domingo, 28 de abril de 2013

"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 ormed 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 ReserveUniversity 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.
Image Credit: Erick Ramon and Justin Simon/NASA
Source: The Daily Galaxy via University of California/Berkeley

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