Saturn’s moon Titan is distinctive, in part because of its orangish 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 instead comes from things like radar images. 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 means not biological) molecules.
The New Horizons mission to Pluto showed that the dwarf planet also has a haze. It’s less prominent in Pluto’s lean atmosphere, but it’s there (it’s actually similar to the one on Neptune’s moon Triton). Because Pluto’s atmosphere isn’t that different from Titan’s upper reaches, 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 Pluto’s haze may require a different explanation. On both bodies, the atmosphere contains methane, carbon monoxide and nitrogen. But if Titan’s process on Pluto operated at the same speed, it wouldn’t create enough haze particles to match what we measured there. Since Pluto’s atmosphere is even colder than the upper atmosphere on Titan, that haze particle chemistry should work slower on Pluto.
So could another process be important? To play 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 in the presence of ultraviolet radiation, forming some simple organic compounds, such as on Titan. But those chemicals remain scattered. To produce haze, you have to create particles that incorporate these compounds, and that’s where things diverge.
On Pluto, it starts with hydrogen cyanide (one hydrogen, one carbon, one nitrogen), which can freeze into tiny ice particles in the upper atmosphere. These begin to sink down due to gravity. As they settle, they act as seeds, allowing other simple gas-phase organic compounds to condense on their surface. In this way, they can contribute to building haze particles without all the reactions to build more complex molecules like on Titan.
Closer to Pluto’s surface, the particles settle more slowly and temperatures rise. If the hydrogen cyanide ice particles were naked, the model indicates they would likely sublimate and turn back into a gas. However, the layer of other organics that surround them 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 are also capable of freezing on their own, adding 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 is based on simple organic ice particles rather than the formation of progressively larger organic molecules.
This would actually have some implications for the temperatures in Pluto’s atmosphere. Compared to Titan’s nebula particles, these ice particles should absorb less incoming solar energy and be less effective at returning energy to space. The researchers say that to work this out will require better estimates of the optical properties of this mixture of particles, but it will require some rethinking of 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 coating process. So these two worlds would be very different from Titan – and not just because they look like white snowballs instead of a smooth, orange cloud cloud.
Natural Astronomy, 2020. DOI: 10.1038/s41550-020-01270-3 (About DOIs).