Abstract EANA2025-157

The measurement of the abundance of water vapor and the deuterium to hydrogen ratio on the atmosphere of Venus, is crucial to understand its role on the evolution of habitability with time [1] and on the formation of the sulfuric acid clouds, that completely cover the planet [2]. Constraints on the current and past climate can be obtained by measuring water vapor at different altitudes, using ground-based high spectral resolution spectrographs, in the Near-Infrared (NIR) [3], and future space-based missions such as EnVision (ESA) [4]. Moreover, water vapor has been proposed to be a species possibly connected with volcanic plume degassing on Venus [5]. The amount of water vapor on volcanic plumes can help constrain the abundance of water on the planet’s interior, which can provide important clues regarding climate evolution and a possible past ocean [5].

Water vapor can be measured at several altitudes using the nightside thermal emission windows at 1.18 μm (0-15 km), 1.74 μm (20-30 km) and 2.3 μm (30-45 km) [6], which sense different altitude ranges. Measurements of the D/H ratio and its temporal and spatial variations are also crucial to understand the evolution of the atmosphere of and its habitability through time [1]. The first determination of the D/H ratio of Venus was made by Pioneer Venus, with an estimate of D/H = 157 ± 30 times the terrestrial ratio [7]. Ground-based observations of Venus’ nightside, with the Canada-Hawaii-France-Telescope, estimated the D/H ratio as 120 ± 40 [8, 9]. This value can be explained either by a lost primordial ocean through photodissociation and hydrogen escape [10], or by a balance between lost by escape and a source, such as volcanic outgassing [11]. Further measurements are needed to distinguish these scenarios.

In this work, we will summarize our current knowledge of water vapor on Venus and highlight some recent modeling and observational efforts. On the modeling part, we will present the sensitivity of Venus’ NIR spectrum to volcanic gas plumes of H2O, using the Planetary Spectrum Generator (PSG) tool [12], and discuss the possibility of detection of these plumes [13]. On the observational part, we present results of ground-based campaigns, in the near-infrared, using the iSHELL/IRTF spectrograph, on Mauna Kea (Hawaii), during 2024-2025. We aim to provide updated constraints on the D/H ratio and of H2O on the atmosphere of Venus.

References. (1) Encrenaz et al. (2023), A&A (2) Krasnopolsky et al. (1994), Icarus (3) Bézard et al. 2007, JGR: Planets (4) Robert et al. (2021), Europlanet Science Congress 2021 (5) Wilson et al. (2024), Space Science Reviews (6) Tsang, C. C. C. et al. (2008), Journal of Quant. Spect. and Rad. T. [7] Donahue et al. (1992), JGR: Planets [8] de Bergh et al. (1991), Science [9] Bézard et al. (1990), Nature [10] Donahue et al. (1982), Science [11] Grinspoon and Lewis (1988), Icarus [12] Villanueva et al. (2018), Journal of Quant. Spect. and Rad. T [13] Dias et al. (2025), Icarus