Near-infrared spectral monitoring of Triton with IRTF/SpeX II: Spatial distribution and evolution of ices

This report arises from an ongoing program to monitor Neptune’s largest moon Triton spectroscopically in the 0.8 to 2.4 μm range using IRTF/SpeX. Our objective is to search for changes on Triton’s surface as witnessed by changes in the infrared absorption bands of its surface ices N 2 , CH 4 , H 2 O , CO, and CO 2 . We have recorded infrared spectra of Triton on 53 nights over the ten apparitions from 2000 to 2009. The data generally confirm our previously reported diurnal spectral variations of the ice absorption bands (Grundy and Young, 2004). Nitrogen ice shows a large amplitude variation, with much stronger absorption on Triton’s Neptune-facing hemisphere. We present evidence for seasonal evolution of Triton’s N 2 ice: the 2.15 μm absorption band appears to be diminishing, especially on the Neptune-facing hemisphere. Although it is mostly dissolved in N 2 ice, Triton’s CH 4 ice shows a very different longitudinal variation from the N 2 ice, challenging assumptions of how the two ices behave. Unlike Triton’s CH 4 ice, the CO ice does exhibit longitudinal variation very similar to the N 2 ice, implying that CO and N 2 condense and sublimate together, maintaining a consistent mixing ratio. Absorptions by H 2 O and CO 2 ices show negligible variation as Triton rotates, implying very uniform and/or high latitude spatial distributions for those two non-volatile ices.