An outline of the papers: Transient surface liquid in Titan’s polar regions from Cassini by Hazes et al. and Shoreline retreat at Titan’s Ontario Lacus and Arrakis Planitia from Cassini Imaging Science Subsystem observations by Turtle et al.
The Cassini probe has been observing Titan since 2004. It has detected lakes of liquid methane at the surface of the planet, suggesting an active methane cycle between the clouds and the surface.
Two of Cassini’s instruments are useful to study the lakes, the RADAR, and the ISS camera. Liquid surfaces react very differently to radar imaging, and in ISS images lakes appear as dark patches with well-defined borders (lakes on Titan have also been inferred from the measurement of “specular reflection” at low incident angles).
With these two Cassini instruments, Hayes et al. (2011) and Turtle et al. (2011) have mapped changes in the position and shape of some of Titan’s lakes. Some smaller ponds seem to have disappeared entirely, while the shores of the large Ontario Lacus have receded by several kilometres in some regions.
These changes are interpreted as seasonal variations along Titan’s methane cycle by the authors. Lake water from the summer storms is slowly lost to evaporation and infiltration. A season lasts about 7 years of Titan, and the Cassini observations cover the middle and end of the northern summer (2002-2009).
A study of the local topography gives a very nice confirmation that the cause of the changes in the shoreline is due a change in the lake level: where the slope is shallower, the retreat is faster. Like its namesake on Earth, Ontario has some very shallow banks. The authors calculate that the observed retreat, several kilometres at places, corresponds to a drop in lake level of only about 4 metres.
FEATURE IMAGE: Detail from Figure 8 of Hayes et al.