Pluto, Titan and Triton all have low-temperature environments with an N2, CH4 and CO atmospheric composition in which solar radiation drives an intense organic photochemistry. Titan is rich in atmospheric hazes, and Cassini–Huygens observations showed that their formation is initiated with the production of large molecules through ion-neutral reactions. New Horizons revealed that optical hazes are also ubiquitous in Pluto’s atmosphere, and it is thought that similar haze formation pathways are active in this atmosphere as well. However, we show here that Pluto’s hazes may contain a major organic ice component (dominated by C4H2 ice) from the direct condensation of the primary photochemical products in this atmosphere. This contribution may imply that haze has a less important role in controlling Pluto’s atmospheric thermal balance compared to Titan. Moreover, we expect that the haze composition of Triton is dominated by C2H4 ice.
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The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.
The codes used in this study are described in detail in previous relevant publications (see references). They are not publicly available owing to their undocumented intricacies.
Gladstone, G. R. et al. The atmosphere of Pluto as observed by New Horizons. Science 351, 1280–1286 (2016).
Vuitton, V., Yelle, R. V., Klippenstein, S. J., Horst, S. M. & Lavvas, P. Simulating the density of organic species in the atmosphere of Titan with a coupled ion-neutral photochemical model. Icarus 324, 120–197 (2019).
Wong, M. L. et al. The photochemistry of Pluto’s atmosphere as illuminated by New Horizons. Icarus 287, 110–115 (2017).
Waite, J. H. Jr. et al. The process of tholin formation in Titan’s upper atmosphere. Science 316, 870–875 (2007).
Lavvas, P., Yelle, R. V. & Griffith, C. A. Titan’s vertical aerosol structure at the Huygens landing site: constraints on particle size, density, charge, and refractive index. Icarus 210, 832–842 (2010).
Lavvas, P. et al. Energy deposition and primary chemical products in Titan’s upper atmosphere. Icarus 213, 233–251 (2011).
Gao, P. et al. Constraints on the microphysics of Pluto’s photochemical haze from New Horizons observations. Icarus 287, 116–123 (2017).
Lellouch, E. et al. Detection of CO and HCN in Pluto’s atmosphere with ALMA. Icarus 286, 289–307 (2017).
Määttänen, A. et al. Nucleation studies in the Martian atmosphere. J. Geophys. Res. 110, E02002 (2005).
Rannou, P. & West, R. Supersaturation on Pluto and elsewhere. Icarus 312, 36–44 (2018).
Lavvas, P. et al. Aerosol growth in Titan’s ionosphere. Proc. Natl Acad. Sci. USA 110, 2729–2734 (2013).
Anderson, C. M. & Samuelson, R. E. Titan’s aerosol and stratospheric ice opacities between 18 and 500 μm: vertical and spectral characteristics from Cassini CIRS. Icarus 212, 762–778 (2011).
Cheng, A. F. et al. Haze in Pluto’s atmosphere. Icarus 290, 112–133 (2017).
Young, L. A. et al. Structure and composition of Pluto’s atmosphere from the New Horizons solar ultraviolet occultation. Icarus 300, 174–199 (2018).
Stern, S. A. et al. Evidence for possible clouds in Pluto’s present-day atmosphere. Astron. J. 154, 43 (2017).
Luspay-Kuti, A. et al. Photochemistry on Pluto – I. Hydrocarbons and aerosols. Mon. Not. R. Astron. Soc. 472, 104–117 (2017).
Behmard, A. et al. Desorption kinetics and binding energies of small hydrocarbons. Astrophys. J. 875, 73 (2019).
Lavvas, P., Sander, M., Kraft, M. & Imanaka, H. Surface chemistry and particle shape: processes for the evolution of aerosols in Titan’s atmosphere. Astrophys. J. 728, 80 (2011).
Vaden, T. D., Imre, D., Beránek, J., Shrivastava, M. & Zelenyuk, A. Evaporation kinetics and phase of laboratory and ambient secondary organic aerosol. Proc. Natl Acad. Sci. USA 108, 2190–2195 (2011).
Steffl, A. et al. Pluto’s ultraviolet spectrum, airglow emissions, and surface reflectance. EPSC Abstracts 13, EPSC-DPS2019-1213-1 (2019).
Haynes, W. M. (ed.) The CRC Handbook of Chemistry and Physics 94th edn, Ch. 3 (Taylor & Francis Group, 2013).
Khanna, R. K., Ospina, M. J. & Zhao, G. Infrared band extinctions and complex refractive indices of crystalline C2H2 and C4H2. Icarus 73, 527–535 (1988).
