Uncertainty in phosphine photochemistry in the Venus atmosphere prevents a firm biosignature attribution

¹ Institut für Planetenforschung, Deutsches Zentrum für Luft- und Raumfahrt, Rutherfordstraße 2, 12489 Berlin, Germany
e-mail: wunderlich.fabian@googlemail.com
² Zentrum für Astronomie und Astrophysik, Technische Universität Berlin, Hardenbergstraße 36, 10623 Berlin, Germany
³ Institut für Geologische Wissenschaften, Freie Universität Berlin, Malteserstr. 74-100, 12249 Berlin, Germany

Received: 29 October 2021
Accepted: 28 March 2023

Abstract

Context. The possible detection of phosphine (PH₃) in the clouds of Venus has raised the question as to which processes could produce such large abundances of PH₃. Previous studies suggested that abiotic processes including photochemical production cannot explain the claimed PH₃ concentrations. However, the photochemistry of phosphorus-bearing species in the atmosphere of Venus is not well known.

Aims. We aim to assess the abiotic production of PH₃ considering the effect of uncertainties in the chemical rate coefficients of phosphorus-containing reactions.

Methods. Using a photochemical column model, we simulated Venus-like conditions and varied the chemical rate coefficients with a Monte Carlo (MC) approach in order to estimate the associated error in the PH₃ abundances throughout the atmosphere.

Results. Current uncertainties and missing data in photochemical rate coefficients lead to a variation of about six orders of magnitude in the modelled PH₃ abundance on Venus, assuming photochemical production of PH₃ from tetraphosphorus hexoxide (P₄O₆) pathways. Our results suggest an abiotically produced upper limit of 2 ppb PH₃ between 50 and 60 km. These concentrations are in the range of a recent reanalysis of Atacama Large Millimeter Array (ALMA) data, suggesting planet-averaged abundances in PH₃ of 1–4 ppb above 55 km. Future observations of phosphorus monoxide (PO) on Venus would be beneficial for increasing our confidence in assessing PH₃ as a biosignature.

Conclusions. We conclude that due to the large uncertainties in phosphorus chemistry, even a firm detection of several ppb PH₃ in the Venus atmosphere would not necessarily mean a biological origin.