On the surface composition of Triton’s southern latitudes

• Published in: Icarus (New York, N.Y. 1962) N.Y. 1962), 2016-03, Vol.267, p.255-266
• Main Authors: Holler, B.J., Young, L.A., Grundy, W.M., Olkin, C.B.
• Format: Article
• Language: English
• Subjects: Absorption; Band spectra; Carbon monoxide; Evolution; Ices; Ices, IR spectroscopy; Latitude; Longitude; Spectra; Spectroscopy; Triton
• ISSN: 0019-1035; 1090-2643
• DOI: 10.1016/j.icarus.2015.12.027

•We analyze 63 near-infrared spectra of Triton from IRTF/SpeX.•Volatile ices exhibit stronger longitudinal variability than non-volatile ices.•Triton’s southern latitudes are dominated by non-volatile ices.•Absorption from CH4 increased between 2002 and 2014.•The 2.405μm feature is consistent with ethane absorption. 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 (0.67–2.55μm) 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 2.405μm, 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 2.405μm band shows little variability over one Triton rotation and is 97±44° and 92±44° out of phase with the 1.58μm and 2.35μm CO bands, respectively. This evidence indicates that the 2.405μm 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 significant 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.