The Butterfly Nebula, an example of a star forming region in the Tarantula Nebula. The white bar is 2 light years, or about 120,000 AU (astronomical units). A bright central star, obscured by dust, modifies the oxygen isotopes in the nebula by photodissociation of carbon monoxide. This is another example of an environment where oxygen isotopes in the molecular cloud could be modified prior to the formation of a planetary system. Credit: ASA and ESA
In search of the origins of our solar system, an international team of researchers, including planetary scientist and cosmochemist James Lyons of Arizona State University, compared the composition of the sun with the composition of the oldest materials that formed in our system: refractory inclusions in not transformed meteorites.
By analyzing the oxygen isotopes (varieties of an element with a few extra neutrons) of these refractory inclusions, the research team found that the composition differences between the sun, planets, and other materials in the solar system were inherited from the protosolar molecular cloud that existed before the solar system. The results of their study were recently published in Advances in science.
“It has recently been shown that variations in the isotopic compositions of many elements in our solar system were inherited from the protosolar molecular cloud,” said lead author Alexander Krot of the University of Hawaii. “Our study shows that oxygen is no exception.”
An example of a star-forming region in NGC 3324 in the Carina Nebula, in which neighboring large stars both shape the shape of the nebula and change the distribution of oxygen isotopes through photodissociation of carbon monoxide by ultraviolet light. The results of the work presented here speak for the change of oxygen isotopes in a molecular cloud environment. The white bar is 5 light years or 300,000 AU (astronomical units, distance between the earth and the sun). Photo credit: NASA, ESA, Hubble Heritage Team
Molecular Cloud or Solar Nebula?
When scientists compare oxygen isotopes 16, 17, and 18, they find significant differences between the earth and the sun. It is believed that this is due to the processing of carbon monoxide by ultraviolet light, which is broken apart, resulting in a large change in oxygen isotope ratios in water. The planets are formed from dust, which inherits the changed oxygen isotope ratios through interactions with water.
What scientists don’t know is whether the UV processing took place in the parent molecular cloud that collapsed to form the proto-solar system, or later in the cloud of gas and dust that formed the planets, known as the solar nebula .
Artistic rendering of the protosun and the solar nebula. Oxygen isotopes can also be changed in this environment by ultraviolet light (gold arrows). Short-lived radiogenic isotopes made of aluminum (maroon wave arrows) may also have been injected into the solar nebula. The insets show electron backscatter images from two of the calcium-aluminum inclusions analyzed for this study and the approximate location where these high-temperature condensates formed. The new results presented here suggest that the change in oxygen isotopes occurred primarily in the parent’s molecular cloud and not in the solar fog. The earth and everything on earth has received an isotopic composition of oxygen derived from the molecular cloud from which the solar system was formed. The white scale shows three AU (Astronomical Units). Photo credit: NASA JPL-Caltech / Lyon / ASU
To determine this, the research team turned to the oldest component of meteorites, the so-called calcium-aluminum inclusions (CAIs). They used an ion microprobe, electron backscatter images, and X-ray element analysis at the Department of Geophysics and Planetary Sciences at the University of Hawaii to carefully analyze the CAIs. They then built in a second isotope system (aluminum and magnesium isotopes) to limit the age of the CAIs, and for the first time established the connection between the frequency of oxygen isotopes and the mass of 26 aluminum isotopes.
From these aluminum and magnesium isotopes, they concluded that the CAIs were formed around 10,000 to 20,000 years after the molecular cloud collapsed.
“This is extremely early in the history of the solar system,” said Lyons, associate research professor at ASU’s School of Earth and Space Exploration. ”
While more measurements and modeling are needed to fully assess the impact of these results, they do have implications for the inventory of organic compounds available during the solar system and later as planets and asteroids form.
“Any restriction on the amount of ultraviolet processing of material in the solar mist or in the parent’s molecular cloud is critical to understanding the inventory of organic compounds that make up life on earth,” Lyons said.
Reference: “Oxygen isotope heterogeneity in the early solar system inherited from the protosolar molecular cloud” by Alexander N. Krot, Kazuhide Nagashima, James R. Lyons, Jeong-Eun Lee and Martin Bizzarro, October 16, 2020, Advances in science.
DOI: 10.1126 / sciadv.aay2724
