Saturn’s moon Titan is unique, partly because of its orange and hazy atmosphere. The surface features are virtually invisible because the haze is so opaque in the visible part of the spectrum. What we know comes from things like radar images. Haze is the product of chemical reactions in the upper atmosphere driven by ultraviolet radiation. These molecules then cascade into larger, more complex organic (reminder: this does not mean biology) molecules.
The New Horizons mission to Pluto shows that the dwarf planets are also misty. In Pluto’s thin atmosphere, it is not so prominent, but it does exist (actually similar to the one on Neptune’s moon Triton). Because Pluto’s atmosphere is no different from the upper reaches of Titan’s atmosphere, people have always believed that the same chemical reactions are responsible.
However, a new study led by Panayotis Lavvas of the University of Reims-Champaign-Arden suggests that the haze of Pluto may require different explanations. On both objects, the atmosphere contains methane, carbon monoxide, and nitrogen. However, if Titan̵
So some other process might be important? To solve this idea, the research team used atmospheric chemistry model simulations, including the physics of particles sinking to the surface of Pluto. Simulations show that when the reaction occurs, ultraviolet radiation will form some simple organic compounds, such as on Titan. But those chemicals are still dispersed. In order to create haze, you need to make particles that incorporate these compounds, and this is where things diverge.
On Pluto, things start with hydrogen cyanide (one hydrogen, one carbon, one nitrogen), which can freeze into tiny ice particles in the upper atmosphere. These began to sink downward due to gravity. When they settle, they act as seeds, allowing other simple organic compounds in the gas phase to condense on their surface. In this way, they can help generate haze particles without having to perform all the reactions to generate more complex molecules like on Titan.
Near the surface of Pluto, particles settle more slowly and the temperature rises. If the hydrogen cyanide ice particles are exposed, the model suggests that they may sublime and become gas. However, other organic layers surrounding them insulate and preserve them. Particle collisions also become important, forming larger clumps of particles. In addition to this particle coating behavior, other simple organic matter can also freeze on its own, thus contributing more particles.
The final result of this model is the vertical distribution of chemical and haze particles, which is more consistent with the measurements of Pluto’s atmosphere. Compared with Titan, this explanation relies on simple organic ice particles rather than the formation of larger and larger organic molecules.
This will actually have some effect on the temperature of Pluto’s atmosphere. Compared with Titan’s haze particles, these ice particles absorb less incident solar energy and are less efficient at releasing energy back into space. Researchers say that solving this problem can better estimate the optical properties of this particle mixture, but it requires some rethinking of the Pluto climate model.
As for Triton’s haze, they said it may be a more extreme version of the Pluto process. When the temperature of the moon is even lower, the initially formed ice particles will dominate, and have less effect in the coating process of the mixed particles. Therefore, both worlds are very different from Titans-not only because they look like white snowballs, rather than smooth orange puffs.
Natural Astronomy, 2020. DOI: 10.1038 / s41550-020-01270-3 (about DOI).