About 4.6 billion years ago, meteorites and other ancient building blocks began to coalesce to form the early Earth in a process called accretion. During that process, frequent, violent collisions with other planetary bodies and radioactivity kept the early Earth hot, creating an ocean of molten magma sitting over a molten metallic core. Despite the havoc these collisions created, at least one may have been responsible for forming the Moon, delivering the carbon and other volatile elements that make Earth habitable today.
In a new paper in Science Advances, DCO members Damanveer Grewal, Rajdeep Dasgupta, Kyusei Tsuno, and colleagues (all at Rice University, USA), used high-temperature and high-pressure experiments to investigate how sulfur affects whether carbon and nitrogen end up in the core or mantle during planetary formation. The researchers discovered that higher sulfur concentrations in the core allow carbon to exist in the outer portions of a planet in greater proportion, where it’s available for life. Through computer modeling, they estimate that an immature planetary “embryo” with a sulfur-rich core, about the size of Mars, slammed into Earth and delivered a major portion of the carbon, nitrogen, and sulfur that enabled life to evolve on Earth. Based on its size, they propose that the object may also be responsible for the creation of the Moon.
The researchers began this project to find a way to explain the skewed carbon to nitrogen ratios on Earth. Rocks from Earth’s mantle and crust have more carbon and less nitrogen than scientists would expect, based on the makeup of the meteorites that were the planet’s starting materials. Previously, researchers proposed that the missing nitrogen may have sunk into the metallic core during formation. But lab experiments have shown that carbon is more likely to segregate into the core than is nitrogen. Alternatively, researchers suggested that nitrogen escaped Earth from the atmosphere, but isotopic analyses ruled out that idea. Additionally, the early atmosphere likely was rich in carbon, not nitrogen.
Instead of losing nitrogen, Earth could have gained extra carbon when other not-quite-mature bodies, called planetary embryos, slammed into the young planet. “Generally, people believe that there could have been an accretion of three to four planetary embryos during the last stages of the Earth,” said Grewal, “We know that there was at least one planetary embryo that accreted to the Earth and created the Moon.”
The researchers expanded their testing to see how different concentrations of sulfur affect whether nitrogen and carbon end up in the core or in the mantle of a planetary embryo using laboratory simulations of embryo formation under high-temperature and high-pressure conditions. They mixed metal, to represent the metallic core, and silicate, the main component of the mantle, and combined them in a capsule made of graphite. Then they heated the capsule to 1600 to 1800 degrees Celsius and squeezed it to almost 70 000 times the atmospheric pressure of Earth. The capsules contained zero, low, or high levels of sulfur. The researchers then measured how much carbon, nitrogen, and sulfur ended up in the metal and silicate portions inside the capsule.
As in previous experiments, without sulfur, more carbon than nitrogen entered the core. But the more sulfur the researchers added, the more carbon stayed in the silicate portion of their simulated embryo. The amount of nitrogen entering the core remained approximately the same, regardless of the sulfur content.
Using these data, the researchers ran computer simulations to see what kind of planetary embryo would create the current ratios of carbon, nitrogen, and sulfur that exist on Earth. “We determined that if that planet has a sulfur-rich core – anywhere from 20 to 35 percent sulfur – the size of that body would be somewhat close to a planet the size of Mars,” said Dasgupta. If such an embryo delivered these volatile elements with minimal interaction of the core of the embryo with the growing Earth’s silicate, such an impact would explain Earth’s skewed carbon to nitrogen ratio.
Previously, scientists have estimated that the impact that created the Moon was about the size of Mars, making it possible that this same planetary body also delivered volatile elements to Earth. “This planetary embryo that could have formed the Moon, could actually have been life-giving rather than destructive,” said Grewal.
To support this idea, scientists will now need to examine the concentrations of non-volatile elements as well as different isotopes, to see if they fit the same pattern.
The finding that such a large impact could increase the habitability of a planet has implications for where else we might find life in the solar system. If a planet doesn’t need to retain its original volatile elements during formation and can instead receive them later from impacts, then this process potentially expands the number of planets that could support life. “What our studies show is that there may be more than one way to establish the budgets of life-essential elements in rocky planets,” said Dasgupta. It also spurs the question: Are these impacts necessary for life?
Additionally, the study establishes the earliest possible starting point for when Earth’s carbon cycle could have begun. Other researchers propose that the cycle began with the movement of tectonic plates and the sinking of the edges of plates into the mantle through subduction, but theoretically the deep carbon cycle could begin once the mantle had established a reservoir of carbon. “Our understanding of the deep carbon cycle is premised on the fact that there is a vast amount of carbon in the mantle portion of our planet, but how that mantle budget of carbon got established remains an enigma,” said Dasgupta.
Top photo: An impact with a planetary body the size of Mars with a sulfur-rich core and carbon and nitrogen in the mantle, could be responsible for the sulfur, carbon, and nitrogen concentrations that we see on Earth today. Credit: Rajdeep Dasgupta