On the surface composition of Triton's southern latitudes

Original · EN

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 μm) spectra using the SpeX instrument at NASA's Infrared Telescope Facility (IRTF). Triton has spectral features in this wavelength region from N₂, CO, CH₄, CO₂, and H₂O. Absorption features of ethane (C₂H₆) and ¹³CO are coincident at 2.405 μm, a feature that we detect in our spectra. We calculated the integrated band area (or fractional band depth in the case of H₂O) in each nightly average spectrum, constructed longitudinal distributions, and quantified temporal evolution for each of the chosen absorption bands. The volatile ices (N₂, CO, CH₄) show significant variability over one Triton rotation and have well-constrained longitudes of peak absorption. The non-volatile ices (CO₂, H₂O) show poorly-constrained peak longitudes and little variability. The longitudinal distribution of the 2.405 μm band shows little variability over one Triton rotation and is 97±44∘ and 92±44∘ out of phase with the 1.58 μm and 2.35 μm CO bands, respectively. This evidence indicates that the 2.405 μm band is due to absorption from non-volatile ethane. CH₄ 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.