Abstract
Pluto and its largest moon Charon display a variety of surfaces, whose thermal and energetic properties are largely unknown. Previous thermal measurements of the Pluto–Charon system yield multiple solutions because most of them did not resolve Pluto from Charon. In addition, recent modelling studies suggest that the atmospheric haze of Pluto could substantially contribute to its mid-infrared emission, thus adding further degeneracy. Here we measure separate Pluto and Charon thermal light curves over 15–25.5 μm with JWST and retrieve the thermophysical and emissivity properties of the different terrains on each. We also detect and measure the thermal emission of Pluto’s haze. The observed fluxes indicate that Pluto’s haze is composed of Titan-like organic particles as well as hydrocarbon and nitrile ices and demonstrate that the haze largely controls Pluto’s atmospheric balance. As a result, Pluto’s temperatures, climate and general circulation should therefore be substantially affected by the haze across seasons.
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Data availability
The JWST data are publicly available via the MAST archive at https://mast.stsci.edu/portal/Mashup/Clients/Mast/Portal.html. Raw data of JWST observations and raw data with colour-corrected flux are provided in Supplementary Data Tables 1 and 2.
Code availability
Data reduction software are available upon request. The 1-D haze model used in this study is described in detail in previously published studies23. The code is not publicly available due to its undocumented intricacies, but is available on demand (contact X.Z. for access to the model). The Pluto and Charon surface models are available upon request.
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Acknowledgements
This work is based on observations made with the NASA/ESA/CSA James Webb Space Telescope. The data were obtained from the Mikulski Archive for Space Telescopes at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127 for JWST. These observations are associated with programme #1658 ‘Pluto’s climate system with JWST’. X.Z. acknowledges support from the NASA Exoplanet Research grant number 80NSSC22K0236 and the NASA Interdisciplinary Consortia for Astrobiology Research (ICAR) grant number 80NSSC21K0597. N.P.A. acknowledges the Ministry of Science, Innovation, and Universities (MICIU) in Spain and the State Agency for Research (AEI) for funding through the ATRAE programme, project number ATR2023-145683. Resources supporting this work were provided by the NASA High-End Computing (HEC) Program through the NASA Advanced Supercomputing (NAS) Division at Ames Research Center.
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T.B. designed the research project, analysed the JWST data and wrote the paper. E.L. performed the data reduction and photometry with important contributions from B.H., J.S. and I.W. X.Z. and L.W. performed the 1D simulations with the haze model. All authors provided insightful comments on the main text and Supplementary Information.
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Nature Astronomy thanks Leslie Young and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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Extended data
Extended Data Fig. 1 Sample of observations in the four filters.
(observation 03, Oct 05, 2022). Top: pixel values in a 10”x10” RA,DEC field centered on Pluto’s expected position (MT_RA, MT_DEC). Background emission is evident. Bottom: background-corrected images. The value of the constant subtracted background and of the resulting maximum signal (MJy/sr) due to Pluto-Charon are indicated. Charon (to the north-east of Pluto) is much fainter than Pluto in F1500W, but brighter in F1800W, F2100W and F2550W because of its higher surface temperature.
Extended Data Fig. 2 Illustration of PSF fitting for the four filters.
(observation 03, Oct 05, 2022). Top: Background-subtracted images from Extended Data Fig. 1, on a 51×51 pixel grid centered on an initial estimate of Pluto’s position. Middle: Reconstructed image. Solution values for fPluto and fCharon (that is, multiplicative factors for the PSF at Pluto and Charon) and for a residual background correction (MJy/sr) are indicated. Bottom: Observed minus model residuals. The color bars indicate the flux levels in MJy/sr. The fitted positions of Pluto and Charon are indicated by the circle and cross, respectively.
Extended Data Fig. 3 Pluto and Charon brightness temperature.
Brightness temperatures are shown at the four pivot wavelengths (15.051, 17.970, 20.802, and 25.232 µm).
