How Much Life Would Be Required to Create the Phosphine Signal on Venus?

Recently we shared the news that astronomers may have found signs of life on Venus.

Phosphine gas detected in the clouds of Venus – a potential indicator of life or “biosignature.” Now some gases might be a false positive for biosignatures because they can be created by other chemical processes on a planet like photochemical processes in the atmosphere or geological processes beneath the surface that create a given gas. For example, methane can also be a biosignature, and we’ve been hunting it down on Mars, but we know that methane can also be created geologically. Finding phosphine in Venusian clouds is truly remarkable because we don’t presently know of any way to create phosphine abiotically or without life being a part of the equation. Question is – how much life??

Once a biosignature is discovered, a method to rule out false positives is to look at the concentration of the gases in question and see if a plausible amount of life could generate the gas. Phosphine gas in Venusian clouds was detected at concentrations of 20 ppb (parts per billion). If the required biomass to create this concentration of gas is high, then an otherwise unfamiliar abiotic process may still be at work. Because while Venus may have life, requiring high concentrations of life on a world generally thought to have zero surface habitability starts lowering your alien credibility.

“We find that the typical biomass densities predicted by our simple model are several orders of magnitude lower than the average biomass density of Earth’s aerial biosphere.” – Lingam and Loeb 2020

In other words, far less life would have to live in the clouds of Venus to create the level of phosphine we’ve detected than the amount of life living in the clouds of our own planet – a plausible amount of life. That is really exciting because it means that we can still count life as a possible source of the phosphine gas. A small amount of possible life giving off a signal we can see from Earth letting us know it’s there. Were the amount of required biomass really high, we might then have to look for other abiotic processes we’re not aware of as it is less likely that high concentrations of life exists on Venus.

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