Saturn’s moon Titan is characteristic, in part for its orange-hazy and hazy atmosphere. It is virtually impossible to see surface features because the haze is so opaque in the visible part of the spectrum; what we know about it comes rather from things like radar images. The haze is the product of chemical reactions in the upper atmosphere, powered by ultraviolet radiation. It then falls into larger and more complex organic (memory: it does not mean biological) molecules.
The New Horizons mission to Pluto showed that the dwarf planet also had a haze. It is less prominent in the sparse atmosphere of Pluto, but it is there (it is actually similar to that on Neptune’s moon Triton). Because Pluto’s atmosphere is not so different from the upper parts of Titan’s atmosphere, the same chemistry was thought to be responsible.
But a new study led by Panayotis Lavvas at the University of Reims Champagne-Ardenne shows that the fog of Pluto may need a different explanation. On both bodies the atmosphere contains methane, carbon monoxide and nitrogen. But if Titan’s process is working at the same rate on Pluto, it will not make enough haze particles that match what we measured there. Since Pluto’s atmosphere is even colder than the upper atmosphere on Titan, the chemistry of the haze particles must run slower on Pluto.
So could another process be important? To play around with this idea, the research team used model simulations of atmospheric chemistry, including the physics of particles that settle on Pluto’s surface. The simulation shows reactions to the presence of ultraviolet radiation that form some organic compounds, such as on Titan. But the chemicals remain distributed. To produce haze, you have to make particles that contain these compounds, and this is where things differ.
On Pluto, things start with hydrogen cyanide (one hydrogen, one carbon, one nitrogen), which can freeze into small ice particles in the upper atmosphere. It starts to sink downwards due to gravity. As they settle, they act as seeds, allowing other simple organic compounds to condense on their surface during the gas phase. In this way, they can contribute to the formation of haze particles without all the reactions to build more complex molecules like on Titan.
Closer to Pluto’s surface, the particles drop more slowly, raising temperatures. If the hydrogen cyanide particles were naked, the model indicates that it would probably sublimate and turn back into a gas. However, the layer of other organic matter that surrounds it isolates and preserves it. Particle collisions also become important and form larger particle clumps. In addition to this behavior of particle coatings, some of the other simple organic substances can also freeze on their own, contributing more particles.
The end result in the model is a vertical profile of chemistry and haze particles that is much more in line with the measurements of Pluto’s atmosphere. Compared to Titan, this statement is based on simple organic ice particles rather than the formation of larger and larger organic molecules.
This could actually affect the temperature in Pluto’s atmosphere. Compared to Titan’s haze particles, these ice particles must absorb less incoming solar energy and be less efficient at sending energy out into space. According to the researchers, the calculation of the optical properties of this mixture of particles will be better estimated to work it out, but it needs to be reconsidered a bit about the Pluto climate model.
As for Triton’s haze, they say it’s probably a more extreme version of the Pluto process. With even colder temperatures on that moon, the initially formed ice particles would dominate, leaving a smaller role for the mixed particle coating process. Both of these worlds would therefore differ greatly from Titan – and not just because they look like white snowballs, rather than a smooth, orange cloud.
Natural Astronomy, 2020. DOI: 10.1038 / s41550-020-01270-3 (About DOIs).