Refining Saturn’s deuterium-hydrogen ratio via IRTF/TEXES spectroscopy^★

¹ Department of Physics & Astronomy, University of Leicester, University Road, Leicester, LE1 7RH, UK
e-mail: jsdb3@le.ac.uk
² Southwest Research Institute, San Antonio, Texas, TX, USA
³ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA, 91109, USA

Received: 22 April 2020
Accepted: 11 January 2021

Abstract

The abundance of deuterium in giant planet atmospheres provides constraints on the reservoirs of ices incorporated into these worlds during their formation and evolution. Motivated by discrepancies in the measured deuterium-hydrogen (D/H) ratio on Jupiter and Saturn, we present a new measurement of the D/H ratio in methane for Saturn from ground-based measurements. We analysed a spectral cube (covering 1151–1160 cm⁻¹ from 6 February 2013) from the Texas Echelon Cross Echelle Spectrograph (TEXES) on NASA’s Infrared Telescope Facility (IRTF) where emission lines from both methane and deuterated methane are well resolved. Our estimate of the D/H ratio in stratospheric methane, 1.65_−0.21^+0.27 × 10⁻⁵ is in agreement with results derived from Cassini CIRS and ISO/SWS observations, confirming the unexpectedly low CH₃D abundance. Assuming a fractionation factor of 1.34 ± 0.19 we derive a hydrogen D/H of 1.23_−0.23^+0.27 × 10⁻⁵. This value remains lower than previous tropospheric hydrogen D/H measurements of (i) Saturn 2.10(±0.13) × 10⁻⁵, (ii) Jupiter 2.6(±0.7) × 10⁻⁵ and (iii) the proto-solar hydrogen D/H of 2.1(±0.5) × 10⁻⁵, suggesting that the fractionation factor may not be appropriate for stratospheric methane, or that the D/H ratio in Saturn’s stratosphere is not representative of the bulk of the planet.