Ice Mineralogy Across and Into the Surfaces of Pluto, Triton, and Eris

TL;DR

This study combines spectral observations and modeling to analyze the ice mineralogy and methane-nitrogen mixing ratios on Pluto, Triton, and Eris, revealing surface composition details and challenging previous expectations about depth-dependent composition.

Contribution

It provides new measurements of methane abundances and demonstrates the importance of modeling methane in two phases, advancing understanding of icy dwarf planet surfaces.

Findings

Methane abundances are 9.1%, 7.1%, 8.2% on Pluto at different longitudes.

Triton has methane abundances of about 5%, with no significant variation.

Eris shows a methane abundance of 10%, with no trend with depth.

Abstract

We present three near-infrared spectra of Pluto taken with the IRTF and SpeX, an optical spectrum of Triton taken with the MMT and the Red Channel Spectrograph, and previously published spectra of Pluto, Triton, and Eris. We combine these observations with a two-phase Hapke model, and gain insight into the ice mineralogy on Pluto, Triton, and Eris. Specifically, we measure the methane-nitrogen mixing ratio across and into the surfaces of these icy dwarf planets. In addition, we present a laboratory experiment that demonstrates it is essential to model methane bands in spectra of icy dwarf planets with two methane phases - one highly-diluted by nitrogen and the other rich in methane. For Pluto, we find bulk, hemisphere-averaged, methane abundances of 9.1 \pm 0.5%, 7.1 \pm 0.4%, and 8.2 \pm 0.3% for sub-Earth longitudes of 10\degree, 125\degree, and 257\degree. Application of the Wilcoxon rank sum test to our measurements finds these small differences are statistically significant. For Triton, we find bulk, hemisphere-averaged, methane abundances of 5.0 \pm 0.1% and 5.3 \pm 0.4% for sub-Earth longitudes of 138\degree and 314\degree. Application of the Wilcoxon rank sum test to our measurements finds the differences are not statistically significant. For Eris, we find a bulk, hemisphere-averaged, methane abundance of 10 \pm 2%. Pluto, Triton, and Eris do not exhibit a trend in methane-nitrogen mixing ratio with depth into their surfaces over the few cm range probed by these observations. This result is contrary to the expectation (Grundy & Stansberry 2000) that since visible light penetrates deeper into a nitrogen-rich surface than the depths from which thermal emission emerges, net radiative heating at depth would drive preferential sublimation of nitrogen leading to an increase in the methane abundance with depth.