Peculiar Ice Patterns in Craters In Bird Planitia and Viking Terra on Pluto

Introduction. New Horizons upended our ideas of volatile transport on Pluto. Pre-encounter models focused on the seasonal migration of N₂ between summer and winter poles. These models predicted that in some regions, N₂ ice would periodically cover and then expose bare terrains, while other areas might remain permanently covered in N₂ ice. Instead, New Horizons showed a variety of volatile transport modes: glacial flow; the clearly permanent N₂ ice deposit named Sputnik Planitia; the diurnal sublimation/condensation cycle in the lower atmosphere evident in the radio occultation; seasonal N₂ sublimation fronts in surface ices; and the suggestion from combined occultation and modeling that the south pole may stay clear of N₂ ice over its long winter. N₂ moves between the surface and atmosphere and across the surface, but at what timescales?

Observations. A clue comes from a collection of craters that show distinctive and varied ice patterns on their rims and in their basins [1]. These craters are found at low northern latitudes west of Sputnik Planitia and north-to-northwest of Elliot Crater, in Bird Planitia and Viking Terra, 115 to 145° East longitude and 10 to 30° North latitude. For example, we show six craters in Table 1. While some of the north-facing crater slopes are bright due to more direct insolation, much of the brightness variation is correlated to deposits of N₂-rich N₂/CH₄ ice or CH₄-rich frost [2].

Discussion. In bare regions, higher insolation can lead to higher temperatures, inhibiting condensation. In areas covered by N₂-rich ice or CH₄-rich frost, higher insolation can lead to increased sublimation. Using the digital elevation model [1], we calculated insolation patterns in these craters, accounting for the local incidence angle and shadowing. The first question is: are there any timescales over which we see that the ice seen today lies where there had been less incident sunlight? If so, what are the time scales? If not, what are some other mechanisms control the ice deposition?

Acknowledgments: We thank all the scientists and engineers who made NASA New Horizons mission to Pluto possible. This work was funded by NASA NFDAP 80NSSC23K0666.

References: [1] Schenk, P. M. et al. (2018) Icarus, 314, 400–433. [2] Earle A. M. et al. (2018) Icarus, 314, 195–209.