Three Different Ways to Explain the Sulfur Depletion in the Clouds of Venus. (arXiv:2101.08582v1 [astro-ph.EP])

Three Different Ways to Explain the Sulfur Depletion in the Clouds of Venus. (arXiv:2101.08582v1 [astro-ph.EP])
<a href="http://arxiv.org/find/astro-ph/1/au:+Rimmer_P/0/1/0/all/0/1">Paul B. Rimmer</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Jordan_S/0/1/0/all/0/1">Sean Jordan</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Constantinou_T/0/1/0/all/0/1">Tereza Constantinou</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Woitke_P/0/1/0/all/0/1">Peter Woitke</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Shorttle_O/0/1/0/all/0/1">Oliver Shorttle</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hobbs_R/0/1/0/all/0/1">Richard Hobbs</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Paschodimas_A/0/1/0/all/0/1">Alessia Paschodimas</a>

The depletion of SO$_2$ and H$_2$O in and above the clouds of Venus (45 — 65
km) cannot be explained by known gas-phase chemistry and the observed
composition of the atmosphere. We apply a full-atmosphere model of Venus to
investigate three potential explanations for the SO$_2$ and H$_2$O depletion:
(1) varying the below-cloud water vapor (H$_2$O), (2) varying the below-cloud
sulfur dioxide (SO$_2$), and (3) the incorporation of chemical reactions inside
the sulfuric acid cloud droplets. We find that increasing the below-cloud
H$_2$O to explain the SO$_2$ depletion results in a cloud top that is 20 km too
high, above-cloud O$_2$ three orders of magnitude greater than observational
upper limits and no SO above 80 km. The SO$_2$ depletion can be explained by
decreasing the below-cloud SO$_2$ to 20 ppm. The depletion of SO$_2$ in the
clouds can also be explained by the SO$_2$ dissolving into the clouds, if the
droplets contain hydroxide salts. These salts buffer the cloud pH. The amount
of salts sufficient to explain the SO$_2$ depletion entail a droplet pH of
$sim 1$ at 50 km. Since sulfuric acid is constantly condensing out into the
cloud droplets, there must be a continuous and pervasive flux of salts of
~1e-13 mol cm$^{-2}$ s$^{-1}$ driving the cloud droplet chemistry. An
atmospheric probe can test both of these explanations by measuring the pH of
the cloud droplets and the concentrations of gas-phase SO$_2$ below the clouds.

The depletion of SO$_2$ and H$_2$O in and above the clouds of Venus (45 — 65
km) cannot be explained by known gas-phase chemistry and the observed
composition of the atmosphere. We apply a full-atmosphere model of Venus to
investigate three potential explanations for the SO$_2$ and H$_2$O depletion:
(1) varying the below-cloud water vapor (H$_2$O), (2) varying the below-cloud
sulfur dioxide (SO$_2$), and (3) the incorporation of chemical reactions inside
the sulfuric acid cloud droplets. We find that increasing the below-cloud
H$_2$O to explain the SO$_2$ depletion results in a cloud top that is 20 km too
high, above-cloud O$_2$ three orders of magnitude greater than observational
upper limits and no SO above 80 km. The SO$_2$ depletion can be explained by
decreasing the below-cloud SO$_2$ to 20 ppm. The depletion of SO$_2$ in the
clouds can also be explained by the SO$_2$ dissolving into the clouds, if the
droplets contain hydroxide salts. These salts buffer the cloud pH. The amount
of salts sufficient to explain the SO$_2$ depletion entail a droplet pH of
$sim 1$ at 50 km. Since sulfuric acid is constantly condensing out into the
cloud droplets, there must be a continuous and pervasive flux of salts of
~1e-13 mol cm$^{-2}$ s$^{-1}$ driving the cloud droplet chemistry. An
atmospheric probe can test both of these explanations by measuring the pH of
the cloud droplets and the concentrations of gas-phase SO$_2$ below the clouds.

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