In 2014, the Cassini-Huygens mission discovered bright spots in the seas and lakes of Titan, a moon of Saturn, that moved over time. For a long time, scientists could not understand what this strange phenomenon was. It seems that now the explanation has been found: about large icebergs (though not made of water ice).
Titan is the second largest satellite in the solar system, with a diameter of about 5,150 kilometers, and a surface area larger than Eurasia and Africa combined. It orbits Saturn, which means it is so far from the Sun that it receives a hundred times less solar energy per unit area than the Earth. Therefore, its surface temperature is almost -180 degrees. Accordingly, its surface is made up of ice, and the local analog of sand dunes even includes mothballs. There are many rivers, lakes and seas on the satellite. But they are filled not with water, but with liquid ethane, propane, and methane. The total volume of hydrocarbons in these seas is several times larger than all known reserves of this kind on Earth.
Titan’s research is extremely difficult because its mostly nitrogen atmosphere is four times denser than Earth’s (the pressure on the surface is one and a half times higher than Earth’s). In such a dense and cold atmosphere, dense haze and clouds constantly prevail. It is very difficult to observe through them in the visible and some other ranges. In 2005, the Huygens lander made the first (and so far the last) soft landing outside Mars’ orbit there, seriously increasing our knowledge of Titan, but many things remain unclear. Since 2014, the Cassini spacecraft has detected some bright spots in Titan’s seas using radar, and these spots have been changing their location over time. This caused quite a surprise. The idea of floating ice was obvious, but what kind of ice was it?
The fact is that the main component of the seas of Saturn’s moon is liquid ethane with an admixture of liquid methane, with a density of about 0.6 tons per cubic meter. That is, it is a very light liquid. However, solid, mostly ethane ice does not float particularly well in such conditions – it must be heavier than the liquid surrounding it. In addition, liquid ethane and methane have a much lower surface tension than water, which makes it even more difficult for dense objects to float in it. Finally, pure ethane is formed only at temperatures below -182.
However, Titan’s atmosphere is so dense that seasonal temperature fluctuations even at the poles are extremely small. Thus, stable solid ethane is unlikely to be present there. Previously, the possibility of floating hydrocarbon ice floes in Titan conditions was justified by their high porosity: if it exceeds five percent, such ice can float. However, it remains unclear what such porous ice can consist of.
The authors of the new paper, published in Geophysical Research Letters, approached the problem from a different angle. They note that in the atmosphere of Titan, molecules much heavier than both nitrogen and methane (the two main gases of the local gas shell) must inevitably form. So, they should fall to the surface, like the snow on Earth. Only local snow will be composed largely of nitriles (compounds of carbon, hydrogen, and nitrogen), triple-bonded hydrocarbons, and benzene.
Using calculations, the researchers found that such snow, consisting of hydrogen cyanide, can subsequently form ice with high porosity of up to 25-60 percent by volume. In this case, if it is an ethane-rich sea, this ice will be able to float in it. The authors of the paper note that ice of an alternative composition will have significant problems with long-term navigation in the seas of Titan.
At the same time, they note, such porous ice is unlikely to be formed directly from hydrocarbon “snowflakes” falling into seas and lakes. They should be too small to swim long enough not to drown. It is more likely that such “snow” falls on the shores of the Titan seas and manages to form larger clumps there, and then the waves wash away the loose ice that has formed and break off large areas of it, as happens with icebergs in the Earth’s seas.
Such a scenario, scientists say, can explain another unusual feature of the seas and lakes of Saturn’s moon: the absence of large waves on them. In the zone saturated with floating ice, the formation and stable existence of large waves is seriously hampered.
It is worth noting that such works are far from theoretical. Titan is one of the most interesting places in the solar system for research. And it’s not just about its exotic hydrocarbon seas, but also about what lies beneath them. In the depths of the satellite there are large subsurface water oceans. In the future, it is wise to study them in relation to life. However, the missions required for this require heavy equipment. In 2005, the Huygens landed on a surface that seemed solid during mission planning but turned out to be close to a swamp in practice. Improved understanding of the composition of the Titan surface is essential for the success of future landings there.
