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

Kavli Affiliate: Sara Seager

| First 5 Authors: William Bains, Janusz J. Petkowski, Paul B. Rimmer, Sara Seager,

| Summary:

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.

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