Monitoring trace species and cloud opacity in the lower atmosphere of Venus using IRTF/iSHELL

Our results. We present preliminary new simultaneous measurements of cloud mode 3 aerosol opacity, CO, H₂O abundances and the D/H ratio below the nightside clouds of Venus (30-45 km), using IRTF/iSHELL observations in the K3 mode (2.26-2.55 µm), obtained on 6th-25th February 2025. The 1.5x5’’ slit was used, with an effective spectral resolution of ~25 000. Scans of Venus were performed by placing the slit next to the limb of the planet and then drifiting it across the disk towards the terminator. We investigated possible correlations between mode 3 opacity, CO and H₂O abundances, and with the D/H ratio, and compared with previous studies [1,2,3,4].

Trace species and cloud opacity correlations. Our goal is to investigate correlations between trace gases and cloud morphologies, to constrain models of upwelling and downwelling, cloud formation and dissipation in Venus’ atmosphere, which are still poorly understood [5,6,7]. Simultaneous cloud and trace species maps allows us to study the coupled dynamics, chemistry and cloud microphysics, with some hints of correlations in previous studies [8,9]. Maps of CO and OCS were found to be anti-correlated in latitude [2,10] and may constrain circulation due to the Hadley-cell and photochemistry processes.

The vertical gradient of H₂O. Moreover, we investigated the vertical gradient of H₂O and the D/H ratio (0-45 km), using available IRTF/iSHELL data from the 1.74 µm and 1.18 µm emission windows. Measuring H₂O below the clouds is crucial to understand its role in the greenhouse effect, on the formation of sulfuric acid clouds [11] and on surface-atmosphere reactions [12], The D/H ratio can interpreted as a tracer of atmospheric evolution. Its high value, ~ 100 D/H (Earth) (VSMOW) [1, 10], could point to a loss of a primordial ocean [13], or alternatively it could reflect a balance between loss by escape and supply by a source, such as volcanic outgassing [14].

Our program aims to provide monitoring of trace species abundances and cloud opacities, which can be investigated in the context of previous observations (Venus Express, Akatsuki) and will be useful for future missions (EnVision, VERITAS), to improve our understanding of the chemical and dynamical cycles in Venus’atmosphere and the divergent evolution between these two planets.

Data treatment. The data reduction, including wavelength calibration, flat-fielding, sky subtraction, telluric correction and flux calibration were performed using the standard Spextool 5.0.3 software [15,16], following the standard procedure from previous observations [17]. Straylight subtraction was done by simulation of the dayside crescent using the Planetary Spectrum Generator [18]. These were appropriately scaled for each nightside spectra. The scaler was determined using the saturation of spectrally resolved CO lines in the 2.32-2.35 µm range, following [17]. Retrievals of minor species abundances were obtained using the ASIMUT-ALVL radiative transfer code, extensivelly used for forward modeling and retrieval of minor species in Earth and Mars atmospheres [19,20,21,22].

Model Input. For atmospheric input, we used the trace species abundance profiles taken from [10] and the temperature-pressure profile from [23]. Linelists were taken from the HITRAN2020 molecular database and are broadened by CO_2.. The CO₂ line profiles were modeled using a sublorentzian lineshape [24]. The opacity is assumed constant in the entire 2.3 µm window, as an approximation [25]. For H₂O, an updated linelist of spectroscopic parameters is used, in which priority is given to the most recent CO₂-collisional parameters computed by [26] for the 1.18 µm, 1.38 µm, 1.74 µm and 2.3 µm windows. More recent calculations from [27] at 2.7 µm and 6 µm were then added. When CO₂-specific parameters were not available, air-collisional parameters from HITRAN2020 were used, with the broadening parameters multiplied by the empirical factor of 1.7, which currently represents the best approximation for CO₂-collisional parameters. We used the cloud particle profiles defined in [28]: mode 1, mode 2, mode 2p and mode 3, with effective radii of 0.49 µm, 1.18 µm, 1.56 µm and 4.25 µm [28]. We defined a cloud model that includes particles of 75 % H₂SO₄ and 25 % H₂O by weight [29].

References. (1) Marcq et al. 2023, Icarus; (2) Arney et al. 2014, JGR:Planets; (3) Bézard et al. 2007, JGR; (4) Pollack et al. 1993, Icarus; (5) Stolzenbach et al. 2023, Icarus; (6) McGouldrick et al. 2021, The Planetary Science Journal; (7) Bierson et al. 2020; (8) Bell et al. 1991; (9) Tsang et al. 2010; (10) Marcq et al. 2006; (11) Krasnopolsky et al. 1994; (12) Fegley et al. 2003; (13) Donahue et al. 1999; (14) Grinspoon et al. 1993; (15) Cushing et al. 2004; (16) Vacca et al. 2003; (17) Marcq et al. 2021; (18) Villanueva et al. 2018; (19) Vandaele et al. 2006; (20) Drummond et al. 2008; (21) Aoki et al. 2021; (22) Trompet et al. 2023; (23) Zasova et al. 2006; (24) Bézard et al. 2011; (25) Tonkov et al. 1996; (26) Ducreaux et al. 2025; (27) Régalia et al. 2019; (28) Haus et al. 2010; (29) Palmer Williams 1975.

Funding. JAD acknowledges funding through the research grants UID/04434/2025 and a fellowship grant 2022.09859.BD.