Measuring Isotopic Ratios on Venus’ Dayside using iSHELL/IRTF Observations

The measurement of isotopic ratios on Venus is crucial to track atmospheric loss though time, constrain planetary origin and trace surface-atmosphere interactions [1]. The deuterium to hydrogen ratio (D/H) is found to be 157 times greater than that of Earth's oceans [2, 3], suggesting Venus once had a larger quantity of water. The carbon and oxygen isotopic ratios, ¹²C/¹³C and ¹⁶O/¹⁸O, have been measured by the Pioneer Venus and Venera missions [4, 5], as well as through ground-based infrared spectroscopy [6, 7, 8, 9]. They are consistent with Earth’s ratios within uncertainties (¹²C/¹³C ~ 89, ¹⁶O/¹⁸O ~ 500), suggesting little escape or photochemical alteration of CO₂ through time.

We present new simultaneous measurements of bulk-averaged ratios of ¹⁶O/¹⁸O, ^¹6O/¹⁷O and ¹²C/¹³C, measured on CO₂, on Venus’ dayside, from high spectral resolution (R ~ 80 000) iSHELL/IRTF observations, using the H3 mode (1.64-1.82 µm), performed on the 2^nd February 2024. Venus angular diameter was 12.2'', with an illuminated fraction of 86.1 %. Wavelength calibration, flat fielding, dark correction and sky subtraction were performed using the Spextool data reduction software [10, 11]. Absolute flux calibration could not be performed since the SNR of our standard star was too low.

We selected spectral order 314 (1.660–1.667 µm) to estimate the ¹⁶O/¹⁸O ratio, order 316 (1.650–1.657 µm) for the ¹⁶O/¹⁷O ratio, and order 311 (1.676–1.683 µm) to determine the ¹²C/¹³C ratio. Pairs of adjacent isotopologue lines were selected, with a minimum telluric and solar contamination. Forward models were simulated using ASIMUT-ALVL [12] for a range of possible isotopic ratios (scalers 0.85-1.15). For each line pair, the line depth ratio method [7, 13, 14] was used to obtain the best fit isotopic ratio, by comparison of the measured line depth ratio with the modeled one through a simple linear interpolation.

The following preliminary weighted averaged isotopic ratios were computed: ¹⁶O/¹⁸O ~ 507 ± 50, ¹⁶O/¹⁷O ~ 2745 ± 152 and ¹²C/¹³C ~ 90 ± 6, which are consistent with previously reported values [7, 8, 9]. The main limitations of our current measurements arise from uncertainties in line intensities (approximately 5%, based on the HITRAN database), the lack of an absolute flux calibration source with sufficiently high signal-to-noise ratio and uncertainties in the assumed temperature profile in the forward modeling. Ongoing analysis of data from mode H3 (1.64-1.82 µm), and from mode K3 (2.26-2.55 µm), which covers the OCS isotopologues absorption features at 2.42-2.46 µm, will be explored to get an estimate of the ratios of ³⁵Cl/³⁷Cl and ³²S/³⁴S, respectively,  opening an opportunity to shed some light on current volcanic outgassing processes, photochemistry and sulphur cycle dynamics.

References. (1) Avice et al. 2022, Space Science Reviews (2) Donahue et al., 1997, Science (3) Bézard et al. 2007, JGR Planets (4) Hoffman et al. 1980, JGR Space Physics (5) Istomin et al. 1980, 23rd COSPAR meeting, Budapest, Hungary (6) Kuiper et al. 1962, Comm. Lunar Planet. Lab (7) Beźard et al. 1987, Icarus (8) Hedelt et al. 2011, A&A (9) Iwagami et al. 2015, Planetary and Space Science (10) Cushing et al. 2004, PASP 116 (11) Vacca et al. 2003, PASP 115 (12) Vandaele et al. 2006, ESRIN, Italy (13) Krasnopolsky et al. 2010, Icarus (14) Encrenaz et al. 2012, A&A

Funding. JAD acknowledges funding through the research grants UIDB/04434/2020 and UIDP/04434/2020 and a fellowship grant 2022.09859.BD. SR acknowledges funding by the Belgian Science Policy Office (BELSPO) with the financial and contractual coordination by the ESA Prodex Office (PEA 4000144206).