Grundy, W. M. et al. Pluto’s haze as a surface material. Icarus 314, 232–245 (2018).
Bertrand, T. & Forget, F. 3D modeling of organic haze in Pluto’s atmosphere. Icarus 287, 72–86 (2017).
Protopapa, S. et al. Pluto’s global surface composition through pixel-by-pixel Hapke modeling of New Horizons Ralph/LEISA data. Icarus 287, 218–228 (2017).
Khare, B. N. et al. Optical constants of organic tholins produced in a simulated Titanian atmosphere: from soft x-rays to microwave frequencies. Icarus 60, 127–137 (1984).
Horst, S. M. Titan’s atmosphere and climate. J. Geophys. Res. E 122, 432–482 (2017).
Wahlund, J.-E. et al. On the amount of heavy molecular ions in Titan’s ionosphere. Planet. Space Sci. 57, 1857–1865 (2009).
Gudipati, M. S., Jacovi, R., Couturier-Tamburelli, I., Lignell, A. & Allen, M. Photochemical activity of Titan’s low-altitude condensed haze. Nat. Commun. 4, 1648 (2013).
Strazzulla, G. & Johnson, R. E. in Comets in the Post-Halley Era Vol. I (eds Newburn, R. L. Jr et al.) 243–275 (Springer, 1991).
Zhang, X., Strobel, D. F. & Imanaka, H. Haze heats Pluto’s atmosphere yet explains its cold temperature. Nature 551, 352–355 (2017).
Schmitt, B., Quirico, E., Trotta, F. & Grundy, W. M. in Solar System Ices (eds Schmitt, B. et al.) 199–240 (Springer, 1998).
Strobel, D. F. & Zhu, X. Comparative planetary nitrogen atmospheres: density and thermal structures of Pluto and Triton. Icarus 291, 55–64 (2017).
Krasnopolsky, V. A., Sandel, B. R. & Herbert, F. Properties of haze in the atmosphere of Triton. J. Geophys. Res. E 97, 11695–11700 (1992).
Pollack, J. B., Schwartz, J. M. & Rages, K. Scatterers in Triton’s atmosphere: implications for the seasonal volatile cycle. Science 250, 440–443 (1990).
Herbert, F. & Sandel, B. R. CH4 and haze in Triton’s lower atmosphere. J. Geophys. Res. A 96, 19241–19252 (1991).
Rages, K. & Pollack, J. B. Voyager imaging of Triton’s clouds and hazes. Icarus 99, 289–301 (1992).
Zhou, L., Kaiser, R. I. & Tokunaga, A. T. Infrared spectroscopy of crystalline and amorphous diacetylene (C4H2) and implications for Titan’s atmospheric composition. Planet. Space Sci. 57, 830–835 (2009).
Anderson, C. M., Samuelson, R. E. & Nna-Mvondo, D. Organic ices in Titan’s stratosphere. Space Sci. Rev. 214, 125 (2018).
Imanaka, H., Cruikshank, D. P. & McKay, C. P. Photochemical hazes in planetary atmospheres: solar system bodies and beyond. In Proc. 47th Meeting of the Division for Planetary Sciences 416.18 (American Astronomical Society, 2015).
Hörst, S. M. et al. Haze production rates in super-Earth and mini-Neptune atmosphere experiments. Nat. Astron. 2, 303–306 (2018).
He, C. et al. Photochemical haze formation in the atmospheres of super-Earths and mini-Neptunes. Astron. J. 156, 38 (2018).
He, C. et al. Carbon monoxide affecting planetary atmospheric chemistry. Astrophys. J. Lett. 841, L31 (2017).
Hörst, S. M. & Tolbert, M. A. The effect of carbon monoxide on planetary haze formation. Astrophys. J. 781, 53 (2014).
Lavvas, P. P., Coustenis, A. & Vardavas, I. M. Coupling photochemistry with haze formation in Titan’s atmosphere, part II: results and validation with Cassini/Huygens data. Planet. Space Sci. 56, 67–99 (2008).
Douglas, K. et al. Low temperature studies of the removal reactions of 1CH2 with particular relevance to the atmosphere of Titan. Icarus 303, 10–21 (2018).
Douglas, K. et al. Low temperature studies of the rate coefficients and branching ratios of reactive loss vs quenching for the reactions of 1CH2 with C2H6, C2H4, C2H2. Icarus 321, 752–766 (2019).
