How challenging is it to bring forth a planet like Earth? To gather the right combination of rock, metal, and water in a temperate region not too distant from a star? For a considerable time, planetary scientists believed Earth was a fortunate accident, enriched with water and lighter “volatile” elements—such as nitrogen and carbon—by asteroids that had ventured in from the outer fringes of the early Solar System, where these materials were abundant. However, a series of recent studies, including two published today in Science, propose that all the necessary components were much closer at hand during Earth’s inception.
These discoveries, founded on meticulous chemical examinations of meteorites, suggest that the disks forming planets around other stars should also be well-supplied with the ingredients for damp, rocky planets that might be conducive to life. Alessandro Morbidelli, a planetary scientist at the Côte d’Azur Observatory not involved in the recent research, remarks, “It makes the enrichment in volatile elements of a planet more generic.” Even if a fledgling planet doesn’t receive a delivery from the far reaches of the nascent planetary system, he notes, “it doesn’t change habitability.”
Until recently, scientists could only discern the early isotopic markers in metals like chromium, titanium, and molybdenum, which endured the heat of the young Sun. A close resemblance between the isotope ratios found in noncarbonaceous chondrites and those found in the same metals on Earth indicated that much of Earth’s basic material originated from the same neighboring region as those meteorites.
Nonetheless, the search for isotopic proof that Earth’s lighter volatile elements also arose from nearby sources yielded no results. Rayssa Martins, a doctoral student in geochemistry at Imperial College London, notes that people began to believe that this evidence simply didn’t exist. Hence, the traditional belief in the origin of these elements persisted: many were assumed to come from a distant source, like an outer region of the disk, where they might have condensed and then been drawn inward by the gravitational pull of a forming planet, such as Jupiter.
Now, however, the meteorite distinction has been identified in two moderately volatile elements, potassium and zinc. The results suggest that a substantial portion, though not all, of the planet’s volatiles also originated from the noncarbonaceous reservoir. According to Nicole Nie, a planetary scientist at the California Institute of Technology and the lead author of the study focused on potassium, “This is a game changer for cosmochemistry.”
The research was a formidable task. A 2020 paper had identified what appeared to be ancient isotopic signatures in meteorite potassium, but it only examined two potassium isotopes, omitting the much scarcer potassium-40, which can be easily confused with calcium or argon signatures in mass spectrometry. With just two isotopes, it was impossible to confirm whether what the team observed reflected the chemical composition of the early disk. Therefore, Nie and her team measured all three potassium isotopes in 32 meteorites. They discovered that the potassium in the noncarbonaceous rocks exhibited isotopic patterns quite similar to those observed on Earth. “That was really surprising,” she remarks. Collectively, the findings propose that roughly 80% of Earth’s potassium originated from nearby sources.