On the surface composition of Triton's southern latitudes

TL;DR

This study analyzes Triton's surface ices over 12 years, revealing volatile ices vary with rotation while non-volatile ices remain stable, indicating seasonal surface changes related to solar illumination shifts.

Contribution

It provides the first detailed longitudinal and temporal analysis of Triton's surface ices using near-infrared spectra from 2002 to 2014.

Findings

Volatile ices show significant variability over Triton's rotation.

Non-volatile ices exhibit little to no variability.

Ethane absorption is identified as a distinct non-volatile component.

Abstract

We present the results of an investigation to determine the longitudinal (zonal) distributions and temporal evolution of ices on the surface of Triton. Between 2002 and 2014, we obtained 63 nights of near-infrared (0.67-2.55 $\mu$m) spectra using the SpeX instrument at NASA's Infrared Telescope Facility (IRTF). Triton has spectral features in this wavelength region from N$_2$, CO, CH$_4$, CO$_2$, and H$_2$O. Absorption features of ethane (C$_2$H$_6$) and $^{13}$CO are coincident at 2.405 $\mu$m, a feature that we detect in our spectra. We calculated the integrated band area (or fractional band depth in the case of H$_2$O) in each nightly average spectrum, constructed longitudinal distributions, and quantified temporal evolution for each of the chosen absorption bands. The volatile ices (N$_2$, CO, CH$_4$) show significant variability over one Triton rotation and have well-constrained longitudes of peak absorption. The non-volatile ices (CO$_2$, H$_2$O) show poorly-constrained peak longitudes and little variability. The longitudinal distribution of the 2.405 $\mu$m band shows little variability over one Triton rotation and is 97$\pm$44$^{\circ}$ and 92$\pm$44$^{\circ}$ out of phase with the 1.58 $\mu$m and 2.35 $\mu$m CO bands, respectively. This evidence indicates that the 2.405 $\mu$m band is due to absorption from non-volatile ethane. CH$_4$ absorption increased over the period of the observations while absorption from all other ices showed no statistically signifcant change. We conclude from these results that the southern latitudes of Triton are currently dominated by non-volatile ices and as the sub-solar latitude migrates northwards, a larger quantity of volatile ice is coming into view.