Molecular Monday: Venus’s (Formerly) Unknown Absorber

Today’s post is part of a bi-weekly series here on Planet Pailly called Molecular Mondays, where we take a closer look at the atoms and molecules that make up our physical universe.

Okay, I know I said March would be Mars Month here on Planet Pailly, but for today’s episode of Molecular Mondays, we really must talk about the latest news from Venus. Our best lead for finding life on Venus has just dried up.

The idea of life on Venus has always been a long shot, but for the last few decades planetary scientists have been puzzled by a mysterious something in the Venusian atmosphere. A something that absorbs large quantities of light in the ultraviolet and near-ultraviolet part of the spectrum. This unknown UV absorbing substance has come to be known as the “unknown absorber.”

In his book Venus Revealed, planetary scientist David Grinspoon hypothesizes that the unknown absorber could perhaps maybe possibly be a “photosynthetic pigment” similar to chlorophyll. If so, that would mean there are little, photosynthetic microorganisms swarming about in Venus’s atmosphere, gobbling up UV radiation and converting it into usable energy. This hypothesis is extremely unlikely—Grinspoon makes that abundantly clear—but we could never rule the idea out entirely.

Except, unfortunately, we can now rule this idea out entirely. The unknown absorber has been identified. It’s not a photosynthetic molecule. It’s not even a particularly complicated molecule. It’s just a simple sulfur/oxygen compound called disulfur dioxide.

The story goes like this: sulfur monoxide (SO), which is fairly common in Venus’s atmosphere, combines with itself to form disulfur dioxide (S₂O₂). Specifically, it creates two different versions (or isomers) of disulfur dioxide called cis-OSSO (which has its oxygen atoms oriented in the same direction, as pictured above) and trans-OSSO (which has its oxygen atoms oriented in opposite directions). Then when cis- and trans-OSSO absorb ultraviolet light, they break back down into sulfur monoxide, and the cycle begins anew.

So our best hope for finding life on Venus appears to be gone. Oh well. It was a long shot anyway. I still have high hopes for finding life (probably fossilized life) on Mars.