Lavvas, P., Griffith, C. A. & Yelle, R. V. Condensation in Titan’s atmosphere at the Huygens landing site. Icarus 215, 732–750 (2011).
Gladstone, G. R., Pryor, W. R. & Stern, S. A. Lyα@Pluto. Icarus 246, 279–284 (2015).
Hinson, D. P. et al. An upper limit on Pluto’s ionosphere from radio occultation measurements with New Horizons. Icarus 307, 17–24 (2018).
Strobel, D. F. & Summers, M. E. in Neptune and Triton (ed. Cruikshank, D. P.) 1107–1148 (Univ. Arizona Press, 1995).
Pruppacher, H. R. & Klett, J. D. Microphysics of Clouds and Precipitation (Kluwer Academic Publishers, 1997).
Moses, J. I., Allen, M. & Yung, Y. L. Hydrocarbon nucleation and aerosol formation in Neptune’s atmosphere. Icarus 99, 318–346 (1992).
Fisenko, S. P., Kane, D. B. & El-Shall, M. S. Kinetics of ion-induced nucleation in a vapor-gas mixture. J. Chem. Phys. 123, 104704 (2005).
Fray, N. & Schmitt, B. Sublimation of ices of astrophysical interest: a bibliographic review. Planet. Space Sci. 57, 2053–2080 (2009).
Dykyi, J. et al. Vapor pressure of chemicals. In Landolt–Börnstein, Numerical Data and Functional Relationships in Science and Technology, Group IV: Physical Chemistry, Vol. 20 (Ed. Martienssen, W.) (Springer, 1999).
Wohlfarth, C. & Wohlfarth, B. Surface Tension of Pure Liquids and Binary Liquid Mixtures (Springer, 1997).
Guez, L., Bruston, P., Raulin, F. & Régnaut, C. Importance of phase changes in Titan’s lower atmosphere. Tools for the study of nucleation. Planet. Space Sci. 45, 611–625 1997).
Krause, P. F. & Friedrich, H. B. Infrared spectra and dielectric properties of crystalline hydrogen cyanide. J. Phys. Chem. 76, 1140–1146 (1972).
Krasnopolsky, V. Titan’s photochemical model: further update, oxygen species, and comparison with Triton and Pluto. Planet. Space Sci. 73, 318–326 (2012).
Willacy, K., Allen, M. & Yung, Y. A new astrobiological model of the atmosphere of Titan. Astrophys. J. 829, 79 (2016).
Hörst, S. M. & Tolbert, M. A. In situ measurements of the size and density of Titan aerosol analogs. Astrophys. J. Lett. 770, L10 (2013).
He, J., Acharyya, K. & Vidali, G. Sticking of molecules on nonporous amorphous water ice. Astrophys. J. 823, 56 (2016).
Henderson, B. & Gudipati, M. S. Direct detection of complex organic products in ultraviolet (Lyα) and electron-irradiated astrophysical and cometary ice analogs using two step laser ablation and ionization mass spectrometry. Astrophys. J. 800, 66 (2015).
Hudson, R. L., Gerakines, P. A. & Moore, M. H. Infrared spectra and optical constants of astronomical ices: II. Ethane and ethylene. Icarus 243, 148–157 (2014).
Lucarini, V., Peiponen, K.-E. Saarinen, J. J. & Vartiainen, E. M. Kramers–Kroning Relations in Optical Material Research (Springer, 2005).
Masterson, C. M. & Khanna, R. K. Absorption intensities and complex refractive indices of crystalline HCN, HC3N, and C4N2 in the infrared region. Icarus 83, 83–92 (1990).
Beyer, K. D. & Ebeling, D. D. UV refractive indices of aqueous ammonium sulfate solutions. Geophys. Res. Lett. 25, 3147–3150 (1998).
Martonchik, J. V. & Orton, G. S. Optical constants of liquid and solid methane. Appl. Opt. 33, 8306–8317 (1994).
Mason, N. J. et al. VUV spectroscopy and photo-processing of astrochemical ices: an experimental study. Faraday Discuss. 133, 311–329 (2006).
Wu, Y.-J. et al. Spectra and photolysis of pure nitrogen and methane dispersed in solid nitrogen with vacuum-ultraviolet light. Astrophys. J. 746, 175 (2012).
Cruz-Diaz, G. A., Muñoz Caro, G. M., Chen, Y.-J. & Yih, T.-S. Vacuum-UV spectroscopy of interstellar ice analogs. II. Absorption cross-sections of nonpolar ice molecules. Astron. Astrophys. 562, A120 (2014).
