A thin section of a meteorite under a microscope, featuring chondrocytes of complex texture. Chondrules are among the oldest materials in the solar system. Credit: Nicole Zeke Ni.
Earth’s potassium arrived via Meteorite Delivery Service to find new research led by Nicole Ni and Da Wang of Carnegie.
Their work, published in the journal Science, shows that some primitive meteorites contain a different mix of potassium isotopes than do other chemically treated meteorites. These results can help explain the processes that shaped our solar system and determined the formation of its planets.
“The extreme conditions found in stellar interiors enable stars to manufacture elements using nuclear fusion,” explained Ni, a former Carnegie researcher now at Caltech. “Each stellar generation sows the raw material from which subsequent generations are born and we can trace the history of this material through time.”
Some of the material produced in the interiors of stars can be ejected into space, where it accumulates as a cloud of gas and dust. More than 4.5 billion years ago, one such cloud collapsed in on itself to form our Sun.
The remnants of this process formed a rotating disk around the newborn star. Eventually, planets and other solar system bodies coalesced from these remnants, including parent bodies that later broke off to become asteroids and meteorites.
“By studying the variations in the isotopic record preserved within meteorites, we can trace the source materials from which they were formed and build a geochemical timeline for the evolution of our solar system,” added Wang, who is currently at Chengdu University of Technology.
Each element has a unique number of protons, but its isotopes have varying numbers of neutrons. The distribution of different isotopes of the same element throughout the solar system is a reflection of the composition of the material cloud from which the sun was born. Many stars contributed to the so-called solar molecular cloud, but their contributions were not uniform, which can be determined by studying the isotopic content of meteorites.
Wang and Ni—along with Carnegie colleagues Anat Shahar, Zachary Turano, Richard Carlson, and Connell Alexander—measured the ratios of three potassium isotopes in samples from 32 different meteorites.
Potassium is particularly interesting because it’s what’s called a mildly volatile element, named for having relatively low boiling points that make it vaporize fairly easily. As a result, it’s hard to look for pre-sun patterns in isotopic ratios of volatile materials—they don’t linger in hot star-forming conditions long enough to maintain an easy-to-read record.
“However, using highly sensitive and appropriate instruments, we found patterns in the distribution of potassium isotopes that we inherited from presolar materials and differed among meteorite types,” said Ni.
They found that some of the more primitive meteorites called carbonaceous chondrites, which formed in the outer solar system, contain more potassium isotopes that were produced by massive stellar explosions, called supernovae. Whereas other meteorites—the ones that strike Earth most often, are called non-carbonaceous chondrites—have the same potassium isotope ratios we see on our home planet and elsewhere in the inner solar system.
“This tells us that, like bad cake batter, there was no equal distribution of material between the outer reaches of the solar system where carbonaceous chondrites formed, and the inner solar system, where we live,” Shahar concluded.
For years, planetary scientists and Carnegie have worked on Earth to uncover the origins of volatile elements on Earth. Some of these elements may have been transported here all the way from the outer solar system on the backs of carbonaceous chondrites. However, because the presolar potassium isotope pattern found in non-carbonaceous chondrites is identical to that seen on Earth, these meteorites are the most likely source of potassium on our planet.
“It was only recently that scientists challenged a long-held belief that conditions in the solar nebula that gave birth to our sun were hot enough to burn all the volatile elements,” Shahar added. “This research provides new evidence that volatiles could have survived the formation of the Sun.”
More research is needed to apply this new knowledge to our models of planet formation and see if it modifies any ancient beliefs about how Earth and its neighbors appeared.
This work was supported by a NASA NESSF Fellowship, Carnegie Postdoctoral Fellowships, and a Carnegie × Postdoc (P2) Grant.
The Carnegie Institution for Science (carnegiescience.edu) is a private, nonprofit organization based in Washington, D.C., with three research departments on both coasts. Since its founding in 1902, the Carnegie Institution has been a leading force in basic scientific research. Carnegie scholars are leaders in the life and environmental sciences, Earth and planetary sciences, and astronomy and astrophysics.
Meteorites have inherited a nuclear structure abnormality of the potassium-40 produced in supernovae, Science
Astrobiology and Astrochemistry