Near-Infrared Spectral Monitoring of Triton with IRTF/SpeX II: Spatial Distribution and Evolution of Ices

TL;DR

This study uses infrared spectroscopy to monitor Triton's surface ices over a decade, revealing seasonal and diurnal variations in N2, CH4, CO, H2O, and CO2 ices, and providing insights into their spatial distribution and evolution.

Contribution

It presents the first long-term spectral monitoring of Triton, demonstrating seasonal changes and contrasting behaviors of different surface ices, with new evidence on ice distribution and sublimation patterns.

Findings

Nitrogen ice shows strong diurnal and seasonal variations, especially on the Neptune-facing hemisphere.

Methane ice varies differently from nitrogen ice, challenging previous assumptions about their behavior.

CO ice varies similarly to nitrogen ice, indicating co-condensation and sublimation processes.

Abstract

This report arises from an ongoing program to monitor Neptune's largest moon Triton spectroscopically in the 0.8 to 2.4 micron 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 N2, CH4, H2O, CO, and CO2. We have recorded infrared spectra of Triton on 53 nights over the ten apparitions from 2000 through 2009. The data generally confirm our previously reported diurnal spectral variations of the ice absorption bands (Grundy & 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 N2 ice: the 2.15 micron absorption band appears to be diminishing, especially on the Neptune-facing hemisphere. Although it is mostly dissolved in N2 ice, Triton's CH4 ice shows a very different longitudinal variation from the N2 ice, challenging assumptions of how the two ices behave. Unlike Triton's CH4 ice, the CO ice does exhibit longitudinal variation very similar to the N2 ice, implying that CO and N2 condense and sublimate together, maintaining a consistent mixing ratio. Absorptions by H2O and CO2 ices show negligible variation as Triton rotates, implying very uniform and/or high latitude spatial distributions for those two non-volatile ices.