Pluto’s SHERPAS: Studying HazE Radiative Processes and Atmosphere-Surface interactions – SHERPAS

The New Horizons 2015 encounter with the Pluto system unveiled a remarkably active world, with a highly variegated surface displaying glaciers and dunes made of volatile N2, CH4, CO ices, and a chemically-rich atmosphere with extensive haze layers. These exotic, and sometimes enigmatic observations raised new fundamental questions on the evolution of the atmosphere and surface of Pluto and of similar worlds (cold objects with tenuous condensable atmospheres, e.g. Triton and other Trans-Neptunian objects) and call upon modeling efforts to complete their analysis and understand the associated mechanisms at play. In our SHERPAS project, we aim at using Pluto as a natural laboratory to study planetary climate physics and dynamics. In particular, our objectives are to understand:
(1) What controls the thermal profile of Pluto’s atmosphere, in particular the unexpected 40K cooling above the stratosphere and the 3km-deep cold layer above the surface. Is it the organic haze? Does hydrocarbons condensation in the atmosphere plays a role? we recently acquired JWST observations of Pluto, which will provide us with clues regarding the radiative impact of the organic haze. We will also explore the thermal profile of Triton’s atmosphere, which remains largely unexplained.
(2) What processes trigger the formation of atmospheric waves and observed haze layers. Are the waves dominated by a topographic forcing or by the diurnal sublimation and condensation (“breathing”) of nitrogen ice deposits? How do these waves affect the state of such tenuous atmospheres (e.g. temperatures and winds) ?
(3) What surface-atmosphere interactions form the icy periodic bedforms observed at Pluto’s surface. Is it rather sublimation or condensation that dominates the formation of these structure? How do they compare to other similar ones in the solar system, especially on Earth and Mars?
To achieve these goals, we will develop a new-generation global climate model, capable of simulating the atmosphere and surface of Pluto, Triton, and even other trans-Neptunian objects. This model will contain an ultra-parallelizable dynamic core, making it possible to speed up calculations by a factor of 200 compared to what is achieved today with the current Pluto model, which is crucial for simulating a Plutonian year (248 years). We will develop a complete radiative transfer scheme for Pluto and Triton, the microphysics of organic haze and clouds, the impact of micro-climates on slopes, and that of atmospheric waves on winds. The atmospheric model will also be coupled with a surface model to simulate the paleoclimates of Pluto and Triton over more than 100 million years. Finally, an exosphere model will be added to simulate the possible local and non-global atmospheres of Eris and Makemake when these objects approach their perihelion. These developments will directly and strongly benefit the scientific investigations of this project, and also beyond. In particular, the Generic model, simulating the climate of exoplanets, will be able to benefit from all the schemes that we will develop (microphysics of haze and hydrocarbons, radiative transfer, subgrid-scale slopes and wave scheme, paleoclimate model).
We will combine the expertise of 3 laboratories (LESIA, LMD and LPG), and will compare our model results with available observations to interpret them. We will conduct comparative planetology studies, in particular between Pluto, Earth and Mars for surface ice structures, between Pluto, Triton and Mars for gravity waves, and between Pluto, Triton, Titan and early Earth for the radiative impact of the haze. These comparisons will allow us to assess the universality or uniqueness of the phenomena encountered on Pluto.
The SHERPAS project will recruit: 1 two-year postdoctoral researcher at LESIA; 1 PhD at LPG, 1 research engineer at LMD.