mode) (Janssen et al.,2009, 2016). We used this large brightness temperature data-set to investigate the seasonal evolution of the surface temperature probed by the microwave radiometer in two frequently observed regions in the northern polar region of the moon : the sea Ligeia Mare and its nearby solid terrains.

Over the sea, the low loss tangent of liquid methane and ethane let us to observe the seafloor (Mastrogiuseppe et al., 2014) and to probe the complete column of liquid at 2.2cm. Hence in order to estimate the sea effective temperature, we used a 2 layers emissivity model over the sea - developed in (Le Gall et al., 2016) - taking into account the bathymetry (Hayes, 2016), the sea composition (Mitchell et al., 2015) and the geometry of observation. Despite the arrival of the boreal summer at the end of the mission, we report a decrease of temperature over Ligeia Mare of about 0.7±0.2 K. In contrast, the nearby solid terrains slowly warm by about 1.4±0.3 K through the boreal spring over the course of the Cassini mission. The slow summer warming of solids terrains retrieved at 2.2 cm is in very good agreement with the observation by Cassini’s Composite Infrared Spectrometer (Jennings et al., 2019) and support the idea that evaporation of liquid at the surface takes place after the vernal equinox, possibly after precipitation around the equinox (Turtle et al., 2018 ; Dhingra et al., 2019, 2021), delaying the increase of temperature even if the summer approaches. Moreover, the evolution of temperature in Ligeia Mare observed by the radiometer suggests that evaporative cooling takes place at the sea surface after the equinox.

Comparing the seasonal sea temperature variation to prediction from an ocean circulation model (Tokano & Lorenz, 2016) indicate the that the onset of convection in the sea is likely to happen in Ligeia Mare, the temperature at depth decreases as cool liquid from the surface sinks, lowering the temperature sensed by the radiometer in the early boreal summer. Overall, this work highlights the key role of methane hydrology in controlling the surface and submarine temperatures in the boreal pole of Titan.