Atmospheric Modelling for Neptune's Methane D/H Ratio - Preliminary Results

TL;DR

This study models Neptune's atmosphere using high-resolution infrared spectra to refine the D/H ratio in methane, providing insights into Neptune's formation history and its relation to solar system bodies.

Contribution

It introduces a refined measurement of Neptune's methane D/H ratio using updated line lists and atmospheric modeling, supporting formation theories.

Findings

D/H ratio in Neptune's methane is approximately 3.0×10^{-4}

Results align with previous measurements, confirming model consistency

Supports the hypothesis that Neptune formed inside its current orbit

Abstract

The ratio of deuterium to hydrogen (D/H ratio) of Solar System bodies is an important clue to their formation histories. Here we fit a Neptunian atmospheric model to Gemini Near Infrared Spectrograph (GNIRS) high spectral resolution observations and determine the D/H ratio in methane absorption in the infrared H-band ($\sim$ 1.6 {\mu}m). The model was derived using our radiative transfer software VSTAR (Versatile Software for the Transfer of Atmospheric Radiation) and atmospheric fitting software ATMOF (ATMOspheric Fitting). The methane line list used for this work has only become available in the last few years, enabling a refinement of earlier estimates. We identify a bright region on the planetary disc and find it to correspond to an optically thick lower cloud. Our preliminary determination of CH$_{\rm 3}$D/CH$_{\rm 4}$ is 3.0$\times10^{-4}$, which is in line with the recent determination of Irwin et al. (2014) of 3.0$^{+1.0}_{-0.9}\sim\times10^{-4}$, made using the same model parameters and line list but different observational data. This supports evidence that the proto-solar ice D/H ratio of Neptune is much less than that of the comets, and suggests Neptune formed inside its present orbit.