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Pluto has a heterogeneous surface, despite a global haze deposition rate of~1 micrometer per orbit (Cheng et al., 2017; Grundy et al., 2018). While therecould be spatial variation in the deposition rate, this has not yet beenrigorously quantified, and naively the haze should coat the surface moreuniformly than was observed. One way (among many) to explain this contradictionis for atmospheric pressure at the surface to drop low enough to interrupt hazeproduction and stop the deposition of particles onto part of the surface,driving heterogeneity. If the surface pressure drops to less than 10^-3 - 10^-4microbar and the CH4 mixing ratio remains nearly constant at the observed 2015value, the atmosphere becomes transparent to ultraviolet radiation (Young etal., 2018), which would shut off haze production at its source. If the surfacepressure falls below 0.06 microbar, the atmosphere ceases to be global, andinstead is localized over only the warmest part of the surface, restricting thelocation of deposition (Spencer et al., 1997). In Pluto's current atmosphere,haze monomers collect together into aggregate particles at beginning at 0.5microbar; if the surface pressure falls below this limit, the appearance ofparticles deposited at different times of year and in different locations couldbe different. We use VT3D, an energy balance model (Young, 2017), to model thesurface pressure on Pluto in current and past orbital configurations for fourpossible static N2 ice distributions: the observed northern hemispheredistribution with (1) a bare southern hemisphere, (2) a south polar cap, (3) asouthern zonal band, and finally (4) a distribution that is bare everywhereexcept inside the boundary of Sputnik Planitia. We also present a sensitivitystudy showing the effect of mobile N2 ice...(cont.)

Modeling Pluto's Minimum Pressure: Implications for Haze Production