Fig. 2.

Ammonia cycle in the atmosphere of Venus. See SI Appendix, 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, semisolid (and hence nonspherical) 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. IV: 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 farther below the cloud decks, where ammonium sulfate decomposes to SO₃, NH₃, and H₂O. VII: Spores released at this stage may be unviable (gray ovals), but any surviving could also be eventually transported back to the clouds.