One of the great mysteries of Saturn’s largest moon, Titan, is the temporary smoothness of its hydrocarbon lakes in places, a state that sometimes last for days or weeks. Planetary geologists have come up with two potential explanations for this smoothness.
The first is a lack of waves. Perhaps there just isn’t enough wind on Titan to drive waves across the entire surface of its lakes. And where there are no waves, the lakes are almost flat, varying in height by no more than a few millimeters, which is why these regions appear so bright in radar images.
This seems unlikely because the weather on Titan appears to be windier than this.
Spongy Snow
The second explanation is that the lakes support “magic islands” that somehow float on the surface of the liquid. These islands must be made of solid ices that form in the atmosphere and then accumulate on the lake surface.
But this also seems unlikely because the ices on Titan are denser than the liquids that make up the lakes. So these ices should sink (unlike on Earth where water ice is less dense than liquid water and so floats).
What planetary astronomers need is some new insight that breaks this theoretical deadlock.
Now they have it thanks to the work of Xinting Yu at the University of Texas at San Antonio and colleagues, who say that ice could float on Titan, provided it forms in a special, sponge-like way.
Astronomers have long known that Titan is unique in the Solar System for its thick atmosphere of nitrogen and methane, which is about 60 percent thicker than Earth’s, and the lakes on its surface.
One of the interesting features of methane is that it is easily broken down by sunlight. Astronomers believe this breakdown leads to complex photochemistry in Titan’s atmosphere, where the products react with each other and with nitrogen.
So Yu and co begin by listing all the simple common ices likely to form in this way. They come up with ten solids ranging from larger alkanes and alkenes, like propane and ethylene, to nitriles like hydrogen cyanide and so on.
Most of these substances form as solids, which must then rain down on the surface in the form of a kind of photochemical smog. On dry surfaces, they build up slowly over time creating layers that astronomers can use to date the age of craters.
The question that Yu and co investigate is what happens to these ices when they land on liquid.
The team first calculate the density of these ices and compare this to the density of the liquid lakes. Sure enough, all the solid ices are denser than the liquid lakes and so ought to sink to the bottom, forming sedimentary layers.
The team then investigate mechanisms that could keep the ice afloat. One possibility is that ices on Titan don’t form as solid blocks but more like snowflakes on Earth. That’s significant because the porosity of snowflakes makes their density much lower than the ice by itself, in same case up to 90 percent less dense.
If Titan’s ices form as snowflakes, or some other sponge-like structure, then their density could be less than the liquid. And in that case, they would float.
But for how long? To match the observational evidence, these sponge-like structures would have to float for hours and possibly weeks.
That’s problematic. When porous substances land on a liquid, the pores fill with fluid. Yu and co calculate that this ought to happen in a matter of seconds for the kinds of ices that form on Titan. And if so, they can’t possibly form magic islands that float for days or weeks.
But there is another possibility. Perhaps the sponge-like ices form with closed pores rather than opens ones. In that case, the pores would fill more slowly and the ice could float for much longer.
Rock Rafts
That’s not as unlikely as it sounds. On Earth, a porous volcanic rock known as pumice often forms into ocean rafts that float for weeks or months. The rock is much denser than water but the closed system of pores keep it afloat. “In this case, the physical process that dictates the flotation timescale of pumice is gas diffusion instead of water infiltration,” say the team.
Something similar may be happening on Titan. If so, then the ices must form into relatively large sponge-like structures — at least millimeters in size — with enough porosity to make this possible. But how this might occur is not known.
Nevertheless, Yu and co think it the most likely explanation for Titan’s mysterious radar reflections. “Porosity-induced flotation of millimeter-sized and larger particles is the only plausible mechanism for floating solids to explain Titan’s transient radar-bright magic islands,” they say.
That’s interesting work that provides some insight into the complexity of Titan’s environment. There’s clearly much more to learn here. Yu and co’s work is purely theoretical and the results will need to be confirmed and reproduced in a lab. Creating Titan-like snow will be an exciting goal.
Then there is the possibility of gathering more detail from the moon itself.
NASA is currently working on a mission called Dragonfly--an advanced helicopter packed with sensors that will explore Titan. Dragonfly is currently scheduled to arrive in 2034. It will surely be worth the wait and may help confirm — or refute — Yu and co’s ideas about Titan’s sponge-like snow and magic islands.
Ref: The Fate of Simple Organics on Titan's Surface: A Theoretical Perspective : arxiv.org/abs/2401.02640
