* Astronomy

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RE: Chondrules

Blobrana · L

Chondrules may have formed from high-pressure collisions in early solar system

A normally staid University of Chicago scientist has stunned many of his colleagues with his radical solution to a 135-year-old mystery in cosmochemistry. "I'm a fairly sober guy. People didn't know what to think all of a sudden," said Lawrence Grossman, professor in geophysical sciences.
At issue is how numerous small, glassy spherules had become embedded within specimens of the largest class of meteorites - the chondrites. British mineralogist Henry Sorby first described these spherules, called chondrules, in 1877. Sorby suggested that they might be "droplets of fiery rain" which somehow condensed out of the cloud of gas and dust that formed the solar system 4.5 billion years ago.
[url=http://news.uchicago.edu/article/2013/07/08/cosmochemist-discovers-potential-solution-meteorite-mystery]

Blobrana · L

Solar system mystery 'solved'

Planetary scientists claim they may have discovered how "chondrules", tiny particles found in meteorites, formed at the beginning of the solar system, thus solving the decades-old cosmic conundrum.
Chondrules are spherical particles of molten material found in meteorites but their origins have long been a mystery. No longer than about one millimetre in diameter, they melted at temperatures of more than 1,000 degrees Celsius, while the cooler materials surrounding them only experienced temperatures of a few hundred degrees Celsius.
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Chondrules, millimeter-sized spherules that formed as rapidly-cooled molten droplets, are characteristic of chondrite meteorites. If they formed at low pressure in the solar nebula (the cloud of gas and dust surrounding the infant Sun and from which the planets formed), then they should have lost almost all their inventories of volatile elements, such as sodium, because volatile elements would have boiled off the chondrules when they were molten. Conel Alexander (Carnegie Institution of Washington) and colleagues at Carnegie, the U.S. Geological Survey (Reston), and the American Museum of Natural History (New York) show that there was little sodium loss. They measured the sodium concentrations in numerous crystals of olivine inside chondrules in the Semarkona meteorite. The results show that the variations in concentrations from the centers of crystals to their edges are consistent with crystallisation in a molten droplet that was not losing sodium to the surrounding gas. These results are supported by independent measurements by Alexander Borisov (Russian Academy of Sciences, Moscow) and colleagues at the University of Hannover, Georg-August-University Göettingen, and Köln University, all in Germany. Sodium loss could have been suppressed if the gas surrounding each chondrule had a much higher pressure of sodium than that expected for the solar nebula. Such a high pressure of sodium is most easily explained if chondrules formed in a region with a high density of solids. Alexander and his co-workers argue that such dense regions could have enough mass in a small space to collapse by gravity, perhaps forming planetesimals, the first step in constructing the inner planets.

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Blobrana · L

Comets and asteroids are a small but important part of the Solar System. Scientists have long debated how they formed: did their component grains accrete from the cloud of gas and dust the solar nebula - that encircled the Sun at the beginning of the Solar System or did they undergo melting due to violent impacts and the presence of short-lived radioactivity?
On October 12th delegates attending the Royal Astronomical Society (RAS) meeting Early Solar System Processes on Meteorites will discuss the competing theories. The meeting is being held in honour of the late Robert Hutchison, who was a distinguished meteorite expert at the Natural History Museum and used early Solar System samples to deduce how asteroids formed.
One group of scientists believes that the partial melting of asteroids led to the formation of chondrules, mm-size melt droplets which make up many meteorites. Another camp is convinced that asteroids grew from cold particles in the solar nebula and that the chondrules formed beforehand. There are currently two main ways of studying this question: one is to analyse the composition and age of primitive meteorites, the other makes use of similarly primitive grains collected from Comet Wild-2 by the NASA Stardust space probe, which returned a sample to Earth in January 2006.

Like other comets, Wild-2 began life in the earliest stages of the Solar System, more than 4500 million years ago. At that time the rocky, metallic and icy material that ultimately formed the planets started to form larger bodies. Ices of water, ammonia, carbon dioxide and carbon monoxide condensed in the cold outer parts of the Solar System. Most of the ices ended up as part of the gas giant planets Jupiter, Saturn, Uranus and Neptune but some remained in much smaller bodies 1-10 km across that eventually formed the cores (nuclei) of comets.
Comets that have spent most of their lives at a great distance from the Sun contain the least processed material in the Solar System, whereas in contrast an increasing number of scientists believe that asteroid surfaces were extensively melted. For example, one theory put forward by Dr Ian Sanders of Trinity College Dublin is that the presence of radioactive material created enough heat to partially melt the building blocks of planets and asteroids (planetesimals) found in the early Solar System. When these objects collided they would have created great clouds of molten droplets, the predecessors of the material found in asteroids today.
Meeting chair Dr John Bridges of the University of Leicester is part of the international team studying grains from Wild-2 and hence the early history of the Solar System.

There has been a vigorous debate for many years about how the earliest planetesimals formed either from cold accumulation of dust and gas from the nebula and interstellar space or through impact and radioactivity-induced melting. By studying Comet Wild-2 and primitive meteorites we are starting to reveal the true nature of a violent early Solar System where many of the earliest planetary building blocks underwent repeated collisions and melting episodes - Dr John Bridges.

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Blobrana · L

A University of Toronto scientist has found unexpectedly `young` material in meteorites - a discovery that breaks open current theory on the earliest events of the solar system.
A paper published today in the August issue of Nature reports that the youngest known chondrules - the small grains of mineral that make up certain meteorites - have been identified in the meteorites known as Gujba and Hammadah al Hamra.

Gujba fell as a a bright fireball that landed in a corn field near the village of Bogga Dingare in Yobe, Nigeria, on April 3,1984. It was estimated to have weighed about 100 kg, but most of the mass was broken up into small pieces and dispersed.

Hammadah al Hamra, was found in Lybia in October 18 1997.

Researchers who have studied chondrules generally agree that most were formed as a sudden, repetitive heat, likely from a shock wave, condensed the nebula of dust floating around the early Sun. Thinking that an analysis of the chondrules in Gujba and Hammadah al Hamra would be appropriate for accurately dating this process, University of Toronto geologist Yuri Amelin, together with lead author Alexander Krot of the University of Hawaii, studied the chondrules - mineralogical structure and determined their isotopic age.

"It soon became clear that these particular chondrules were not of a nebular origin. And the ages were quite different from what was expected. It was exciting." - Yuri Amelin.

Amelin explains that not only were these chondrules not formed by a shock wave, but rather emerged much later than other chondrules.

"They actually post-date the oldest asteroids. We think these chondrules were formed by a giant plume of vapour produced when two planetary embryos, somewhere between moon-size and Mars-size, collided" - Yuri Amelin.

What does this mean in the grand scheme of things?
The evolution of the solar system has traditionally been seen as a linear process, through which gases around the early sun gradually cooled to form small particles that eventually clumped into asteroids and planets. Now there is evidence of chondrules forming at two very distinct times, and evidence that embryo planets already existed when chondrules were still forming.

"It moves our understanding from order to disorder. But I`m sure that as new data is collected, a new order will emerge" - Yuri Amelin.

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