To sink or swim in Titan’s lakes

Flotation of aerosols as a film on the hydrocarbon lakes of Saturn’s moon Titan may explain the lakes’ stillness, and could influence the atmospheric hydrocarbon cycle.

Titan is unique in the Solar System in that it is a satellite with a dense atmosphere. Saturn’s largest moon is therefore often compared to Earth. One of Titan’s most important characteristics is probably the existence of an approximately 1,000 km thick layer of haze in its lower atmosphere, which gives it its brownish colour¹. This haze consists of photochemical organic aerosols formed in the atmosphere; these aerosols eventually settle to the ground. The Cassini–Huygens mission identified lakes and seas of liquid hydrocarbons (predominantly methane and ethane) in the polar regions of Titan^2,3 (Fig. 1). Writing in Nature Geoscience, Cordier and Carrasco⁴ report the plausible formation of patchy films of aerosols deposited on the surfaces of these lakes, similar to microlayers of oil or surfactant on water on Earth, which could impede wave generation.

Fig. 1: Lakes and seas in the North polar region of Titan.

NASA/JPL-Caltech/ASI/USGS

This false-colour composite of Cassini radar images at >50° N shows the distribution of bodies of liquid hydrocarbons (blue–black areas). The largest sea, Kraken Mare, is approximately 1,100 km long and around 400,000 km² in area (ref. ³).

Observations by the Cassini spacecraft have suggested that Titan’s lakes are curiously unaffected by waves. In particular, surface flatness has been detected in various locations, including on Ligeia Mare⁵, Jingpo Lacus and Kraken Mare⁶ in the North polar region (Fig. 1). Some of these observations were made after the solstice, when winds might have been expected to be high, at least in the upper layers of the atmosphere.

Cordier and Carrasco⁴ present theoretical considerations on the floatability of Titan’s aerosols, and their potential influence on surface smoothness. To do that, they first identify three types of material that may be deposited on Titan’s lakes and contribute to film production: aerosol, either as dry particles or embedded in liquid droplets; crystallized organic analogues of snowflakes on Earth; and large molecules produced in the upper atmosphere, which could be aerosol precursors. The actual composition of such materials is not well known, so Cordier and Carrasco assume that they include both liquidophilic and liquidophobic particles. They calculate that liquidophilic particles, defined by a contact angle of less than 90° with the liquid, will sink to depth. In contrast, the liquidophobic particles, with a contact angle between 90° and 180°, are found to have capillary forces sufficient to support their weight. Hence these particles are able to float and potentially form a layer on the surfaces of Titan’s lakes and seas.

Once aerosols have formed a layer on a lake’s surface, such a film may be fairly persistent. Precipitation from Titan’s atmosphere is small, and aerosol films may be difficult to destroy by tides, rain or wind. These effects could, however, induce migration or fragmentation of films. Cordier and Carrasco suggest that breakage of the surface film by nitrogen bubbles from the lake bottom (rather than the bubbles themselves⁷) could explain lake surface features informally termed ‘magic islands’, bright anomalies that appear and disappear from one Cassini flyby to another⁸. They also show that conditions on Titan are much more favourable for a surface film to damp waves than on Earth; hence they propose that aerosol flotation may explain the absence of waves on the lakes observed by Cassini^5,6. The presence of floating aerosol films on Titan’s lakes could also modify the exchanges between the sea and the atmosphere. They may reduce the evaporation rate and therefore cloud formation. In turn, the atmospheric hydrocarbon cycle is likely to adjust.

The scenario proposed by Cordier and Carrasco⁴ of a floating film of aerosols on Titan’s lakes could explain some of the peculiar features of these lakes, such as their smooth surfaces and ‘magic islands’. Future observations and laboratory experiments on organic aerosols would enable further constraint of the exotic atmospheric and hydrological processes on Titan.