Ground-based measurements of HCl abundance at the cloud top of Venus

The atmosphere of Venus can be vertically divided into three regions with different chemical conditions. High temperature and pressure and the absence of effective photolysis processes are dominant in the lower atmosphere up to 60 km. The middle atmosphere between 60 and 110 km is controlled by photochemistry driven by solar UV radiation. In the upper atmosphere above 110 km, dissociation, ionization, and ionospheric reactions are important processes.

HCl is the primary chlorine reservoir in the Venus’ atmosphere below 110 km. Highly reactive chlorine species (ClO_x) is produced by solar UV photolysis of HCl and has been proposed to play an important role in catalysis of CO and O recombination to CO₂, thereby stabilizing the CO₂ atmosphere. Chlorine chemistry is also linked to sulfur chemistry and its understanding is necessary to explain the observed vertical distribution of SO₂.

Interestingly, there is a large inconsistency between the HCl abundances measured by spacecraft and ground-based telescopes. The SOIR instrument onboard Venus Express measured its abundance as less than ~50 ppb at the cloud top (~70 km) increasing with altitude, reaching to 1 ppm in the upper atmosphere (~110 km) [Mahieux et al., 2015]. Such a vertical trend conflicts with the results obtained by sub-mm ground-based observations which inferred a vertically constant profile (up to ~80 km) [Sandor and Clancy, 2012]. Near-infrared ground-based observations also showed the HCl abundance at the cloud top as ~500 ppb [Iwagami et al., 2008; Krasnopolsky, 2010], which are nearly one order of magnitude larger than the SOIR results. The reason for this inconsistency has not been understood yet.

In order to re-examine HCl abundance at the cloud top, we carried out a high-resolution spectroscopy of Venus’ dayside at wavelengths of 3.580-3.934 μm with IRTF/iSHELL on August 5-7, 2018 and August 18-20, 2020 (UT). Venus was near its greatest eastern and western elongations, respectively, in the observation periods. Taking the full advantages of iSHELL’s high spectral resolution of R ~ 75,000, iSHELL resolved individual HCl lines with sufficient separation from terrestrial lines. We analyzed three cross-dispersed echelle orders (orders 141, 142, and 144). For each order, retrievable lines of HCl³⁵, HCl³⁷, and O¹⁶C¹²O¹⁸ were included. With using radiative transfer modeling, HCl³⁵ and HCl³⁷ abundances were derived after cloud top altitude was retrieved from several O¹⁶C¹²O¹⁸ lines. Our preliminary results showed that HCl abundance at the cloud top is at least larger than 100 ppb and does not vary with the observation period (i.e., no difference between the morning and evening hemispheres).

In this presentation, we show latitudinal distribution of HCl abundance and its isotopic (HCl³⁵/HCl³⁷) ratio at the cloud top, retrieved from the iSHELL spectra and compare them with the previous studies.