Tanguy Bertrand (NASA) : «Pluto's atmosphere dynamics and surface-atmosphere interactions»»

Jeudi 16 Janvier 2020, 13:00 - 14:00
par Mickaël BONNIN

Pluto’s tenuous atmosphere is mainly nitrogen and is in solid-gas equilibrium with the surface nitrogen ice and thus belongs to the category of thin atmospheres with those of Mars, Triton and Io. Over the past three decades, different Earth-based observations hinted at an exotic and dynamical atmosphere as they revealed (1) a much warmer atmosphere (70-100 K) than the surface (40 K), with a strong inversion in the first 20 km above the surface, (2) a threefold increase of surface pressure since 1988, and (3) global-scale oscillations in the vertical density and temperature profiles. In 2015, the observations made by the New Horizons spacecraft revealed the presence of magnificent haze layers, possibly due to gravity waves arising from N2 sublimation and orographic forcings. Surface-atmosphere interactions were also suggested by observed surface features, such as wind streaks and linear dunes, further highlighting the dynamical activity of Pluto’s surface and atmosphere.
New Horizons also revealed a complex distribution of the main volatile ices (N2, CH4, and CO), including the thousand-kilometers nitrogen ice-sheet in Sputnik Planitia, a combination of N2, CO and CH4 deposits at mid-latitudes, massive methane-rich deposits forming the Bladed Terrain at low latitudes, a methane mantle at high latitudes, CH4 snow-capped mountains near the equator, etc.  
To understand all these observations, I have developed and used a hierarchy of models able to simulate Pluto’s climate and volatile transport over multiple timescales: (1) A Global Climate Model to represent the evolution of the 3D atmospheric circulation, the transport of gases and surface ices (N2, CH4 and CO) over up to several tens of Earth years (2) a 2D volatile transport model able to simulate the N2, CH4 and CO cycles over several tens of thousands of years (tuned using the GCM) and 3) A long-term Pluto evolution model combining the volatile transport model simulations with the variations of Pluto’s orbit and obliquity to simulate the evolution of the volatile reservoirs over up to 50 million Earth years. Such tools are based on universal equations, with the minimum of ad-hoc hypothesis. 
At the seminar, I will review our knowledge of Pluto’s dynamics, volatile transport and surface-atmosphere interactions, and I will put forward what we have learned and what remains difficult to understand and predict with these models