Ground-based observations using IRTF/iSHELL for monitoring trace gases near the cloud tops of Venus

The Venusian atmosphere serves as a natural laboratory for understanding atmospheric chemistry that cannot be observed in the terrestrial atmosphere. Different chemical processes occur at different altitudes: photochemistry driven by solar UV radiation in the upper atmosphere, cloud formation- and dissipation-related chemistry in the cloud layer, and thermodynamic equilibrium controlled by high temperatures in the lower atmosphere. Another categorization can be made in terms of chemical cycles, such as, sulfur oxides (SO_x), carbon oxides (CO_x), and chlorides (Cl_x). Observational constraints on the spatial and temporal variability of trace gases associated with these cycles can help us better understand atmospheric chemistry.

We have observed high-resolution (R ~ 75,000) spectra of the Venusian dayside using iSHELL mounted on the 3.2-m NASA Infrared Telescope Facility (IRTF) on Mauna Kea since 2018. The iSHELL instrument is a cross-dispersed high-resolution echelle spectrograph operating over the wavelength range of 1.06–5.3 μm. Twenty standard observing modes with different wavelength coverages are available by selecting one of the six cross-dispersing gratings (J, H, K, L, L’, and M) and adjusting the tilt position. By fully exploiting the capabilities of iSHELL, we can retrieve the abundances of HDO, CO, HF, HCl, OCS, and their isotopologues near the Venusian cloud tops. To date, we have accumulated 23 days of successful observations over four observing periods (2018, 2020, 2023, and 2025). Among them, our first results focusing on the HCl abundance near the cloud tops have already been published (Sato and Sagawa, 2023).

Because the observations were conducted using various observing modes and slit lengths after the publication, we revisited and refined the data calibration methods and radiative transfer model. For example, we developed a method to estimate the slit position on the Venusian disk from guide images of Venus taken with the 3.46-μm filter by fitting the limb of the Venusian dayside disk, independent of the slit length (5”, 15”, or 25”). In addition, we estimated the instrument response function for each observing mode using multiple Ar-Th lamp emission lines fitted with a combination of two Gaussian functions: one symmetric and the other asymmetric. The molecular absorption database used in the radiative transfer model was updated to HITRAN2024. In this presentation, we provide an overview of our observations conducted so far. We will present initial results on the retrieval of CO and its isotopologues near the cloud tops.