Vega X-Ray Fluorescence Spectroscopy: Chemical Composition of Venus Clouds

Abstract:

Vega 1 and 2 were twin Russian missions, each of those deployed a landing probe and a balloon to Venus in June 1985 and used the Venus gravity to be directed to and flyby through comet Halley in March 1986. XRF (Andreychikov, B.M., et al., 1987, Cosmic Res. 25, 721) was designed to measure Cl, S, P, Fe collected on the filter at 62 to 47 km on Venus (Fig. 1). The deposit increased linearly down to 51.5 km with equal quantities of sulfur and chlorine and no phosphorus. Phosphorus dominates below 51.5 km with traces of sulfur and no chlorine. Interpretation of the results was made by Krasnopolsky (1989, Icarus 80, 202-210).

The abundance of Fe is low and corresponds to ≈1% of FeCl₃ in the cloud mass loading. (Recently Perez-Hoyos et al. (2018, JGRE 123, 145-162) calculated the NUV absorption using erroneously the FeCl₃ spectrum in ethyl acetate instead of that in H₂SO₄.)

If phosphorus exists in the atmosphere, then thermochemistry predicts dimer P₄O₆ as its major component. It can react with H₂SO₄ and form phosphoric acid H₃PO₄ that precipitates and gradually loses water converting into phosphoric anhydride P₂O₅. It exists as aerosol down to 25 km and then evaporates. There are two facts in favor of this explanation: the observed aerosol extended down to 33 km where the nephelometer was switched off, and the similar aerosol altitude distributions was observed by Venera 8.

The significant excess of Cl over Fe requires another chlorine aerosol species, in addition to FeCl₃. AlCl₃ is the best candidate that could condense in the middle cloud layer with the densities close to those measured. Composition of the clouds based on the Vega XRF data is given in the table.

According to Greaves et al. (2020), the observed absorption line at 266.94 GHz originates near 56 km and is a sum of PH₃ (90%) and SO₂ (10%). Expected contributions of the species must be proportional to products of their mixing ratio and line strength, that is, 2×10^-8 and 1.23×10^-22 cm for PH₃ and 10^-5 and 3.15×10^-23 cm for SO₂. (Line strengths are from HITRAN.) Hence the expected effect of SO₂ exceeds that of PH₃ by two orders of magnitude.