Pluto's atmosphere is fairly hazy.
Enlarge / Pluto’s atmosphere is fairly hazy.

Saturn’s moon Titan is distinctive, in part for its orange-ish and hazy atmosphere. It’s virtually impossible to see surface features because the haze is so opaque in the visible portion of the spectrum; what we know of it comes from things like radar imagery, instead. The haze is the product of chemical reactions in the upper atmosphere, driven by ultraviolet radiation. These then cascade into larger and more complex organic (reminder: that doesn’t mean biological) molecules.

The New Horizons mission to Pluto showed that the dwarf planet, too, has a haze. It’s less prominent in Pluto’s meager atmosphere, but it is there (it’s actually similar to the one on Neptune’s moon Triton). Because Pluto’s atmosphere isn’t that different from the upper reaches of Titan’s atmosphere, it has been thought that the same chemistry is 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 worked at the same rate on Pluto, it wouldn’t make enough haze particles to match what we’ve measured there. As Pluto’s atmosphere is even colder than the upper atmosphere on Titan, that haze particle chemistry should be running slower on Pluto.

So could some other process be important? To play around with this idea, the research team used model simulations of atmospheric chemistry, including the physics of particle settling toward Pluto’s surface. The simulation shows reactions the presence of ultraviolet radiation forming some simple organic compounds, as on Titan. But those chemicals remain dispersed. In order to produce haze, you need to make particles incorporating these compounds, and that’s 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 start to settle downward thanks to gravity. As they settle, they act as seeds, allowing other simple organic compounds in the gas phase to condense onto their surface. In this way, they can contribute to building haze particles without all the reactions to build more complex molecules as on Titan.

Nearer Pluto’s surface, the particles settle more slowly and the temperatures increase. If the hydrogen cyanide ice particles were naked, the model indicates they would likely sublimate, turning back into a gas. The layer of other organics surrounding them, however, insulates and preserves them. Particle collisions also become important, forming larger particle clumps. In addition to this particle-coating behavior, some of the other simple organics are also able to 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 larger and larger organic molecules.

This would actually have some consequences for 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 emitting energy back to space. The researchers say that working this out would take better estimates of the optical properties of this mixture of particles, but would require some rethinking of the Pluto climate model.

As for Triton’s haze, they say it’s likely 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. So both these worlds would differ strongly from Titan—and not just because they look like white snowballs rather than a smooth, orange puff of cloud.

Nature Astronomy, 2020. DOI: 10.1038/s41550-020-01270-3 (About DOIs).