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New Geochemical Discoveries Reaffirm Earth as a Privileged Planet

Privileged Planet

Each gas component in our atmosphere has a “partial pressure.” The partial pressure of carbon dioxide (CO2) in the atmosphere is represented as pCO2. You could think of it as the percentage of CO2 in the atmosphere compared to the total amount of gas.

If you believe that pCO2 has a major influence on Earth’s surface temperature, then a new PNAS paper has revealed, without calling it that, a significant new fine-tuning parameter for Earth. The paper argues that although CO2 is released into the atmosphere through outgassing (volcanos), thereby increasing Earth’s surface temperature, chemical weathering of continental and seafloor rock into the oceans (where carbon is precipitated out as carbonate rock) serves as a natural sink of CO2. That is, the oceans remove COfrom the atmosphere, thereby decreasing temperature. All of this serves to keep Earth’s temperature stable:

Continental and seafloor weathering buffer Archean surface temperatures to 0–50 °C. This result holds for a broad range of assumptions about the evolution of internal heat flow, crustal production, spreading rates, and the biotic enhancement of continental weathering.

Many origin-of-life theorists hope that the early Earth’s atmosphere was rich in methane so that organic molecules could be produced. But this new fine-tuning parameter removes an argument that methane must have been present on the early Earth to keep the climate warmer:

The seafloor weathering feedback is important, but less dominant than previously assumed. Consequently, the early Earth would not have been in a snowball state due to pCO2 drawdown from seafloor weathering. In principle, little to no methane is required to maintain a habitable surface climate, although methane should be expected in the anoxic Archean atmosphere once methanogenesis evolved.

What’s really interesting is that they say their proposed mechanism of climate stabilization is not just for Earth but for any rocky planet located within the circumstellar habitable zone. If they’re right, we’re not looking at Earth being a “privileged planet” but Earth living in a “privileged universe.”

The latter extrapolation deserves some serious skepticism, however. Most likely this is just a “privileged planet” argument. Why?

Their model uses so many particulars of Earth (e.g., Earth’s mantle composition, ocean composition, continental crustal composition, oceanic crustal composition, weathering patterns, and carbonate deposition patterns) that it’s highly premature to conclude that any rocky planet in the habitable zone would be so privileged to enjoy this same temperature-buffering mechanism. For example, Earth seems peculiar due to its high water content — something that geology still struggles to explain.

Would other rocky planets enjoy the same? Nobody knows. Until we discover rocky planets out there in the habitable zone that are very, very, very Earthlike, the extrapolation is unwarranted.

Indeed, another article, out today, reports that Earth is so unique in its high phosphorous content that it may be unlikely that extraterrestrial life exists anywhere else in the universe:

Amid efforts to find alien life, scientists have not yet confirmed the existence of an extraterrestrial civilization. Findings of a new study suggest this has something do with the element phosphorus lacking in the cosmos. … Astronomers have been hunting for phosphorus in the universe because of the role it plays in life on Earth. If the element is lacking in other parts of the cosmos, it could be difficult for alien life to exist.

A new study presented at the European Week of Astronomy and Space Science meeting now suggests that life as we know it is more unusual than previously thought because the universe substantially lacks phosphorus.

Despite this general lack of phosphorous in the universe, it is the 11th most abundant element on Earth, and it’s vital for life. Remember CHONPS — the acronym you learned in high school to remember the elements most important for life? In case you forgot, the “P” stands for phosphorus, which is vital for the construction of DNA and RNA in every life form, and is also an indispensable component of the energy-carrying molecule in all living things, ATP. Phosphorylation, the addition of a phosphoryl group (PO32-), is also necessary for the function of most enzymes, as well as numerous other biomolecules. Yet our home planet is strangely and almost inexplicably rich in phosphorous.

The study proposes that Earth got its phosphorus due to its proximity to a supernova explosion. Perhaps, but although phosphorous is common on Earth (making up 0.07 percent of our planet), it’s only a trace element in the solar system as a whole. If Earth was close to that supernova explosion, so was the rest of our solar system, and you’d expect it to be rich in phosphorus as well. But it isn’t. Earth is special even within our solar system for its high phosphorus content, and this richness in life-giving phosphorus is very difficult to explain.

As we’ve said many times before, Earth is a indeed privileged planet.

Editor’s note: This post has been updated.

Photo source: StockSnap, via Pixabay.