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A major ice component in Pluto’s haze


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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Fig. 1: Thermal structures of Pluto and Titan.
Fig. 2: HCN in Pluto’s atmosphere.
Fig. 3: Organic ice haze in Pluto’s atmosphere.
Fig. 4: Transition from spheres to aggregates in Pluto’s haze.
Fig. 5: C2 hydrocarbons in Pluto’s atmosphere.
Fig. 6: Pluto’s haze.
Fig. 7: Organic ice haze in Triton’s atmosphere.
Fig. 8: Triton’s haze.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

Code availability

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.


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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.

Author information




P.L. designed and performed the research, and wrote the manuscript. E.L. and M.A.G. provided comparison of model results with the ALMA observations. D.F.S., A.F.C., L.A.Y. and G.R.G. provided insight on the treatment of the New Horizons observations. All authors discussed the manuscript.

Corresponding author

Correspondence to P. Lavvas.

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Extended data

Extended Data Fig. 1 HCN Nucleation.

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.

Extended Data Fig. 2 Average particle properties for Pluto and Triton.

Simulated average bulk radius (black) and corresponding density (blue) of haze particles in Pluto’s (solid lines) and Triton’s (dashed lines) atmospheres.

Extended Data Fig. 3 Particle mass density.

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.

Extended Data Fig. 4 Refractive index of CH4 ice.

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.

Extended Data Fig. 5 Refractive index of H2O ice.

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.

Extended Data Fig. 6 Refractive index of C4H2 ice.

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).

Extended Data Fig. 7 Refractive index of C2H4 ice.

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).

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