Phosphine Gas in the Cloud Decks of Venus

Brief

Greaves et al. (2020) report millimeter-wave spectroscopic detections of the PH₃ 1-0 rotational transition in Venus's atmosphere using the James Clerk Maxwell Telescope (June 2017, SNR 4.3) and ALMA (March 2019, SNR 13.3 whole-planet). Radiative transfer modeling against the JCMT spectrum yields a best-fit phosphine abundance of ~20 ppb, with a range of 20-30 ppb depending on unmeasured CO₂ pressure-broadening coefficients. The team evaluated ~75 thermodynamic reactions under thousands of conditions (temperatures 270-1500 K; pressures 0.25-10,000 bar), finding free-energy deficits of 10-400 kJ/mol against PH₃ formation; photochemical routes fall short of the required ~10⁶-10⁷ molecules cm⁻² s⁻¹ production flux by four to six orders of magnitude. The authors conclude that unknown photochemistry, geochemistry, or biological production remains the only viable class of explanation.

Metadata

Category

Search

Venue

Nature Astronomy

Type

Peer-reviewed

Year

2020

Authors

Jane S. Greaves, Anita M. S. Richards, William Bains, Paul B. Rimmer, Hideo Sagawa, David L. Clements, Sara Seager, Janusz J. Petkowski, Clara Sousa-Silva, Sukrit Ranjan, Emily Drabek-Maunder, Helen J. Fraser, Annabel Cartwright, Ikechukwu Aniebo, Giusi Micela, Steven Miller, Claudio Provost, Hitoshi Sagawa, Jonathan Morse, Thomas Friberg

DOI

10.1038/s41550-020-1174-4

arXiv

2009.06593

Access

Open access

Length

2.3 M

Instruments

James Clerk Maxwell Telescope (JCMT), Atacama Large Millimetre/submillimetre Array (ALMA)

Data sources

JCMT spectral observations June 2017, ALMA interferometric observations March 2019, Venus International Reference Atmosphere (VIRA), Vega descent probe phosphorus abundance data

Tags

biosignature, atmospheric spectroscopy, astrobiology, Venus, phosphine, SETI-adjacent

Key points

• JCMT (June 2017) detected PH₃ 1-0 absorption at SNR 4.3 (whole planet); ALMA (March 2019) confirmed at SNR 13.3, with mid-latitude zones reaching SNR 14.5, two independent facilities, two independent reduction pipelines.p.6
• Best-fit PH₃ abundance is ~20 ppb from JCMT radiative transfer; accounting for CO₂ pressure-broadening uncertainty (0.186-0.286 cm⁻¹/atm), the range extends to ~30 ppb, with an additional ±6 ppb from line-to-continuum uncertainty.p.9
• Thermodynamic analysis of ~75 reactions across thousands of P-T-composition conditions finds PH₃ formation energetically disfavored by 10-400 kJ/mol throughout any plausible Venusian atmosphere, surface, or subsurface.p.9
• Maximum photochemical PH₃ production rates fall short of observed abundance by factors of 10⁴-10⁶; the bottleneck is scarcity of H radicals in Venus's low-H₂O atmosphere.p.10
• Lightning production falls short by ≥10⁷×; sustaining ppb-level PH₃ volcanically would require >200× Earth's volcanic activity, inconsistent with orbiter topography.p.11
• The closest spectral contaminant (SO₂ transition offset +1.3 km/s) contributes <10% to the integrated line depth and shifts the centroid by <0.1 km/s; an SO₂ explanation would require cloud temperatures twice those observed.p.7
• Phosphine is undetected at polar latitudes (3σ upper limit −0.29 × 10⁻⁴ l:c), with the detection boundary agreeing within ~10° of the proposed upper Hadley-cell boundary at ~60° latitude.p.11
• PH₃ atmospheric lifetime is ≤10³ years at all altitudes, requiring continuous replenishment at a flux of ~10⁶-10⁷ molecules cm⁻² s⁻¹, comparable to (but below) the output of some terrestrial anaerobic ecosystems at 10⁷-10⁸ cm⁻² s⁻¹.p.10

Verbatim

• “The presence of phosphine is unexplained after exhaustive study of steady-state chemistry and photochemical pathways, with no currently-known abiotic production routes in Venus' atmosphere, clouds, surface and subsurface, or from lightning, volcanic or meteoritic delivery .”

  p.1

• “Atmospheric PH 3 at ~20 parts-per-billion abundance is inferred.”

  p.1

• “We find that PH 3 formation is not favored even considering ~75 relevant reactions under thousands of conditions encompassing any likely atmosphere, surface, or subsurface properties (temperatures of 270-1500 K, atmospheric and subsurface pressures of 0.25-10,000 bar, wide range of concentrations of reactants).”

  p.9

• “We find that PH 3 -production by Venusian lightning would fall short of few-ppb abundance by factors of 10 7 or more.”

  p.11

• “If no known chemical process can explain PH 3 within the upper-atmosphere of Venus, then it must be produced by a process not previously considered plausible for Venusian conditions.”

  p.11

• “We conclude that the candidate detection of phosphine is robust, for four main reasons.”

  p.7

Most interesting

• ALMA's exclusion of all baselines shorter than 33 m, required to suppress interferometric artifacts from Venus's bright disk, caused spatial filtering losses of 60-92% depending on latitude, meaning the reported ALMA line depths are lower bounds on true abundance.
• Any spectral contaminant mimicking the PH₃ 1-0 feature would have to coincide in rest-wavelength to within ~10⁻⁶, a constraint that eliminates every species in current molecular databases.
• Trace phosphine in Earth's atmosphere exists only at parts-per-trillion levels and is uniquely tied to microbial or anthropogenic activity, its detection threshold on Venus at ~20 ppb is roughly ten thousand times higher than Earth's background.
• The total observing time required to reach this result was under 10 hours on-source across both facilities, a modest allocation for a claim of this significance.
• Phosphine's polar non-detection aligns within ~10° of the theoretical upper Hadley-cell boundary at ~60° latitude, a zone independently proposed as the most stable environment for hypothetical Venusian aerial microbes due to 70-90 day circulation timescales.
• Formation of phosphorous acid (H₃PO₃) as an intermediate route to PH₃ was quantitatively ruled out: the chemistry would require an atmosphere composed almost entirely of hydrogen, a condition physically unrealized on Venus.