The Pluto climate system observed by JWST

1. Introduction

The New Horizons flyby of Pluto in 2015 unveiled a world with variegated surface composition [1], a remarkable topography and active geology [2,3], and a chemically-rich atmosphere with an extensive haze [4,5]. Pluto possesses a complex climatic system driven by the redistribution cycles of volatile ices N₂, CH₄, CO [6]. A large part of the equatorial regions is volatile-free and covered by dark materials, likely organic materials resulting from a combination of surface ice irradiation and sedimentation of the photochemical haze [7]. Charon’s surface is not, morphologically and compositionally, as variegated as Pluto’s [8]. Its spectrum is dominated by H₂O ice and NH₃-bearing species with additional species such as CO₂ and H₂O₂ recently detected from JWST near-IR spectroscopy [9]. Yet, Charon exhibits darker and redder polar regions, possibly resulting from capture, cold-trapping, and subsequent chemical processing of Pluto’s escaping volatiles [10].

Measuring the thermal emission of these icy surfaces at infrared wavelengths yields constraints on surface temperatures, thermal inertias, bolometric and spectral emissivities. Such properties, largely unknown on Pluto and Charon, are diagnostic of grain size, porosity, and composition. On Pluto, they also control the sublimation and condensation of volatiles, and are therefore crucial to understand the climate system. On Charon, emissivity- and thermal inertia-dependent polar temperature is critical to explain its red poles.

Previous thermal measurements of the Pluto-Charon system yield multiple solutions because most of them did not resolve Pluto from Charon. In addition, recent modeling studies suggest that the atmospheric haze of Pluto could significantly contribute to its mid-infrared emission, thus adding further degeneracy.

In October 2022, we measured Pluto and Charon separate thermal light curves over 15-25.5 mm with the JWST/MIRI instrument, in the context of our accepted JWST GO-1 program (#1658) dedicated to the study of Pluto's climate system. Our JWST program also include (i) a deep MIRI/MRS spectrum, to obtain new insights on Pluto's atmosphere composition, and to explore the 5-15 μm reflected spectrum of Pluto (and Charon), searching for hydrocarbon ices and irradiation products, (ii) a MIRI/LRS spectrum to search for non-H2O ice signatures, and (iii) NIRCam filter imaging to map the albedo and methane ice distribution with resolution comparable to HST visible imaging. Data and first analyses will be presented at the meeting with a focus on the MIRI observations.

2. Observations

We observed Pluto and Charon separate thermal emission obtained using the JWST/MIRI Imaging instrument at 15, 18, 21 and 25.5 µm. Data were collected over a single 6.4 day Pluto-Charon mutual orbit in October 2022. Six visits to the Pluto-Charon system were scheduled, separated by 60° in longitude, providing a well-sampled multi-band lightcurve for both objects. The MIRI PSF (FWHM=0.49"-0.80") being comparable to the Pluto-Charon separation (0.68’’-0.78’’), PSF fitting was performed to extract the separate Pluto and Charon fluxes. This is the first detection of Charon’s thermal lightcurve. This lightcurve includes a small fraction of solar reflected light, which was corrected for before thermophysical modelling.

3. Thermophysical model

Observations are analyzed by means of a thermophysical model [11], including the effects of subsurface thermal conduction and a multi-terrain description of Pluto and Charon surfaces in accordance with the latest albedo maps of Pluto and Charon, as in [12]. The Pluto model takes into account the seasonal volatile cycles of N₂ and CH₄. The CH₄ cycle is particularly important because it impacts the surface temperatures and therefore the thermal emission of the CH_4-covered terrains through latent heat exchanges.

4. Results

We applied our thermophysical model to Charon considering two surface units, H₂O ice and the red pole. We obtained very good fits and we retrieved the bolometric emissivity and thermal inertia of each unit. We then applied our thermophysical model to Pluto considering three units respectively covered by N₂ ice, CH₄ ice, and dark materials, based on the albedo map. We obtained very good fits of the lightcurve contrasts (differential flux to mean) in the four filters, and we retrieved the global thermal inertia and the CH₄ ice bolometric emissivity.

For both Pluto and Charon, our thermal inertia results are in line with previously inferred values from Spitzer and Herschel observations [12,13], which encompassed the 5-30 SI range, but are now put on firmer ground thanks to the ability of MIRI to separate Pluto from Charon. A large range of bolometric emissivity for methane ice is obtained depending on the choice of scenario for ice distribution.

Most importantly, we constrain the thermal emission of Pluto’s haze. Our results have strong implications on the haze composition and on the impact of the haze on Pluto’s atmospheric temperatures and its climate at a global scale.

References

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