Saturn’s moon Titan is special for its orange-ish and dim atmosphere. It is virtually impossible to see surface features because the fog in the visible part of the spectrum is so opaque; What we know about it, instead, comes from things like a radar image. Mist is the product of chemical reactions in the upper atmosphere, driven by ultraviolet radiation. This is then cascaded into larger and more complex organic (reminder: does not mean biological) molecules.
The new Horizons mission for Pluto showed that the dwarf planet also has fog. It’s less prominent in Pluto’s meteoric atmosphere, but it’s there (it’s really similar to Neptune’s moon Triton). Because Pluto’s atmosphere is no different from the upper reach of Titan’s atmosphere, it is assumed that the same chemistry is responsible.
But a new study led by Paniotis Lavas at the University of Reims Champagne-Arden shows that Pluto’s mist may need a different explanation. At both bodies, the atmosphere contains methane, carbon monoxide, and nitrogen. But if Titan’s process were to work at the same rate as Pluto, it wouldn’t produce enough mist particles to match what we measured there. As Pluto’s atmosphere is even colder than the atmosphere above Titan, it must run on mist particle chemistry. Slow down On Pluto.
So could some other process be important? To come up with the idea, the research team used sampling simulations of atmospheric chemistry, including the physics of particles settling on the surface of Pluto. The simulation reacts to the presence of ultraviolet radiation, like Titan, forming some simple organic compounds. But those chemicals remain scattered. To produce mist, you need to make particles containing these compounds, and that’s where things vary.
On Pluto, things start with hydrogen cyanide (one hydrogen, one carbon, one nitrogen), which can freeze into small ice particles in the upper atmosphere. This starts to freeze downwards due to gravity. When settled, they act as seeds, allowing other simple organic compounds present in the gas phase to solidify on their surface. In this way, they can contribute to the formation of mist particles without all the reactions to make more complex molecules like Titan.
Near the surface of Pluto, the particles settle more slowly and the temperature rises. If hydrogen cyanide ice particles were naked, the model indicates that they would probably submit li, turning back into gas. The layer of other organisms around, however, insulates and preserves them. Particle collisions also become important, forming larger particle clots. In addition to this micro-coating behavior, some other simple organisms can also freeze on their own, contributing even more particles.
The end result of the model is also the chemistry and profile of the dew particles that are more consistent with Pluto’s atmospheric parameters. Compared to Titan, this clarity is based on simple organic ice particles rather than the formation of larger and larger organic molecules.
This will actually have some consequences for the temperature in Pluto’s atmosphere. Compared to Titan’s dew particles, these ice particles should absorb less incoming solar energy and be less effective in emitting radiation back into space. The researchers say that this work will give a better estimate of the optical properties of these particle mixtures, but will require a little rethinking of the Pluto climate model.
As for Triton’s fog, they say it’s probably a more extreme version of the Pluto process. Despite the colder temperatures on that moon, the initially formed ice particles dominated, leaving a small role for the mixed micro-coating process. So these two worlds will be very different from Titan – and not just because they look like white snowballs instead of green, orange puffs in the clouds.
Nature Astronomy, 2020. DOI: 10.1038 / s41550-020-01270-3 (About DOI).
