Unique Cooling Effect: Data from the NASA's New Horizons probe provides researchers with a possible explanation for Pluto's unusually cold atmosphere. Particles in the atmospheric vapors seem to provide a unique cooling effect by releasing heat into space in the form of infrared radiation.
Image of the atmospheric veil of Pluto by the probe New Horizons. © NASA / JHUAPL / SwRI
People used to think Pluto was a distant, dead ice world. Recently, however, the planet has turned out to be more a dynamic celestial body with bizarre landscapes and possibly active ice volcanoes. Likewise, the dwarf planet has an unusually extensive atmosphere and a blue sky. Similar to the earth.
In 2015, measurements of Pluto's atmosphere were taken for the first time by NASA's New Horizons, and it was found to be about 30 degrees Celsius colder than expected. This was a problem insofar as the gas compositions of a planet's atmosphere usually determine how much heat the planet can hold. Apparently not with Pluto: In that sense, the actual measurements did not match the predictions of the scientists at all.
Atmospheric temperature and gas-density profiles of Pluto. © University of California - Santa Cruz
A possible explanation has now been found by scientists led by Xi Zhang of the University of California at Santa Cruz. Their analysis shows that particles in the haze - a result of chemical reactions in the atmosphere - provide a unique cooling effect.
The ultraviolet radiation of the sun ionizes with nitrogen and methane, whereby hydrocarbon particles form, which sink down and thereby generate ever larger aggregates. Using model simulations, scientists have now been able to show how this phenomenon could affect the temperature of the atmosphere.
Radiative heating and cooling in the atmosphere of Pluto. © University of California - Santa
The result: Compared to the gas molecules, the haze particles absorb more sunlight, which would actually heat up the atmosphere. However, the models of Zhang's team show that the hydrocarbon particles emit much more heat, in the form of infrared radiation, into space.
This results in a cold atmosphere about minus 203 degrees Celsius and not just minus 173, as originally thought.
Pluto is the first known planetary celestial body in our solar system, where the energy balance of the atmosphere is primarily regulated by solids in the form of vapor particles rather than gases, Zhang says.
Even though the model calculations are surprisingly consistent with the measured temperature profile, there's a snag to it. According to Rober West of the California Institute of Technology in Pasadena, the calculated heating and cooling rates are associated with great uncertainties.
We do not know how exactly the haze over Pluto is composed.
West emphasizes that the researchers have adopted a particle composition that behaves in their model so that it can explain the observation. Whether this corresponds to reality, could not yet be verified.
NASA's new James Webb Space Telescope is scheduled to begin service by the end of 2018 and could show whether Zhang's team is right. If the scientists' models are correct, the cooling effect would have to produce detectable infrared emissions, which would confirm the theory.
Unfortunately, today's instruments are too weak to make those measurements.
The James Webb Space Telescope should be sensitive enough to either back up or disprove the model, says West.
If the researchers' hypothesis is confirmed, this could be helpful for the future exploration of hazy atmosphere exoplanets.
Source: Nature, 2017 doi:10.1038
