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Clouds of molten droplets in the early solar system?

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. Dr Bridges comments ‘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.’

Robert Massey | alfa
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