Production of Ammonia Makes Venusian Clouds Habitable and Explains Observed Cloud-Level Chemical Anomalies

Ammonia cycle in the atmosphere of Venus. See SI Section 10 for details. (I) NH3 is produced locally in the clouds from atmospheric N2 and H2O (Table 1) by metabolically active microorganisms (black dots) inhabiting cloud droplets (white circle). (II) The production of NH3 in the droplet raises the droplet pH to -1 to 1 (from -11 on the Hammett acidity scale) by trapping the SO2 and H2O in the droplet as ammonium hydrogen sulfite (NH4HSO3). The production of sulfite salts in the droplet leads to the formation of a large, semi-solid (and hence non-spherical) Mode 3 particle (white decagon). (III) The Mode 3 particle settles out of the clouds where ammonium sulfite disproportionates to ammonium sulfate and ammonium sulfide; the latter decomposes to H2S and NH3, which in turn undergo photochemical reactions to a variety of products. (V) Disproportionation and gas release break up the Mode 3 particles into smaller haze particles and microorganism spores (black ovals), some of which return to the cloud layer (V). (VI) The ammonium sulfate particles fall further below the cloud decks, where ammonium sulfate decomposes to SO3, NH3 and H2O. (VII) Spores released at this stage may be unviable (grey ovals), but any surviving could also be eventually transported back to the clouds.

The atmosphere of Venus remains mysterious, with many outstanding chemical connundra. These include: the unexpected presence of ~10 ppm O2 in the cloud layers; an unknown composition of large particles in the lower cloud layers; and hard to explain measured vertical abundance profiles of SO2 and H2O.

We propose a new hypothesis for the chemistry in the clouds that largely addresses all of the above anomalies. We include ammonia (NH3), a key component that has been tentatively detected both by the Venera 8 and Pioneer Venus probes. NH3 dissolves in some of the sulfuric acid cloud droplets, effectively neutralizing the acid and trapping dissolved SO2 as ammonium sulfite salts. This trapping of SO2 in the clouds together with the release of SO2 below the clouds as the droplets settle out to higher temperatures, explains the vertical SO2 abundance anomaly.

A consequence of the presence of NH3 is that some Venus cloud droplets must be semi-solid ammonium salt slurries, with a pH~1, which matches Earth acidophile environments, rather than concentrated sulfuric acid. The source of NH3 is unknown, but could involve biological production; if so, then the most energy-efficient NH3-producing reaction also creates O2, explaining the detection of O2 in the cloud layers.

Our model therefore predicts that the clouds are more habitable than previously thought, and may be inhabited. Unlike prior atmospheric models, ours does not require forced chemical constraints to match the data. Our hypothesis, guided by existing observations, can be tested by new Venus in situ measurements.

William Bains, Janusz J. Petkowski, Paul B. Rimmer, Sara Seager

Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Atmospheric and Oceanic Physics (
Journal reference: Proceedings of the National Academy of Sciences Dec 2021, 118 (52)
DOI: 10.1073/pnas.2110889118
Cite as: arXiv:2112.10850 [astro-ph.EP] (or arXiv:2112.10850v1 [astro-ph.EP] for this version)
Submission history
From: Janusz Petkowski
[v1] Mon, 20 Dec 2021 20:37:17 UTC (2,164 KB)

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