The outlook for life on any given planet depends not only on where it forms, but also how, according to scientists at Rice University.
Planets like Earth that orbit in the Goldilocks zone of a solar system, with conditions supporting liquid water and a rich atmosphere, are more likely to harbor life. It turns out that how this planet came together also determines whether it has captured and retained certain elements and volatile compounds, including nitrogen, carbon, and water, that give birth to life.
In a study published in Geoscience of nature, Damanveer Grewal, Rice graduate student and lead author, and Professor Rajdeep Dasgupta show the competition between the time it takes for the material to accrete into a protoplanet and the time the protoplanet takes to separate into its distinct layers – a nucleus metallic, a silicate mantle shell, and an atmospheric envelope – in a process called planetary differentiation – is essential in determining which volatile elements the rocky planet is retaining.
Using nitrogen as a proxy for birds, the researchers showed that most of the nitrogen escapes into the atmosphere of protoplanets during differentiation. This nitrogen is then lost to space when the protoplanet cools or collides with other protoplanets or cosmic bodies in the next stage of its growth.
This process depletes nitrogen in the atmosphere and mantle of rocky planets, but if the metal core retains enough, it could still be a significant source of nitrogen during the formation of Earth-like planets.
Dasgupta’s high-pressure lab in Rice captured protoplanetary differentiation in action to show the affinity of nitrogen for metal nuclei.
“We simulated high pressure-temperature conditions by subjecting a mixture of nitrogenous metal and silicate powders to nearly 30,000 times atmospheric pressure and heating them above their melting point,” Grewal said. “The small metallic spots embedded in the silicate glasses of the recovered samples were the respective analogues of the protoplanetary cores and mantles.”
Using this experimental data, the researchers modeled thermodynamic relationships to show how nitrogen is distributed between the atmosphere, the molten silicate, and the nucleus.
“We realized that the fractionation of nitrogen between all of these reservoirs is very sensitive to body size,” Grewal said. “Using this idea, we could calculate how nitrogen would have separated between the different reservoirs of protoplanetary bodies over time to ultimately build a habitable planet like Earth.”
Their theory suggests that the raw materials for Earth quickly developed around planetary embryos the size of the moon and Mars before completing the process of differentiation into the familiar vapor-metal-silicate-gas arrangement.
Typically, they estimate embryos formed within 1 to 2 million years of the start of the solar system, much earlier than the time it took for them to fully differentiate. If the rate of differentiation was faster than the rate of accretion of these embryos, the rocky planets that formed from them would not have been able to accumulate enough nitrogen, and possibly other volatiles, essential for development. conditions conducive to life.
“Our calculations show that the formation of an Earth-sized planet via planetary embryos that grew extremely rapidly before undergoing metal-silicate differentiation sets a unique pathway to meeting Earth’s nitrogen budget,” said Dasgupta, principal investigator at CLEVER Planets, a NASA-funded collaboration. project exploring how elements essential to life might have come together on rocky planets in our solar system or on distant rocky exoplanets.
“This work shows that there is a much greater affinity of nitrogen for the metallic liquid forming the nucleus than previously thought,” he said.
The study follows earlier work, one showing how the impact of a body forming the moon could have given the Earth much of its volatile content, and another suggesting that the planet was drawing more of its nitrogen from local sources in the solar system than was once believed.
In this latest study, Grewal said: “We have shown that protoplanets growing in the inner and outer regions of the solar system accrete nitrogen, and the Earth derives its nitrogen by accreting protoplanets from these two regions. However, it was not known how the Earth’s nitrogen balance was established. “
“We are making a great claim that will go beyond the simple subject of the origin of volatile elements and nitrogen, and will have an impact on a representative sample of the scientific community interested in the formation and growth of planets,” Dasgupta said.