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.