Saturn’s moon Titan is distinctive, in part because of its orange-ish and hazy atmosphere. It is almost impossible to see surface features because the haze is so opaque in the visible part of the spectrum; what we know about it comes from things like radar images instead. The haze is the product of chemical reactions in the upper atmosphere driven by ultraviolet radiation. These then fall into larger and more complex organic (remember: that doesn’t mean biological) molecules.
The New Horizons mission to Pluto showed that the dwarf planet also has a haze. It’s less prominent in Pluto’s meager atmosphere, but it’s there (it’s actually similar to the one on Neptune’s moon Triton). Since Pluto’s atmosphere is not that different from the headwaters 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 in Champagne-Ardenne shows that Pluto’s haze needs a different explanation. The atmosphere on both bodies contains methane, carbon monoxide and nitrogen. But if Titan’s process on Pluto worked at the same rate, it wouldn’t create enough haze particles to match what we measured there. Since Pluto’s atmosphere is even colder than Titan’s upper atmosphere, that nebula chemistry should spin slower on Pluto.
So could a different process be important? To play with this idea, the research team used model simulations of atmospheric chemistry, including the physics of particles settling to the surface of Pluto. The simulation shows reactions to the presence of ultraviolet radiation that forms some simple organic compounds, such as on Titan. But those chemicals continue to spread. To produce haze, you have to make particles that contain these compounds, which is where things diverge.
On Pluto, things start with hydrogen cyanide (a hydrogen, a carbon, a nitrogen), which can freeze into tiny ice particles in the upper atmosphere. These start to sink due to gravity. As they settle, they act as seeds, allowing other simple gaseous organic compounds to condense on their surface. In this way they can help build nebulae without all the reactions to build more complex molecules like on Titan.
Closer to the surface of Pluto, the particles settle more slowly and the temperatures increase. If the hydrogen cyanide ice particles were naked, the model indicates that they would likely sublimate and turn back into a gas. However, the layer of other organic substances around it insulates and preserves them. Particle collisions also become important and form larger particle clumps. In addition to this particle coating behavior, some of the other simple organics 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 consistent with the measurements of Pluto’s atmosphere. Compared to Titan, this explanation relies on simple organic ice particles rather than the formation of ever-larger organic molecules.
This would actually have some implications for the temperatures in Pluto’s atmosphere. Compared to Titan’s haze particles, these ice particles should absorb less incoming solar energy and be less effective at returning energy to space. The researchers say that working this out would require better estimates of the optical properties of this mixture of particles, but the Pluto climate model needs to be rethought.
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 ice particles initially formed would dominate, leaving a smaller role for the mixed particle coating process. So both worlds would be very different from Titan – and not just because they look like white snowballs rather than a smooth orange cloud.
Nature Astronomy, 2020. DOI: 10.1038 / s41550-020-01270-3 (About DOIs).