Phosphine Generation Pathways on Rocky Planets

Abstract

The possibility of life in the venusian clouds was proposed in the 1960s, and recently this hypothesis has been revived with the potential detection of phosphine (PH 3 ) in Venus' atmosphere. These observations may have detected *5-20 ppb phosphine on Venus (Greaves et al., 2020), which raises questions about venusian atmospheric/geochemical processes and suggests that this phosphine could possibly be generated by biological processes. In such a claim, it is essential to understand the abiotic phosphorus chemistry that may occur under Venus-relevant conditions, particularly those processes that may result in phosphine generation. Here, we discuss two related abiotic routes for phosphine generation within the atmosphere of Venus. Based on our assessment, corrosion of large impactors as they ablate near Venus' cloud layer, and the presence of reduced phosphorus compounds in the subcloud layer could result in production of phosphine and may explain the phosphine detected in Venus' atmosphere or on other rocky planets. We end on a cautionary note: although there may be life in the clouds of Venus, the detection of a simple, single gas, phosphine, is likely not a decisive indicator.

Photochemical and thermochemical pathways to S2 and polysulfur formation in the atmosphere of Venus

“…Because of the complexity of the sulfur chemistry in the middle atmosphere, which includes many species of the form S x O y Cl z , and because of the paucity of laboratory rate coefficient data for most of the reactions among these species, high-level ab initio calculations are essential for understanding the chemistry of the Venus atmosphere. Those improvements are needed also to guide the next terrestrial measurements, spacecraft missions (recently announced by the National Aeronautics and Space Administration motivated in part by the studies on the phosphine detection in Venus atmosphere) [59][60][61][62][63] , and to assist with geoengineering of Earth's climate and monitor stratospheric volcanic eruptions' clouds. Furthermore, they are beneficial to better understand the atmosphere of early Earth (pre-oxygenation of Earth's atmosphere) and shall be used in describing Earth-like exoplanet atmospheres in conjunction with future high-resolution spectroscopy measurements of said exoplanets.…”

Sources of Nitrogen-, Sulfur-, and Phosphorus-Containing Feedstocks for Prebiotic Chemistry in the Planetary Environment

“…Phosphine (PH 3 ), the most reduced form of phosphorus, has recently been the source of significant attention due to its potential detection in the clouds of Venus [ 240 ]. The abiotic production of phosphine may be possible on rocky planets from the ablation of large impactors near a thick cloud layer (such as the Venusian clouds) or the presence of reduced phosphorus compounds in a subcloud layer [ 241 ].…”

Phosphine reactivity and its implications for pyrolysis experiments and astrochemistry

“…During pyrolysis or in the ISM, consecutive PH 3 dissociations are expected by homolytic fission of the P-H bond [4,33,34]. The unimolecular processes should compete with other bimolecular reactions of PH 3 [4,28,73,74]. However, the dissociation must be analyzed to infer their role in forming P-bearing molecules in the ISM.…”

“…In the giant planets, phosphorus is found in the PH 3 form, and the volume mixing ratio is about 10-6 on Jupiter [86,87]. However, the presence of such element in the deck clouds of Venus is still under debate [74,[88][89][90][91]. Though PH 3 is not the most thermodynamically favorable form of gas-phase phosphorus at temperatures of the Venus atmosphere, PH 3 formed in the hot deep layers should be brought to the top of the atmosphere through convective transport.…”