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"Was Existence of Our Solar System Triggered by a Supernova?" Meteorite Evidence Offers Clues




The dust grains that eventually coalesced into our solar system's planets bounced around like pinballs over vast distances nearly 4.6 billion years ago. The dust grains came from a rare type of meteorite  known as a carbonaceous chondrite, which fell as a fireball near the village of Pueblito de Allende, in the Mexican state of Chihuahua, on February 8, 1969. Several tons of material were scattered over an area measuring 48 km by 7 km. What makes the Allende meteorite so special is that it's a composite of materials that coalesed 30 nillion years before the Earth was formed when the Sun was still surrounded by the protoplanetary disk.

Specimens of the meteorite were found to contain a fine-grained carbon-rich matrix studded with many chondrules, both matrix and chondrules consisting predominantly of the mineral olivine. Close examination of the chondrules, by NASA researchers and a team from Case Western Reserve University in 2011 revealed tiny black markings, up to 10 trillion per square centimeter, which were absent from the matrix and interpreted as evidence of radiation damage. 

The Allende meteorite also contains fine-grained, microscopic diamonds with strange isotopic signatures that point to an extrasolar origin; these interstellar grains are older than the Solar System and probably the product of a nearby supernova. 

Scientists studying a tiny chunk of the meteorite say it likely formed close to the sun, was ejected near today's asteroid belt, and then returned to the scorching inner reaches thereafter. The results should help astronomers better understand the early days of our solar system, and could shed light on planet-formation processes in general, researchers said.

"This has implications for how our solar system and possibly other solar systems formed and how they evolved," the study’s lead author, Justin Simon, of NASA's Johnson Space Center in Houston, said in a statement. "There are a number of astrophysical models that attempt to explain the dynamics of planet formation in a protoplanetary disk, but they all have to explain the signature we find in this meteorite."

Simon and his colleagues investigated a pea-size piece of the Allende meteorite, a hunk of space rock that crash-landed in Mexico in 1969. The bit they looked at is what's known as a calcium-aluminum-rich inclusion, or CAI, among the first solids to condense from the swirl of gas and dust as the  planets were forming. Studying them can yield clues about our solar system's origins.

The team studied the composition of a 4.57 billion-year-old inclusion in detail with a tiny probe, measuring the concentrations of two different oxygen isotopes in the space rock's various layers. Isotopes are versions of the same element that have different numbers of neutrons in their atomic nuclei.

The concentrations of these two isotopes — oxygen-16 and oxygen-17 — varied from place to place while the solar system was forming. So by analyzing their relative abundances in the different parts of the CAI, the team was able to learn a great deal about its travels — which turned out to be extensive.

"If you were this grain, you formed near the protosun, then likely moved outward to a planet-forming environment, and then back toward the inner solar system or perhaps out of the plane of the disk," Simon said. "Of course, you ended up as part of a meteorite, presumably in the asteroid belt, before you broke up and hit the Earth."

The Allende meteorite findings are consistent with some theories about how dust grains formed and moved about in our solar system's infancy, eventually seeding the formation of planets, researchers said. Heated-up grains from near the sun and cooler dust from farther out were eventually incorporated into asteroids and planets, the theory goes.

"There are problems with the details of this model, but it is a useful framework for trying to understand how material originally formed near the sun can end up out in the asteroid belt," said study co-author Ian Hutcheon, of Lawrence Livermore National Laboratory in Livermore, Calif.

The image at the top of the page is a compositional X-ray image of the rim and margin of a 4.57 billion-year-old calcium-aluminum-rich inclusion (CAI) from the Allende meteorite. Analysis of oxygen isotope abundances clued scientists in to the huge distances this chunk of space rock traveled while the solar system was forming.

The Daily Galaxy via University of California/Berkeley

Image Credit: Erick Ramon and Justin Simon/NASA


"There are problems with the details of this model, but it is a useful framework for trying to understand how material originally formed near the sun can end up out in the asteroid belt," said study co-author Ian Hutcheon, of Lawrence Livermore National Laboratory in Livermore, Calif.

"Sasha Krot and I [Edward R. D. Scott] suggest that there is only one plausible reason why solar nebula condensates and evaporative residues of pre-solar dust could be similarly enriched in 16O. Condensation must have occurred in a region where the gaseous oxygen was dominated by oxygen from evaporated 16O-rich pre-solar dust." http://www.psrd.hawaii.edu/Dec01/Oisotopes.html

So CAIs and chondrules are enriched in 16O because they came from 16O rich dust precursors? That's no explanation, its merely kicking the can down the street.

An alternative hypothesis may actually explain the reason if our Sun formed as a binary pair that spiraled in and merged in a luminous red nova (LRN) at 4.567 Ma. If the LRN created the observed 28Si anomaly of our sun, then peak core temperatures may have reached several billions of Kelvins, enabling the alpha process to create excess 28Si, along with the lighter alpha process elements: 12C, 16O, 20Ne, 24Mg, and r-process SRs. And CAIs may have condensed from a polar flow from the core with a higher relative 16O enrichment.

The significant 28Si isotope enrichment of our solar system compared with older, presolar, mainstream silicon-carbide (SiC) grains in carbonaceous chondrites is evident on a oxygen three-isotope graph where solar values plot to the lower left corner of the grouping of mainstream SiC grains. (Nittler and Hoppe, 2005, Fig. 2) However, glalactic chemical evolution (GCE) predicts a trend over time toward the heavier secondary isotopes, 29Si and 30Si, so our solar system bucks this trend, apparently having been reset by some mechanism. Presolar X-type SiC grains from SNe plot far below even the solar value, suggesting the possibility of a high-degree of supernova contamination to explain away the solar 28Si enrichment; however, supernova grains are only 1% as prevalent as mainstream grains in solar-system chondrites which is insufficient to explain this enrichment. So again we come back to an LRN as the most likely cause of solar enrichment.

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