Phosphorous-Bearing Compounds and Atmosphere-Surface Chemical Interactions on Venus

Abstract:

Although phosphorus (P) has been tentatively detected in the clouds of Venus with 1980’s X-ray fluorescent analysis [1], sources of atmospheric P remain unclear. Here we discuss speciation of P-bearing compounds within the crust and lower atmosphere, and estimate endogenic sources of atmospheric P.

By analogy with the Earth, Mars, and the Moon, phosphates are the most probable P-bearing minerals in Venus' igneous and putative sedimentary and metamorphic rocks. Accessory apatite group minerals are likely the most abundant phosphate minerals. Elevated abundances of phosphates could be manifested in alkaline silicate igneous complexes and carbonatites, which are expected on Venus. Phosphorites could have formed if adequate reservoirs of surface water existed earlier in Venus’ history. Formation of marine phosphorites along shorelines may have been possible and under some circumstances could have been bio-mediated.

Chemical equilibrium models for near-surface atmospheric gases suggest P₄O₆ as the most abundant P-bearing gas [2, 3, our new data]. The contact of this relatively reduced gas with surface phosphates creates a potential for redox reactions that involve P(III) and P(V) compounds. However, chemical equilibrium calculations do not suggest reduction of surface apatite and whitlockite to P₄O₆ gas. At Venus’ surface conditions, these minerals do not react with SO₂ gas to form Ca sulfate and P₄O₆ gas. On Earth, volcanic degassing does not supply P-bearing gases to the atmosphere and our models for Venus do not suggest volcanic delivery of such volatiles. Therefore, Venus’ surface and interior may not supply P to the atmosphere. Interplanetary phosphide-bearing dust sources are possible but cannot account for measurable gas concentrations recently suggested [3].

Concentrations and upper limits of P-bearing gases could be measured with instruments proposed for the DAVINCI+ descent probe [4]. Any in situ measurements of P-bearing gases would assess atmospheric P chemistry and corresponding gas-solid type reactions. Thus, the P sources and sinks could be constrained on modern Venus.

Refs: [1] Andreichikov B. M. et al., 1987, Cosmic Research 25, 554–567. [2] Krasnopolsky V. A., 1989, Icarus 80, 202–210. [3] Bains W. et al., 2020, Astrobiology, submit. [4] Garvin J. et al. 2020, J. Spacecraft Rockets, in review.