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

Abstract

We present eight new 0.8 to 2.4 μm spectral observations of Neptune's satellite Triton, obtained at IRTF/SpeX during 2002 July 15–22 UT. Our objective was to determine how Triton's 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 Triton's nitrogen ice absorption band at 2.15 μm; its strength varies by about a factor of two as Triton rotates. Maximum N 2 absorption approximately coincides with Triton's Neptune-facing hemisphere, which is also the longitude where the polar cap extends nearest Triton's equator. More subtle rotational variations are reported for Triton's CH 4 and H 2O ice absorption bands. Unlike the other ices, Triton's CO 2 ice absorption bands remain nearly constant as Triton rotates. Triton's H 2O ice is shown to be crystalline, rather than amorphous. Triton's N 2 ice is confirmed to be the warmer, hexagonal, β N 2 phase, and its CH 4 is confirmed to be highly diluted in N 2 ice.