Seas and lakes of liquid hydrocarbons are known to exist on the surface of Titan beneath a thick atmosphere of nitrogen and methane. It is a common assumption that ice on a liquid hydrocarbon lake is negatively buoyant, causing any ice which forms on the lake’s surface to sink towards the bottom. This is because methane ice is denser than liquid methane and this results in a behaviour that is opposite of water where water ice is less dense than liquid water. A paper by Roe and Grundy (2012) titled “Buoyancy of ice in the CH4-N2 system” suggests that contrary to common assumption, the conditions that exist on Titan can allow ice to float on a liquid hydrocarbon lake.
Cassini delivers this stunning vista showing small, battered Epimetheus and smog-enshrouded Titan, with Saturn's A and F rings stretching across the scene. Credit: NASA/JPL/Space Science Institute
Given that nitrogen is the primary constituent of Titan’s atmosphere and nitrogen is soluble in a hydrocarbon mixture, a substantial presence of dissolved nitrogen is expected in the hydrocarbon lakes on Titan. In this study, heavier hydrocarbons are ignored and a liquid solution with a mole fraction abundance of 30.7 percent nitrogen and 69.3 percent methane at a temperature of 88.2 degrees Kelvin is used. In order for ice to start forming, the liquid solution has to be cooled to 78.1 degrees Kelvin where the ice that is formed will have a mole fraction abundance of 16.5 percent nitrogen and 83.5 percent methane. At 78.1 degrees Kelvin, the liquid solution has a density of 0.574 grams per cubic centimetre.
Two methods are used to estimate the density of the ice that is formed. The first method assumes an ideal solution where the weighted mean of the densities of nitrogen and methane are summed up to give a density of 0.564 grams per cubic centimetre. The second method uses a lattice replacement assumption where the lattice structure of the methane ice remains unchanged while a nitrogen molecule replaces a methane molecule and this gives a density of 0.549 grams per cubic centimetre. In either case, the density of the ice is less than the density of the liquid solution and the ice will float. Hence, the decrease in density from the decrease in nitrogen abundance of the ice is larger than the increase in density from the freezing of the liquid solution.
A liquid solution of nitrogen and methane is a simplification since Titan’s lakes contain a mixture of other hydrocarbons and a more accurate study will need to consider these other hydrocarbons. The seasonal variation in surface temperature on Titan is unlikely to be sufficient to allow ice to form on Titan’s lakes. However, it is still possible for ice to be present on the surface of a lake on Titan after a hail storm or a torrential downpour of methane. A hail storm that is large enough can even provide sufficient cooling to freeze the surface of the lake. Raindrops of methane arriving at the surface of a lake from a torrential downpour are expected to be cooler than the local atmosphere and contain some amount of dissolved nitrogen. As the lake consists of a mixture of heavier hydrocarbons, this newly precipitated liquid solution of nitrogen and methane is expected to be less dense and will form a floating layer on the lake’s surface. The volume of this layer can be greatly boosted by drainage from surrounding terrain. Cooling delivered by winds can cause ice to form on this cooler and less dense surface layer of the lake.
When ice floats on the surface of a lake, it isolates the rest of the lake from the atmosphere and allows it to remain liquid beneath. Floating ice also affects the rate of evaporation of methane back to the atmosphere and this can have huge effects on the climate and atmosphere of Titan if lake evaporation is a significant contributor to the atmospheric abundance of methane.