The haze of Pluto: exploring its radiative impact on the climate
Authors: Bertrand Tanguy , Lellouch Emmanuel , Zhang Xi , Jovanovic Lora , Gautier Thomas , Carrasco Nathalie , Sciamma-O'Brien Ella , Forget Francois , Rannou Pascal , Charnay Benjamin , Roush Ted , Salama Farid 
Published: October 2020

&lt;p&gt;&lt;strong&gt;Introduction&lt;/strong&gt;&lt;/p&gt;
&lt;p&gt;The thermal profile of Pluto&amp;#8217;s atmosphere has been measured from ground-based observations and from the REX instrument on-board New Horizons [1,2]. From the surface to the top of the atmosphere, the profiles show a 3-km deep ~37 K cold layer above the N&lt;sub&gt;2&lt;/sub&gt; ice-covered surface, a strong positive gradient below 50 km, warming the stratosphere up to 110 K, and a 40 K negative gradient cooling the upper atmosphere to 70 K.&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;Our objective is to study the radiative balance of Pluto&amp;#8217;s atmosphere in 1D with a radiative-convective model, and in 3D with the full Pluto Global Climate Model (GCM), taking into account the radiative impact of haze particles.&lt;/strong&gt;&lt;/p&gt;
&lt;p&gt;We focus on three aspects of the thermal profile with the GCM: (1) It has been suggested that the organic haze has a significant radiative impact and is responsible for the cooling of the upper atmosphere [3]. However, this has not been explored yet in details in a GCM and remains to be tested against JWST observations of Pluto&amp;#8217;s atmosphere thermal emission. (2) The depth of the near-surface cold layer in the GCM is currently only 1 km, versus 3 km in the observations [4,5]. (3) The significant temperature gradient between the equator and the north pole tentatively indicated by recent ALMA observations (PI Lellouch) is not predicted by current models, because the long radiative timescale of Pluto&amp;#8217;s atmosphere should prevent horizontal temperature gradients. However, by radiating in the infrared, haze particles could shorten the radiative timescale and trigger significant temperature gradients.&lt;/p&gt;
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&lt;p&gt;&lt;strong&gt;The Pluto GCM&lt;/strong&gt;&lt;/p&gt;
&lt;p&gt;The GCM is described in detail in [4,5]. It takes into account the sublimation and condensation cycles of N&lt;sub&gt;2&lt;/sub&gt;, CH&lt;sub&gt;4&lt;/sub&gt;, and CO [4], their thermal and dynamical effects, the cloud formation, the vertical turbulent mixing, molecular thermal conduction, and a detailed surface thermal model with different thermal inertia for various timescales (diurnal, seasonal). It also includes a parameterization of the formation of organic haze [6].&lt;/p&gt;
&lt;p&gt;We use the 1D radiative-convection version of the GCM to explore the radiative impact of haze depending on the haze properties. We then use the 3D GCM to explore the effect in a climatic context, with consistent 3D predictions for haze formation and methane abundance, which are used as an input for our radiative transfer calculation.&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;At the conference we will present the results obtained with our model for different types of haze particles (spheres, fractal aggregates) and different datasets of laboratory-generated optical constants.&lt;/strong&gt;&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;We will reveal if the radiative haze can solve the three mysteries mentioned in Section 1, and how it impacts Pluto&amp;#8217;s atmosphere dynamics.&lt;/strong&gt;&lt;/p&gt;
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&lt;p&gt;&lt;strong&gt;Acknowledgements&lt;/strong&gt;&lt;/p&gt;
&lt;p&gt;T.B. was supported for this research by an appointment to the National Aeronautics and Space Administration (NASA) Post-doctoral Program at the Ames Research Center administered by Universities Space Research Association (USRA) through a contract with NASA.&lt;/p&gt;
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&lt;p&gt;&lt;strong&gt;References&lt;/strong&gt;&lt;/p&gt;
&lt;p&gt;[1] Hinson, D. P., et al., Radio occultation measurements of Pluto&amp;#8217;s neutral atmosphere with New Horizons, Icarus, 290, 96&amp;#8211;111, 2017.&lt;/p&gt;
&lt;p&gt;[2] A. Dias-Oliveira et al., Pluto's atmosphere from stellar occultations in 2012 and 2013, ApJ 811, 53, 2015.&lt;/p&gt;
&lt;p&gt;[3] Zhang, X., et al, Haze heats Pluto's atmosphere yet explains its cold temperature, Nature, 2017.&lt;/p&gt;
&lt;p&gt;[4] Forget, F., et al., A post-new horizons global climate model of Pluto including the N&lt;sub&gt;2&lt;/sub&gt;, CH&lt;sub&gt;4&lt;/sub&gt; and CO cycles, Icarus, 287, 54&amp;#8211;71, 2017.&lt;/p&gt;
&lt;p&gt;[5] Bertrand, T., et al., Pluto&amp;#8217;s Beating Heart Regulates the Atmospheric Circulation: Results From High-Resolution and Multiyear Numerical Climate Simulations. JGR: Planets, 125(2), 1&amp;#8211;24, 2020.&lt;/p&gt;
&lt;p&gt;[6] Bertrand, T. and Forget, F.: 3D modeling of organic haze in Pluto&amp;#8217;s atmosphere, Icarus, 287:72, 2017.&lt;/p&gt;
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