Extended Data Fig. 4 Modeling of optical and 15 µm Pluto lightcurves.
(Top left). Modelling of the 2002-2003 optical lightcurve observed by50 using the distribution of Bond albedos from42. A Lunar-Lambert model with A = 0.8 and a phase integral q = 1.04 were adopted. Black: data. Blue: model. (Top right). Blue: optical lightcurve calculated for the geometry of Oct. 2022. Red: adjusted 15 µm albedo lightcurve (see text). (Bottom left). Differential, and corrected for (virtually negligible) thermal emission, 15 µm observed lightcurve, compared to models using (i) an albedo following the visible lightcurve (blue) and (ii) the adjusted 15 µm albedo lightcurve (red). The adjusted 15 µm albedo profile somewhat improves the 15 µm contrasts, and its value at L ~ 342 matches the value indicated by the MIRI/MRS spectrum. (Bottom right) 15 µm observed lightcurve (black) and model fit (red). The green, blue, and cyan curves indicate the respective contributions of: (green) surface thermal emission (blue) solar reflected component (cyan) haze and gas thermal emission, assumed constant with longitude.
Extended Data Fig. 6 Distribution of the terrain units.
(Top) Charon: the albedo value A = 0.2 delimits the two units (H2O ice and the red poles). (Middle) Pluto: Reference scenario, as in27. The albedo values A = 0.38 and A = 0.79 delimit the three units: dark materials (red), methane ice (yellow), nitrogen ice (red). (Bottom) Pluto: Alternative scenario (for simulation Alt_U_0.5), roughly based on New Horizons spectroscopic observations3,4 with N2 ice in Sputnik Planitia, in local depressions in East Tombaugh Regio and in a mid-latitudinal band, while the north pole contains CH4 ice only.
Extended Data Fig. 7 Absolute thermal emission fluxes of Charon.
Observed (black points) and modeled (red curve) fluxes. The purple and blue curve show the contribution of the H2O ice and red pole units, respectively. In order to explain the observed emission peak of Charon’s lightcurves at ~220°E, we find that the H2O ice unit must dominate the thermal emission and be almost as warm as the red pole (despite its higher albedo).
Extended Data Fig. 8 Simulated emission flux for all Pluto “seasonal cycles” simulations.
Top: Best fits of the JWST data of Pluto’s emission flux contrasts at 18, 21 and 25 µm. Black points with error bars: observations (corrected for solar reflected component). Colors: Best fit surface model for the different scenarios. Middle: Same, but for absolute emission fluxes of Pluto, with errors at 3σ. Bottom: Contribution of the CH4 ice unit (triangles) and dark materials unit (squares) to Pluto’s thermal emission for the different simulations.
Extended Data Fig. 9 Vertical profiles of haze particle properties in the 1-D haze model.
Three modes are shown: monomer (red), small sphere (blue), and fractal aggregates (black). Data below 50 km are based on the retrieved results from Fan et al. (2022)37, while data above this altitude are simple estimates based on the microphysical models in Gao et al. (2017)49 and Lavvas et al. (2021)28. See Methods for details. Upper left: UV extinction coefficients (cm−1), with the shaded grey thick line representing Alice observations from New Horizons. Upper right: number density (cm−3). Lower left: particle radius (micron). Lower right: number of monomers per aggregate.
Supplementary information
Supplementary Information
Supplementary text, Figs. 1–8 and Tables 1–6.
Supplementary Data Table 1
Raw data (JWST observations).
Supplementary Data Table 2
Raw data with colour-corrected flux from Figs. 1 and 2 and Supplementary Table 2.
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Bertrand, T., Lellouch, E., Holler, B. et al. Evidence of haze control of Pluto’s atmospheric heat balance from JWST/MIRI thermal light curves. Nat Astron 9, 1300–1308 (2025). https://doi.org/10.1038/s41550-025-02573-z
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DOI: https://doi.org/10.1038/s41550-025-02573-z
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