Research led by Carnegie’s Nicole Nie and Da Wang, published in Science, shows that some primitive meteorites contain a different mix of potassium isotopes than those found in more chemically processed meteorites.
“The extreme conditions that occur inside stars allow them to manufacture elements through nuclear fusion,” he explained in Nie. “Each stellar generation sows the raw material from which subsequent generations are born, and we can trace the history of this material through time,” he noted.
Some of the material produced inside stars can be ejected into space, where it accumulates as a cloud of gas and dust. More than 4.5 billion years ago, one of these clouds collapsed in on itself to form the Sun. The remains of this process formed a spinning disk around the newborn star. Over time, planets and other objects in the solar system coalesced from these debris, including parent bodies that broke away to become asteroids and meteorites.
“By studying variations in the isotopic record preserved in meteorites, we can trace the source materials from which they formed and build a geochemical timeline of the evolution of our solar system,” Wang added. Each element contains a unique number of protons, but their isotopes have different numbers of neutrons. The distribution of the isotopes of an element throughout the solar system reflects the composition of the cloud of material from which the Sun was born. Many stars contributed to this solar molecular cloud, but their contributions were not uniform, which is determined by studying the content isotopic of meteorites.
The key in potassium isotopes
With their team, Wang and Nie measured the ratios of three potassium isotopes from 32 meteorites. Potassium is interesting because it has relatively low boiling points that make it easily evaporate. Thus, it is difficult to look for pre-Sun patterns in the isotopic ratios of volatiles, since they do not remain in hot star formation conditions long enough to keep a readable record.
“Using very sensitive instruments, we found patterns in the distribution of our potassium isotopes inherited from pre-solar materials and they differed between meteorite types,” Nie said. They found that some of the earliest solar system meteorites that formed in the outer solar system, called carbonaceous chondrites, contained more potassium isotopes produced by stellar explosions.
Other meteorites—the ones that hit Earth, the noncarbonaceous chondrites—contain the same potassium isotope ratios seen on Earth and elsewhere in the inner solar system. “This tells us that, like a poorly mixed pie crust, there was no uniform distribution of materials between the outer reaches of the solar system, where carbonaceous chondrites formed, and the inner solar system,” Shahar concluded.
For years, Carnegie Earth and planetary scientists worked to unravel the origins of Earth’s volatile elements. Some may have been transported from the outer solar system in carbonaceous chondrites. However, as the presolar potassium isotope pattern of the non-carbonaceous chondrites matched that seen on Earth, these meteorites are the likely source of the planet’s potassium. “Scientists have recently questioned whether the conditions in the solar nebula that gave rise to the Sun were hot enough to burn up all the volatile elements,” Shahar added.
“This research provides new evidence that volatiles could survive the formation of the Sun.” More research is needed to apply this insight to our models of planet formation and see if they fit any long-held beliefs about how Earth and its neighbors came to be.