Evidence of local CH4 stratification on Pluto from New Horisons LEISA data and a complete N2 ice map.
Abstract
The diurnal, seasonal and astronomical cycles on Pluto trigger sublimation-transport-condensation cycles of the volatile ices (N2, CH4, CO) with different amplitudes and time constants at the surface. The qualitative distribution of the two major volatile ice phases identified on the surface of Pluto, N2-rich:CH4:CO ice and CH4-rich ice, has been mapped by Schmitt et al. (2017) and the spatial transitions between the predominant zone of these phases have been highlighted. The first quantitative composition map has been derived by Protopapa et al. (2017). A surprising observation, but not discussed in Schmitt et al. paper, was that the CH4 qualitative abundance maps obtained from different CH4 bands display relatively different spatial distributions, but with regular evolutions at a given location from the weakest to the strongest CH4 bands. In this talk we demonstrate that these band depth changes reflect a stratification of CH4, either in the CH4 concentration in N2-rich ice and/or in the relative abundance between the N2-rich:CH4:CO and CH4-rich ices, depending on the area. For this we use in addition our 'CH4 state index' based on the spectral position of the CH4 bands and a new N2 ice distribution map including the area where the N2 ice band is too weak to be observed directly. The stratification of CH4 is shown to result from the differential sublimation between N2 and CH4 which tends to concentrate CH4 in N2 ice grains and, according to the phase diagram, produces a CH4-rich phase that accumulates on the surface. We will show that several different configurations appear to exist at the surface of Pluto according to the latitude and the altitude, and that they may be the witness of different stages in the sublimation-condensation cycles or of different timescales. The occurence, the CH4 concentration and the depth of these stratified terrains can be correlated with the nitrogen sublimation fluxes obtained by volatile transport models (Bertrand et al. 2018).
- Publication:
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AAS/Division for Planetary Sciences Meeting Abstracts #50
- Pub Date:
- October 2018
- Bibcode:
- 2018DPS....5050602S