A new constraint on HCl abundance at the cloud top of Venus

Abstract

We provided a new constraint on HCl abundance at the cloud top of Venus by infrared spectroscopy using a cross-dispersed high-resolution echelle spectrograph, iSHELL, mounted on the NASA Infrared Telescope Facility (IRTF). This study investigated the inconsistency in HCl abundance between previous ground-based observations and Venus Express solar occultation measurements. Venusian dayside observations at a solar phase angle of ∼90° were conducted during August 6–7, 2018, and August 18–20, 2020 (UT), when the Venusian afternoon and morning sides were visible, respectively. The high spectral resolving power of ∼80,000 and large Doppler shift (∼13 km/s) enabled the measurement of Venusian lines with less contamination by terrestrial lines. We analyzed the H35Cl P(5) and H37Cl P(6) lines at 2775.8 and 2750.1 cm−1, respectively, in the 1–0 band, together with 16O12C18O P- and R-branch lines of the 20001–00001 band, which fell in the same spectral orders as the HCl lines. The 16O12C18O lines were used to derive the cloud top altitude because the upper clouds significantly impacted the retrieval of HCl abundance. The cloud tops had an equatorially symmetric structure. The average altitude was 70.8 ± 0.6 km in the region equatorward of ±30° and decreased toward higher latitudes. The HCl volume mixing ratio was derived as 0.379 ± 0.013 ppm at a probing altitude of 70.6 ± 1.1 km and showed no significant latitudinal dependence within the range of ±70°. A difference of ∼0.02 ppm between 2018 and 2020 would result mainly from temporal variation. The H35Cl/H37Cl abundance ratio was 3.01 ± 0.16, with no prominent latitudinal dependence. The obtained HCl volume mixing ratio agreed with the results of previous ground-based measurements, which were approximately one order of magnitude larger than those derived from Venus Express solar occultation measurements. The systematic uncertainties in our retrieval analysis cannot explain this significant inconsistency. The impact of diffuse light produced from aerosol scattering on the retrieval method for solar occultation measurements should be investigated further to solve this issue.