Ground-based measurements of the D/H ratio in the deep atmosphere of Venus from 2022 IRTF/iSHELL Observations

The deuterium-to-hydrogen [D/H] ratio in atmospheric water vapour is a key tracer of Venus’s water history. Previous measurements by the Pioneer Venus entry probe [1] together with Fourier Transform Spectrometer observations from ground-based telescopes [2] [3] in the 1980s revealed a D/H ratio in Venus’s deep atmosphere of about 150 times the terrestrial standard. This result was interpreted as evidence of atmospheric escape, which preferentially removes hydrogen and enriches deuterium over geological timescales, suggesting that Venus lost a significant amount of water. However, the reported values have relatively large uncertainties: 150 VSMOW from the Pioneer Venus entry probe [1], while 120 ± 40 VSMOW [2] and 157 ± 15 VSMOW [3] by ground-based telescopes for disk-averaged values. No new measurements have been reported since the last observations in the 1980s. Furthermore, no spatial map of the D/H ratio in the lower atmosphere has ever been obtained.

Unlike the upper atmosphere, which is dominated by photochemistry, the deep atmosphere below the thick cloud layer is dominated by thermodynamic equilibrium chemistry [4]. This region acts as a reservoir, preserving the “memory” of Venus's water inventory. Probing these deep layers is essential for measuring the bulk D/H ratio independently of the cloud microphysics and photodissociation processes at higher altitudes. By resolving the vertical distribution of this ratio from the deep to the upper atmosphere, and linking the bulk value to the escape zone, we can constrain the fractionation factor [5]. This is a key diagnostic for distinguishing between two evolutionary scenarios that could explain the enhanced D/H ratio: a massive escape from a past primordial ocean [1] or a steady-state balance maintained by current volcanic and/or cometary supplies [5]. Furthermore, detecting local anomalies would reveal active volcanic outgassing and thus constrain the proposed scenarios. Given that a 10% variation in bulk D/H can alter the estimated volume of escaped water by an order of magnitude, these deep atmospheric measurements are crucial for reconstructing the history of Venus. To better constrain the D/H ratio in the deep atmosphere, new spatially resolved observations with better sensitivity are therefore required.

In this study, we conduct high-resolution spectroscopy of Venus’s nightside using the iSHELL instrument at NASA InfraRed Telescope Facility (IRTF) to observe H₂O and HDO in the deep atmosphere of Venus and retrieve updated D/H values. The observations were carried out on 11 February 2022, when the apparent diameter of Venus was 41.2” and the Doppler shift between Earth and Venus was 11.31 km s^-1. The observations were conducted under a small illuminated fraction of the disk, approximately 25%, in order to minimize stray light from the bright crescent and enable sensitive measurements of the nightside thermal emission. We observed the nightside of Venus in the K3 band, covering 2.26–2.55 μm, which probes the lower atmosphere at altitudes of approximately 30–40 km. This spectral region allows the simultaneous detection and separation of H₂O and HDO absorption lines. We used the narrowest slit width of 0.375″, corresponding to a resolving power of λ/Δλ ~ 75,000, which is required to resolve individual absorption lines and distinguish Venusian features from telluric contamination. The observing strategy was designed to map the nightside disk by placing the slit at five different positions across Venus, with the slit aligned in the east-west direction.

The observed data were reduced using standard procedures, including sky subtraction, flat-fielding, and spectral calibration. A small contribution of scattered light from the dayside crescent was characterized and removed using observational data obtained from the dayside crescent. The flux calibration was based on radiance predicted by radiative transfer calculations. The HDO/H₂O ratio is retrieved together with the volume mixing ratios of CO, HF, SO₂, and OCS using radiative transfer calculations, and the results will be discussed.

 

References

[1] T.M. Donahue, J.H. Hoffman, R.R. Hodges Jr., A.J. Watson, Venus Was Wet: A Measurement of the Ratio of Deuterium to Hydrogen. Science 216, 630-633 (1982). doi:10.1126/science.216.4546.630

[2] G.L. Bjoraker, H.P. Larson, M.J. Mumma, R. Timmermann, J.L. Montani, Airborne observations of the gas composition of Venus above the cloud tops: measurements of H2O, HDO, HF, and the D/H and 18O/16O isotopic ratios, in AAS/Division for Planetary Sciences Meeting Abstracts #24. Bulletin of the American Astronomical Society, vol. 24, 1992, p. 995 link

[3] C. de Bergh, B. Bézard, T. Owen, D. Crisp, J.P. Maillard, B.L. Lutz, Deuterium on Venus: Observations From Earth. Science 251, 547–549 (1991). doi:10.1126/science.251.4993.547

[4] E. Marcq, F.P. Mills, C.D. Parkinson, A.C. Vandaele, Composition and Chemistry of the Neutral Atmosphere of Venus. Space Sci Rev 214, 10 (2018). doi:10.1007/s11214-017-0438-5

[5] D.H. Grinspoon, Implications of the high D/H ratio for the sources of water in Venus’ atmosphere. Nature 363, 428-431 (1993). doi:10.1038/363428a0