Cruz-Diaz, G. A., Muñoz Caro, G. M., Chen, Y.-J. & Yih, T.-S. Vacuum-UV spectroscopy of interstellar ice analogs. I. Absorption cross-sections of polar-ice molecules. Astron. Astrophys. 562, A119 (2014).
Warren, S. G. & Brandt, R. E. Optical constants of ice from ultraviolet to the microwave: a revised compilation. J. Geophys. Res. D 113, D14220 (2008).
Aboudan, A., Colombatti, G., Ferri, F. & Angrilli, F. Huygens probe entry trajectory and attitude estimated simultaneously with Titan atmospheric structure by Kalman filtering. Planet. Space Sci. 56, 573–585 (2008).
Grundy, W. M., Schmitt, B. & Quirico, E. The temperature-dependent spectrum of methane ice I between 0.7 and 5 μm and opportunities for near-infrared remote thermometry. Icarus 155, 486–496 (2002).
Trotta, F. & Schmitt, B. in The Cosmic Dust Connection (ed. Greenberg, J. M.) 179–184 (Springer, 1996).
Protopapa, S., Grundy, W. M., Tegler, S. C. & Bergonio, J. M. Absorption coefficients of the methane-nitrogen binary ice system: implications for Pluto. Icarus 253, 179–188 (2015).
P.L. acknowledges support from the Programme National de Planétologie of the Institut National des Sciences de l’Univers (projects AMG and TISSAGE). D.F.S. was supported in part by the New Horizons Mission through Southwest Research Institute contract no. 277043Q. A.F.C., L.A.Y. and G.R.G. were supported by NASA through contract no. NASW02008 to Southwest Research Institute.
The authors declare no competing interests.
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Comparison of neutral (solid blue) and ion (dashed blue) nucleation rates at 70 K and at various degrees of HCN super-saturation. The corresponding black curves present the size of the HCN cluster for each case.
Simulated average bulk radius (black) and corresponding density (blue) of haze particles in Pluto’s (solid lines) and Triton’s (dashed lines) atmospheres.
Simulated variation of haze particle mass density with altitude in Pluto’s atmosphere according to our simulations. The solid line corresponds to a heterogeneous coating mass density of 0.5 gcm−3 and the dashed to 1 gcm−3.
Red lines and dots correspond to measurements at IR32,76 and visible77,78 wavelengths, respectively. Our estimate based on the gas phase cross section is shown by the gray line and is consistent with a previous estimate69 (blue dots). The dashed line presents the results when the kUV is fitted with a set of Gaussian curves and the solid black line presents the case of a 2,000 cm−1 blue-shift on kUV. All cases provide results in agreement with the visible observations and consistent with measurements (orange and green lines) in the UV range71,72.
Red lines present the reported (n,k) from measurements over a broad spectral range74. Our estimate based on the gas phase cross section is shown by the gray line. The dashed line presents the results when the kUV is fitted with a set of Gaussian curves and the solid black line presents the case of a 10,000 cm−1 blue-shift on kUV. This shift brings nVIS closer to the observed value.
Red lines and crosses present (n,k) values from measurements at IR wavelengths22 and the red dot the measured nVIS value for the C4H2 liquid21. Our estimate based on the gas phase cross section is shown by the gray line. The dashed line presents the results when the kUV is fitted with a set of Gaussian curves and the solid black line presents the case of a 2,000 cm−1 blue-shift on kUV. The shaded cyan area presents the wavelength range sensitive to haze retrieval from the New Horizons observations14. The average k value within this range (see inset) varies between 4.0 × 10−3 and 6.8 × 10−3 for the solid and dashed line cases, respectively. The corresponding value in this range for tholin-type composition is ~0.2 (dash-triple-dotted line).
Red lines and dot present (n,k) values from measurements at IR and visible wavelengths65. Our estimate based on the gas phase cross section is shown by the gray line. The dashed line presents the results when the kUV is fitted with a set of Gaussian curves and the solid black line presents the case of a 2,000 cm−1 blue-shift on kUV. The shaded cyan area present the wavelength range sensitive to haze retrieval from the Voyager observations37. The average k value within this range varies between 0.65 and 0.72 for the dashed and solid line cases, respectively.
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Lavvas, P., Lellouch, E., Strobel, D.F. et al. A major ice component in Pluto’s haze. Nat Astron 5, 289–297 (2021). https://doi.org/10.1038/s41550-020-01270-3
The Astrophysical Journal (2021)
Monthly Notices of the Royal Astronomical Society (2021)