Modeling Pluto’s minimum pressure: Implications for haze production

Abstract

Pluto has a heterogeneous surface, despite a global haze deposition rate of ∼1μm per orbit (Cheng et al., 2017; Grundy et al., 2018). While there could be spatial variation in the deposition rate, this has not yet been rigorously quantified, and naively the haze should coat the surface more uniformly than was observed. One way (among many) to explain this contradiction is for the atmospheric pressure at the surface to drop low enough to interrupt haze production and stop the deposition of particles onto part of the surface, driving heterogeneity. If the surface pressure drops to less than 10−3 - 10−4μbar and the CH4 mixing ratio remains nearly constant at the observed 2015 value, the atmosphere becomes transparent to ultraviolet radiation (Young et al., 2018), which would shut off haze production at its source. If the surface pressure falls below 0.06 μbar, the atmosphere ceases to be global, and instead is localized over only the warmest part of the surface, restricting the location of deposition (Spencer et al., 1997). In Pluto’s current atmosphere, haze monomers collect together into aggregate particles at beginning at 0.5 μbar; if the surface pressure falls below this limit, the appearance of particles deposited at different times of year and in different locations could be different. We use VT3D, an energy balance model (Young, 2017), to model the surface pressure on Pluto in current and past orbital configurations for four possible static N2 ice distributions: the observed northern hemisphere distribution with (1) a bare southern hemisphere, (2) a south polar cap, (3) a southern zonal band, and finally (4) a distribution that is bare everywhere except inside the boundary of Sputnik Planitia. We also present a sensitivity study showing the effect of mobile N2 ice. By comparing the minima of the modeled pressures to the three haze-disruption pressures, we can determine if or when haze production is disrupted. We find that Pluto’s minimum surface pressure in its current orbit is predicted to be between 0.01 and 3 μbar, and that over the past 10 million years the surface pressure has not fallen below 0.004 μbar. According to our model, southern N2 ice is required for haze aggregation to be interrupted, and southern N2 with very low thermal inertia is required for the possibility of a local atmosphere.