Given its location, we always knew Pluto would be cold. However, when New Horizons sped past it in 2015, it measured the dwarf planet's temperature as even lower than expected. This has now been attributed to haze in Pluto's thin atmosphere, a theory we should soon be able to test, with possible implications for our understanding of conditions on Earth.
Considering the tiny amount of sunlight it receives, planetary scientists anticipated Pluto would have an average temperature of around -173º C (-280º F). Yet New Horizons measured an even chillier -203ºC (-333ºF).
"Pluto is the first planetary body we know of where the atmospheric energy budget is dominated by solid-phase haze particles instead of by gases," said Dr Xi Zhang of the University of California, Santa Cruz, in a statement.
The haze exists because enough ultraviolet radiation reaches Pluto to ionize nitrogen and methane in its upper atmosphere. They then react forming hydrocarbon particles. Initially tiny, these clump together as they fall through the atmosphere, growing in size and therefore changing how they interact with longer wavelength radiation.
In Nature, Zhang's team has modeled how these particles absorb the light they receive from the distant Sun. Pluto's weak gravity means the particles fall fairly slowly, leaving time for heat to radiate away, cooling the atmosphere.
If Zhang is right, Pluto's upper atmosphere should radiate much more in the infrared spectrum than previous models predict. This radiation wouldn't be visible to existing telescopes, particularly since their measurements of Pluto at these wavelengths are mixed with radiation from Charon. However, the James Webb Space Telescope, due to launch in early 2019, should be sensitive enough to test Zhang's work.
Zhang's theory contrasts with initial explanations that attribute New Horizon's observations to water vapor or other gases in Pluto's atmosphere. Although water vapor is a greenhouse gas on Earth, atmospheric scientists proposed that under Pluto's conditions, it could have a cooling effect. However, Zhang points out that to explain the observed temperatures, there would need to be much more water vapor on Pluto than we think is present.
It's very difficult to understand something when you only have a single example to study, so climate science has benefited greatly from observations of the atmospheres of Mars and Venus. Although the exact lessons are yet to emerge, a better understanding of how particles affect Pluto's temperatures may eventually influence studies of our own planet. Most relevantly, it may tell us more about how the atmosphere will respond if we try to cool it by deliberately injecting particles into the upper atmosphere.