Nitrogen, carbon, and water—the key ingredients for life—are generally believed to have come to Earth from the outer reaches of the solar system. Researchers now have found evidence that as much as half of Earth’s nitrogen could have come from much closer to home. This finding may change the way we look at how life formed in our solar system—and how it may form in others.
Our solar system began as clouds of gas and dust in a swirling disk. At the center of the swirl, most material formed the Sun. Farther out, matter accumulated and formed the nuclei of the planets we know today. Volatile elements like nitrogen and carbon have traditionally been thought to have condensed in the outer reaches of the disk, beyond the orbit of today’s Jupiter, and to have been carried to the inner planets by meteoroids.
In the past decade, scientists figured out that Earth’s meteorites can be separated into two distinct categories: those that came from the inner part of the solar system and others that came from beyond Jupiter. Further analysis revealed that isotopes of nonvolatile elements (like molybdenum and tungsten) in these meteorites are also split between “inner” and “outer” categories.
Damanveer Grewal of Rice University, lead author of a new study on the origin of nitrogen on Earth, was considering the fate of volatile elements in the early solar system when he came across data for nitrogen isotopes in iron meteorites. Iron meteorites are remnants of protoplanets—the undeveloped cores of planets that never fully formed. They formed within 300,000 years of the origin of the solar system, as the planets were just starting to nucleate. Grewal thought iron meteorites could serve as a proxy for the seeds of today’s planets and help him tease out clues about how life-essential elements fared early on.
Tracing Nitrogen’s History
Grewal analyzed the nitrogen isotope data in the meteorites. “What I found at this stage was extremely shocking,” he said: The meteorites’ nitrogen isotopes fell into the same inner and outer categories as the nonvolatile elements. “This was too good to be true.”
The iron meteorites from the outer part of the disk were rich in the nitrogen-15 isotope, and those from the inner part were rich in nitrogen-14, implying that nitrogen was present in the inner part of the disk when Earth was young. This suggests that not all of Earth’s nitrogen came from the outer solar system, Grewal and colleagues reported in their paper published in Nature Astronomy.
“So the seeds of the protoplanets never started volatile-free; they always had volatiles in them,” said Grewal. It’s likely the nitrogen was present in some type of organic material with high temperature resistance.
These findings challenge the traditional idea that volatiles like nitrogen and water were brought in from the outer solar system, said Sebastiaan Krijt, an astrophysicist at the University of Exeter not involved in the study. Because Earth’s nitrogen isotope ratio falls between that of the inner and outer solar system reservoirs, “this suggests, in fact, [that] as much as half of Earth’s current nitrogen budget may have been sourced locally in the form of nitrogen-bearing organics and/or dust.”
Krijt said that understanding whether the processes responsible for bringing the ingredients for life to Earth were just a lucky chance or commonplace, as this study suggests, is important. “It sheds light on how often we expect similar conditions to arise on rocky planets in other planetary systems.”
—Lakshmi Supriya ([email protected]), Science Writer