Distribution and energy balance of Pluto’s nitrogen ice, as seen by New Horizons in 2015

Abstract

Pluto's surface is geologically complex because of volatile ices that are mobile on seasonal and longer time scales. Here we analyzed New Horizons LEISA spectral data to globally map the nitrogen ice, including nitrogen with methane diluted in it. Our goal was to learn about the seasonal processes influencing ice redistribution, to calculate the globally averaged energy balance, and to place a lower limit on Pluto's N 2 inventory. We present the average latitudinal distribution of nitrogen and investigate the relationship between its distribution and topography on Pluto by using maps that include the shifted bands of methane in solid solution with nitrogen (which are much stronger than the 2.15-µm nitrogen band) to more completely map the distribution of the nitrogen ice. We find that the global average bolometric albedo is 0.83 ± 0.11, similar to that inferred for Triton, and that a significant fraction of Pluto's N 2 is stored in Sputnik Planitia. We also used the encounter-hemisphere distribution of nitrogen ice to infer the latitudinal distribution of nitrogen over the rest of Pluto, allowing us to calculate the global energy balance. Under the assumption that Pluto's nitrogen-dominated 11.5 µbar atmosphere is in vapor pressure equilibrium with the nitrogen ice, the ice temperature is 36.93 ± 0.10 K, as measured by New Horizons' REX instrument. Combined with our global energy balance calculation, this implies that the average bolometric emissivity of Pluto's nitrogen ice is probably in the range 0.47 -0.72. This is consistent with the low emissivities estimated for Triton based on Voyager results, and may have implications for Pluto's atmospheric seasonal variations, as discussed below. The global pattern of volatile transport at the time of the encounter was from north to south, and the transition between condensation and sublimation within Sputnik Planitia is correlated with changes in the grain size and CH 4 concentration derived from the spectral maps. The low emissivity of Pluto's N 2 ice suggests that Pluto's atmosphere may undergo an extended period of constant pressure even as Pluto recedes from the Sun in its orbit.1

Triton Haze Analogs: The Role of Carbon Monoxide in Haze Formation

“…Triton's atmosphere, due to its extremely cold surface temperature of 38 K (Conrath et al., 1989), is in vapor pressure equilibrium with its surface ices, making atmosphere‐surface interactions active and strongly seasonally dependent (Cruikshank et al., 1993; Hansen & Paige, 1992). Similar activity is observed for Pluto (Lewis et al., 2020), where seasonal sublimation appears to drive winds (Telfer et al., 2018). Measurements of Triton's atmospheric pressure at the surface range from 14 ± 1 μbar in 1989 from Voyager 2 radio science (Tyler et al., 1989) to 19 ± 1.8 μbar from stellar occultations in 1995 and 1997 (Elliot et al., 1998; Olkin et al., 1997), suggestive of seasonal sublimation and deposition.…”

Triton Haze Analogues: the Role of Carbon Monoxide in Haze Formation

“…Triton's atmosphere, due to its extremely cold surface temperature of 38 K (Conrath et al, 1989), is in vapor pressure equilibrium with its surface ices, making atmospheresurface interactions active and strongly seasonally dependent (Hansen & Paige, 1992;Cruik-shank et al, 1993). Similar activity is observed for Pluto (Lewis et al, 2020), where seasonal sublimation appears to drive winds (Telfer et al, 2018). Measurements of Triton's atmospheric pressure at the surface range from 14±1 µbar in 1989 from Voyager 2 radio science (Tyler et al, 1989) to 19±1.8 µbar from stellar occultations in 1995 and 1997 (Olkin et al, 1997;Elliot et al, 1998), suggestive of seasonal sublimation and deposition.…”

Impact Crater Morphometry on Pluto: Implications for Surface Composition and Evolution

“…The Hayabusa group, situated north of the Tartarus group, is characterized by its rounded topography and flatfloored craters and valleys (Howard et al 2017b;White et al 2021). Signs of volatile ices suggest long-term mantling (i.e., the forming of a thick layer) of CH 4 ice in this region (Grundy et al 2016;Howard et al 2017b;Lewis et al 2021). To the west of Sputnik Planitia, at approximately the same latitude as the Hayabusa group, lies the Venera group.…”