A Search for Molecular Oxygen at the Cloud Top of Venus with Subaru/HDS

Venus is often regarded as Earth’s “sister planet”, yet its atmosphere is fundamentally different, being dominated by CO₂ (~96%). How such a CO₂-rich atmosphere is chemically maintained remains a central unresolved problem in Venus science [1]. In the upper atmosphere, CO₂ is dissociated by ultraviolet radiation into CO and O, while the direct recombination reaction, CO + O → CO₂, is inefficient because it is spin-forbidden. Therefore, additional chemical pathways are required to explain the observed scarcity of CO and O₂ near the cloud-top region. Proposed mechanisms include ClOx catalytic cycles and heterogeneous chemistry on aerosol particles, but these processes remain poorly constrained by observations. In this context, measuring or constraining O₂ at the cloud top provides an important test of CO₂ recycling chemistry.

 

Previous searches for O₂ near the cloud-top region of Venus have been limited to a small number of high-resolution visible spectroscopic observations conducted in 1974, 1982, and 1995 [2-4]. These studies reported upper limits of 3 ppm for the 1974 and 1995 observations and 0.3 ppm for the 1982 observation. Since then, no dedicated observational update has been reported, despite major improvements in telescope aperture, spectrograph performance, and data analysis techniques.

 

Here we present a new search for molecular oxygen at the cloud top of Venus using the High Dispersion Spectrograph (HDS) on the Subaru Telescope. Subaru/HDS provides very high spectral resolution, R ~ 160,000, together with broad spectral coverage, allowing us to combine multiple O₂ absorption lines and improve sensitivity to the extremely weak Venusian signal. The observation was performed on 17 June 2025 under a favorable Earth-Venus Doppler-shift geometry, which helps separate Venusian O₂ features from telluric absorption. Although the total integration time was only a few minutes, the large aperture of Subaru enabled spectra with sufficiently high signal-to-noise ratio for this search.

 

In the presentation, we compare the observed spectra with radiative transfer calculations including both terrestrial and Venusian absorption components, and report the resulting constraint on the cloud-top O₂ abundance.

References: [1] Yung & Demore, 1982, Icarus, 51, 199-247. [2] Traub and Carleton, 1974, in Exploration of the Planetary System, 223-228. [3] Trauger and Lunine, 1983, Icarus, 55, 272-281. [4] Mills, 1999, J. Geophys. Res. 104, 30757–30763.