Privileged Planet: evolving this planet is not simple (Introduction)

by David Turell , Monday, May 10, 2021, 23:51 (1294 days ago) @ David Turell

This new study tells how lucky we are:

https://phys.org/news/2021-05-planets-elements-essential-life.html

"Planets like Earth that orbit within a solar system's Goldilocks zone, with conditions supporting liquid water and a rich atmosphere, are more likely to harbor life. As it turns out, how that planet came together also determines whether it captured and retained certain volatile elements and compounds, including nitrogen, carbon and water, that give rise to life.

"In a study published in Nature Geoscience, Rice graduate student and lead author Damanveer Grewal and Professor Rajdeep Dasgupta show the competition between the time it takes for material to accrete into a protoplanet and the time the protoplanet takes to separate into its distinct layers—a metallic core, a shell of silicate mantle and an atmospheric envelope in a process called planetary differentiation—is critical in determining what volatile elements the rocky planet retains.

"Using nitrogen as proxy for volatiles, the researchers showed most of the nitrogen escapes into the atmosphere of protoplanets during differentiation. This nitrogen is subsequently lost to space as the protoplanet either cools down or collides with other protoplanets or cosmic bodies during the next stage of its growth.

"This process depletes nitrogen in the atmosphere and mantle of rocky planets, but if the metallic core retains enough, it could still be a significant source of nitrogen during the formation of Earth-like planets.

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"'We realized that fractionation of nitrogen between all these reservoirs is very sensitive to the size of the body," Grewal said. "Using this idea, we could calculate how nitrogen would have separated between different reservoirs of protoplanetary bodies through time to finally build a habitable planet like Earth."

"Their theory suggests that feedstock materials for Earth grew quickly to around moon- and Mars-sized planetary embryos before they completed the process of differentiating into the familiar metal-silicate-gas vapor arrangement.

"In general, they estimate the embryos formed within 1-2 million years of the beginning of the solar system, far sooner than the time it took for them to completely differentiate. If the rate of differentiation was faster than the rate of accretion for these embryos, the rocky planets forming from them could not have accreted enough nitrogen, and likely other volatiles, critical to developing conditions that support life.

"'Our calculations show that forming an Earth-size planet via planetary embryos that grew extremely quickly before undergoing metal-silicate differentiation sets a unique pathway to satisfy Earth's nitrogen budget," said Dasgupta, the principal investigator of CLEVER Planets, a NASA-funded collaborative project exploring how life-essential elements might have come together on rocky planets in our solar system or on distant, rocky exoplanets.

"'This work shows there's much greater affinity of nitrogen toward core-forming metallic liquid than previously thought," he said."

Comment: Pure luck or pure design.