Saturn’s moon Titan is distinctive, partly due to its orange color and hazy atmosphere. It is almost impossible to see surface features because the haze is very opaque in the visible part of the spectrum; What we know about her comes from things like radar images, instead. Fog is the product of chemical reactions in the upper atmosphere, driven by ultraviolet rays. These then turn into larger, more complex organic molecules (reminder: this is not biologically relevant).
The New Horizons mission to Pluto showed that the dwarf planet also has fog. It’s less prominent in Pluto’s minuscule atmosphere, but it’s there (it’s actually similar to the atmosphere on Neptune’s moon. Triton). Since Pluto’s atmosphere is not much different from the upper reaches of Titan’s atmosphere, the same chemistry has been thought to be responsible.
But a new study led by Panayiotis Lavas At the University of Reims, Champaign Ardenne shows that Pluto fog may require a different interpretation. The atmosphere contains both methane, carbon monoxide, and nitrogen. But if the Titan process worked at the same rate on Pluto, it wouldn’t produce enough fog particles to match what we measured there. Since Pluto’s atmosphere is cooler than Titan’s upper atmosphere, the fog particle chemistry should work Slower On Pluto.
Could some other process be important? To manipulate this idea, the research team used simulations of atmospheric chemistry, including the physics of particles settling toward Pluto’s surface. Simulations show the reactions in the presence of ultraviolet rays that form some simple organic compounds, as in the case of Titan. But these chemicals are still dispersed. In order to produce haze, you need to make molecules that include these compounds, and that’s where things diverge.
On Pluto, things start with hydrogen cyanide (one hydrogen, one carbon, one nitrogen), which can solidify into tiny ice particles in the upper atmosphere. These begin to settle due to gravity. When settled, they act as seeds, allowing other simple organic compounds in the gas phase to condense on their surface. In this way, they can contribute to building fuzzy particles without all the reactions to building more complex particles as in Titan.
Near Pluto’s surface, particles settle more slowly and temperatures increase. If the hydrogen cyanide ice particles were naked, the model indicates that they would likely sublimate, and revert to a gas. However, the layer of other organic matter surrounding it insulates and preserves it. Particle collisions also become important, forming larger clumps of particles. In addition to this particle coating behavior, some other simple organic materials are also able to freeze on their own, which contributes to particulate excess.
The end result in the model is a vertical profile of the chemistry and haze particles that more closely match the measurements of Pluto’s atmosphere. Compared to Titan, this interpretation relies on simple organic ice particles rather than the formation of larger and larger organic molecules.
In fact, this will have some consequences for the temperatures in Pluto’s atmosphere. Compared to Titan’s fog particles, these ice particles should absorb less incoming solar energy and be less effective at sending energy into space. The researchers say working on this will take better estimates of the optical properties of this mixture of particles, but it will require some rethinking of Pluto’s climate model.
As for the Triton fog, they say it’s probably a more extreme version of the Pluto process. With cooler temperatures on this moon, the ice particles initially formed will dominate, leaving a smaller role for the mixed particle plating process. So these two worlds would be very different from Titan – and not just because they look like white snowballs rather than a soft orange puff of cloud.