Exploring Pluto’s climate, atmosphere dynamics and surface-atmosphere interactions : observation and numerical modeling

Speaker : Tanguy Bertrand, Observatoire de Paris

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 that Pluto is one of the most active solid bodies in the Solar System, with a wide array of processes on display, including ongoing tectonism, cryovolcanism, solid-state convection, glacial flow, atmospheric circulation, surface-atmosphere volatile exchange, atmospheric photochemistry, microphysics, and haze and cloud formation. In particular, it showed 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. 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. I will also present new observations of Pluto we plan to acquire with the James Webb Space Telescope. Both observation and modeling methods will provide a broad variety of constraints on Pluto’s composition, chemistry, and climatic evolution and help us in better understanding how Pluto works, and by extension, Triton and other volatile-rich Kuiper-Belt objects.