Letter

Haze heats Pluto’s atmosphere yet explains its cold temperature

Received:
Accepted:
Published online:

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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References

  1. 1.

    et al. The atmosphere of Pluto as observed by New Horizons. Science 351, aad8866 (2016)

  2. 2.

    et al. Structure and composition of Pluto’s atmosphere from the New Horizons solar ultraviolet occultation. Icarus 300, 174–199 (2017)

  3. 3.

    et al. Haze in Pluto’s atmosphere. Icarus 290, 112–133 (2017)

  4. 4.

    et al. The density and thermal structure of Pluto’s atmosphere and associated escape processes and rates. Icarus 228, 301–314 (2014)

  5. 5.

    & Evidence for a molecule heavier than methane in the atmosphere of Pluto. Nature 339, 288–290 (1989)

  6. 6.

    Nonisothermal Pluto atmosphere models. Icarus 84, 1–11 (1990)

  7. 7.

    Pluto’s atmospheric structure: clear vs hazy models. Icarus 108, 255–264 (1994)

  8. 8.

    et al. On the vertical thermal structure of Pluto’s atmosphere. Icarus 120, 266–289 (1996)

  9. 9.

    & Comparative planetary nitrogen atmospheres: density and thermal structures of Pluto and Triton. Icarus 291, 55–64 (2017)

  10. 10.

    Heat balance in Titan’s atmosphere. Planet. Space Sci. 56, 648–659 (2008)

  11. 11.

    et al. Aerosol influence on energy balance of the middle atmosphere of Jupiter. Nat. Commun. 6, 10231 (2015)

  12. 12.

    et al. Aerosol growth in Titan’s ionosphere. Proc. Natl Acad. Sci. USA 110, 2729–2734 (2013)

  13. 13.

    et al. Constraints on the microphysics of Pluto’s photochemical haze from New Horizons observations. Icarus 287, 116–123 (2017)

  14. 14.

    & 3D modeling of organic haze in Pluto’s atmosphere. Icarus 287, 72–86 (2017)

  15. 15.

    et al. Laboratory experiments of Titan tholin formed in cold plasma at various pressures: implications for nitrogen-containing polycyclic aromatic compounds in Titan haze. Icarus 168, 344–366 (2004)

  16. 16.

    et al. in Titan: Surface, Atmosphere and Magnetosphere (eds et al.) 285–321 (Cambridge Univ. Press, 2014)

  17. 17.

    et al. Optical constants of organic tholins produced in a simulated titanian atmosphere: from soft X-ray to microwave frequencies. Icarus 60, 127–137 (1984)

  18. 18.

    et al. The detached haze layer in Titan’s mesosphere. Icarus 201, 626–633 (2009)

  19. 19.

    et al. Complex refractive index of Titan’s aerosol analogues in the 200–900 nm domain. Icarus 156, 515–529 (2002)

  20. 20.

    et al. Optical constants of Titan’s stratospheric aerosols in the 70–1500 cm−1 spectral range constrained by Cassini/CIRS observations. Icarus 219, 5–12 (2012)

  21. 21.

    et al. Global albedos of Pluto and Charon from LORRI New Horizons observations. Icarus 287, 207–217 (2017)

  22. 22.

    et al. A model of Titan’s aerosols based on measurements made inside the atmosphere. Planet. Space Sci. 56, 669–707 (2008)

  23. 23.

    et al. The photochemistry of Pluto’s atmosphere as illuminated by New Horizons. Icarus 287, 110–115 (2017)

  24. 24.

    & Titan’s aerosol and stratospheric ice opacities between 18 and 500 μm: vertical and spectral characteristics from Cassini CIRS. Icarus 212, 762–778 (2011)

  25. 25.

    , & The thermal structure of Titan’s atmosphere. Icarus 80, 23–53 (1989)

  26. 26.

    & CH4 and haze in Triton’s lower atmosphere. J. Geophys. Res. Space Phys. 96, 19241–19252 (1991)

  27. 27.

    et al. Thermal properties of Pluto’s and Charon’s surfaces from Spitzer observations. Icarus 214, 701–716 (2011)

  28. 28.

    et al. The Mid-Infrared Instrument for the James Webb Space Telescope, IX: predicted sensitivity. Publ. Astron. Soc. Pacif. 127, 686–695 (2015)

  29. 29.

    et al. Detection of CO and HCN in Pluto’s atmosphere with ALMA. Icarus 286, 289–307 (2017)

  30. 30.

    et al. Infrared spectra and optical constants of nitrile ices relevant to Titan’s atmosphere. Astrophys. J. Suppl. Ser. 191, 96–112 (2010)

  31. 31.

    N2 escape rates from Pluto’s atmosphere. Icarus 193, 612–619 (2008)

  32. 32.

    et al. Radio occultation measurements of Pluto’s neutral atmosphere with New Horizons. Icarus 290, 96–111 (2017)

  33. 33.

    et al. Radiative forcing of the stratosphere of Jupiter, Part I: atmospheric cooling rates from Voyager to Cassini. Planet. Space Sci. 88, 3–25 (2013)

  34. 34.

    et al. The HITRAN2012 molecular spectroscopic database. J. Quant. Spectrosc. Radiat. Transf. 130, 4–50 (2013)

  35. 35.

    , & Collisional deactivation of laser-excited acetylene by H2, HBr, N2 and CO. J. Chem. Soc. Faraday Trans. 2. 77, 469–476 (1981)

  36. 36.

    , , & Surface chemistry and particle shape: processes for the evolution of aerosols in Titan’s atmosphere. Astrophys. J. 728, 80 (2011)

  37. 37.

    ., ., & Stratospheric aerosols on Jupiter from Cassini observations. Icarus 226, 159–171 (2013); erratum 266, 433–434 (2016)

  38. 38.

    , & Titan’s vertical aerosol structure at the Huygens landing site: constraints on particle size, density, charge, and refractive index. Icarus 210, 832–842 (2010)

  39. 39.

    & Energy transfer between an aerosol particle and gas at high temperature ratios in the Knudsen transition regime. Int. J. Heat Mass Transfer 43, 127–138 (2000)

  40. 40.

    & The gas-grain interaction in the interstellar medium—thermal accommodation and trapping. Astrophys. J. 265, 223–234 (1983)

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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.

Author information

Affiliations

  1. Department of Earth and Planetary Sciences, University of California Santa Cruz, Santa Cruz, California 95064, USA

    • Xi Zhang
  2. Department of Earth & Planetary Sciences and Physics & Astronomy, Johns Hopkins University, Baltimore, Maryland 21218, USA

    • Darrell F. Strobel
  3. SETI Institute, 189 North Bernardo Avenue, Suite 100, Mountain View, California 94043, USA

    • Hiroshi Imanaka
  4. NASA Ames Research Center, Moffett Field, California 94035, USA

    • Hiroshi Imanaka

Authors

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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.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Xi Zhang.

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

Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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