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Haze heats Pluto’s atmosphere yet explains its cold temperature

Abstract

Pluto’s atmosphere is cold and hazy1,2,3. Recent observations1 have shown it to be much colder than predicted theoretically4, suggesting an unknown cooling mechanism1. Atmospheric gas molecules, particularly water vapour, have been proposed as a coolant; however, because Pluto’s thermal structure is expected to be in radiative–conductive equilibrium4,5,6,7,8,9, the required water vapour would need to be supersaturated by many orders of magnitude under thermodynamic equilibrium conditions9. Here we report that atmospheric hazes, rather than gases, can explain Pluto’s temperature profile. We find that haze particles have substantially larger solar heating and thermal cooling rates than gas molecules, dominating the atmospheric radiative balance from the ground to an altitude of 700 kilometres, above which heat conduction maintains an isothermal atmosphere. We conclude that Pluto’s atmosphere is unique among Solar System planetary atmospheres, as its radiative energy equilibrium is controlled primarily by haze particles instead of gas molecules. We predict that Pluto is therefore several orders of magnitude brighter at mid-infrared wavelengths than previously thought—a brightness that could be detected by future telescopes.

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Figure 1: Atmospheric temperature and gas-density profiles of Pluto.
Figure 2: Radiative heating and cooling in the atmosphere of Pluto.
Figure 3: Heat-transfer timescales in the atmosphere of Pluto.
Figure 4: Infrared spectra of Pluto with flux values at Earth’s distance, with and without the effects of haze radiation from Pluto’s atmosphere.

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Acknowledgements

We thank Y. Yung, B. Bézard, E. Lellouch, M. Liang, X. Zhu and P. Gao for discussions. X.Z. acknowledges partial support from NASA Solar System Workings grant NNX16AG08G. D.F.S. acknowledges partial support from NASA’s New Horizons Mission. H.I. acknowledges support from NASA Cassini Data Analysis grant NNX14AF61G and NASA Exoplanet Research grant NNX15AQ73G.

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Authors

Contributions

X.Z. conceived the research, performed the calculations and wrote the manuscript. D.F.S. provided the New Horizons data and the gas-only model and assisted with analysis. H.I. provided the refractive indices of haze particles. D.F.S. and H.I. contributed to manuscript writing.

Corresponding author

Correspondence to Xi Zhang.

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The authors declare no competing financial interests.

Additional information

Reviewer Information Nature thanks R. West and Y. Yung for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Figure 1 Properties of haze in Pluto’s atmosphere as a function of altitude.

Vertical distributions of: a, the mean particle size; b, the observed haze attenuation factor at the Alice far-ultraviolet (FUV) channel; c, particle number density; and d, scaling factors applied to the haze k values in our balanced model.

Extended Data Figure 2 Optical properties of haze in Pluto’s atmosphere as a function of wavelength.

a, Total vertical optical depths of haze for two cases: if the particles in the near-surface layers are Mie spheres, or if they are fractal aggregates. b, Extinction efficiencies for a 10-nm monomer particle and an aggregated particle composed of 10-nm monomers with a volume-equivalent radius of 0.15 μm (ref. 3). Extinction efficiency for the aggregates is defined as the calculated total extinction cross-section divided by the cross-section of equal-volume spheres22. c, Single scattering albedos as a function of wavelength. The contribution of the spectral region from 1 μm to 7 μm (grey lines) to the haze heating rate is less than 1% and is omitted here.

Extended Data Figure 3 Reflectivity I/F values as a function of phase angle.

LORRI observations at an altitude of 45 km (black) and near the surface (blue) are shown with squares3. Simulated phase functions from multiple solutions of aggregates (composed of 10-nm monomers, 30-nm monomers or 50-nm monomers) are scaled to fit the observations.

Extended Data Figure 4 Sensitivity study of haze heating and cooling rates.

a, b, Modelled heating rates. c, d, Modelled cooling rates. Solid lines represent the results obtained if the near-surface haze layers are composed of Mie-sphere particles with radii of 0.5 μm. Dashed lines show the results obtained if the near-surface particles are instead aggregates identical to those at 45 km (consistent with Extended Data Fig. 3). b, d, Zoomed-in versions of a and c, respectively, for the lower 100-km region.

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Zhang, X., Strobel, D. & Imanaka, H. Haze heats Pluto’s atmosphere yet explains its cold temperature. Nature 551, 352–355 (2017). https://doi.org/10.1038/nature24465

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