Near-infrared spectral monitoring of Triton with IRTF/SpeX I: establishing a baseline for rotational variability

We present eight new 0.8 to 2.4 μm spectral observations of Neptuneʹs satellite Triton, obtained at IRTF/SpeX during 2002 July 15–22 UT. Our objective was to determine how Tritonʹs near-infrared spectrum varies as Triton rotates, and to establish an accurate baseline for comparison with past and future observations. The most striking spectral change detected was in Tritonʹs nitrogen ice absorption band at 2.15 μm; its strength varies by about a factor of two as Triton rotates. Maximum N2 absorption approximately coincides with Tritonʹs Neptune-facing hemisphere, which is also the longitude where the polar cap extends nearest Tritonʹs equator. More subtle rotational variations are reported for Tritonʹs CH4 and H2O ice absorption bands. Unlike the other ices, Tritonʹs CO2 ice absorption bands remain nearly constant as Triton rotates. Tritonʹs H2O ice is shown to be crystalline, rather than amorphous. Tritonʹs N2 ice is confirmed to be the warmer, hexagonal, β N2 phase, and its CH4 is confirmed to be highly diluted in N2 ice.