A new article in Nature Astronomy by CLEVER Planets scientists Damanveer Grewal, Rajdeep Dasgupta, and Bernard Marty shows that Earth’s nitrogen came from both the inner and outer regions of the disk that formed our solar system, contrary to an earlier theory.
Nitrogen is a volatile element that, like carbon, hydrogen and oxygen, makes life on Earth possible. Knowing its source offers clues to not only how rocky planets formed in the inner part of our solar system but also the dynamics of far-flung protoplanetary disks.
“Researchers have always thought that the inner part of the solar system, within Jupiter’s orbit, was too hot for nitrogen and other volatile elements to condense as solids, meaning that volatile elements in the inner disk were in the gas phase,” Grewal said in an official press release from Rice University.
Because the seeds of present-day rocky planets, also known as protoplanets, grew in the inner disk by accreting locally sourced dust, it appeared they did not contain nitrogen or other volatiles, necessitating their delivery from the outer solar system. An earlier study by the team suggested much of this volatile-rich material came to Earth via the collision that formed the moon.
But new evidence clearly shows only some of the planet’s nitrogen came from beyond Jupiter.
Iron meteorites are remnants of the cores of protoplanets that formed at the same time as the seeds of present-day rocky planets. In recent years, scientists have analyzed nonvolatile elements in meteorites, including iron meteorites that occasionally fall to Earth, to show that dust in the inner and outer solar system had completely different isotopic compositions.
The CLEVER Planets researchers decided to see if volatile elements in the iron meteorites also exhibited the same dichotomy, and found a distinct nitrogen isotopic signature in the dust that bathed the inner protoplanets within about 300,000 years of the formation of the solar system. All iron meteorites from the inner disk contained a lower concentration of the nitrogen-15 isotope, while those from the outer disk were rich in nitrogen-15.
This suggests that within the first few million years, the protoplanetary disk divided into two reservoirs, the outer rich in nitrogen-15 and the inner rich in nitrogen-14.
“Our work completely changes the current narrative,” Grewal said. “We show that the volatile elements were present in the inner disk dust, probably in the form of refractory organics, from the very beginning. This means that contrary to current understanding, the seeds of the present-day rocky planets — including Earth — were not volatile-free.”
Read more about the study in the official Rice University press release, and check out the full paper here.