The First Solid Grains

Many models of the Solar Nebula suggest that temperatures in its early history were enough to vaporise even the highest temperature dust. As the temperature in the disk cooled this rock gas condensed to form the first solid grains. At the highest temperatures we know the grains would consist mainly of oxides of refractory elements such as calcium, titanium and aluminium. However, as temperatures fell iron and nickel metal grains condensed, and then silicate minerals, such as olivine and pyroxene, which are the commonest minerals found within planets. Only at the lowest temperatures did volatile elements such as sulphur and hydrogen condense as sulphide minerals and ice.

In primitive meteorites, known as chondrites, remnants of the first grains may be present as objects known as calcium-aluminium-inclusions, or CAIs. These mm-sized inclusions consist of high temperature minerals such as hibonite, a calcium and aluminium oxide, and spinel, a magnesium aluminium oxide. These are amongst the first minerals predicted by calculations to condense from nebula gas. Such calculations, however, are complex, particularly if the gas cooled quickly, and are based mainly of the chemical properties of elements derived under very different conditions.

One important aim of the Origins Network is to recreate the very first solid grains in our Solar System. These experiments will be conducted using a specially designed apparatus called the "Nebulotron" which will heat gas under very low pressures to temperatures more than 3000 degrees Kelvin and then allow it to condense under different conditions. The resulting tiny grains, or condensates, will then be analysed by a range of analytical techniques to determine their minerals, shapes and chemistry.

The results of the Nebulotron will tell us about the nature of the first solid grains in our Solar System. They will help us find the earliest grains in meteorites and understand exactly under what conditions these firstcomers formed. They can even be used to compare with dust grains that the next generation of telescopes are observing around other young stars.

Magma Droplets and Gas

Primitive meteorites, the chondrites, contain mm-sized melted droplets known as chondrules. These objects formed as tiny magma droplets and testify to energetic disturbances in the Solar Nebula. The most popular theories for the formation of chondrules suggests these were aggregates of dust in the nebula that were heated and melted in shockwaves.

Chondrules are so abundant in chondrites, often comprising more than 50% by volume, that their chemical make-up plays an important part in the nature of the asteroids from which these meteorites are derived. Chondrules likewise may be important in the chemical compositions of planets, that formed by the piece-wise growth of smaller bodies similar to asteroids. What determines the nature and chemical composition of chondrules is thus very important to understand.

Some models suggest that it is only the nature of the precursor dust aggregates that is important in the chemistry of chondrules, others suggest that molten chondrules within hot gas may swap elements and compounds with the gas, depending on the conditions where they were formed. The Origins Network will test whether this exchange is important through experiments using the Nebulotron.

Chondrules and Matrix

In between the chondrules in primitive meteorites is a matrix of fine-grained minerals in which crystals are often smaller than a millionth of a metre across. This matrix contains many low temperature compounds such as organic molecules and even water. Some grains within matrix are now known to predate our Solar System and some how survived being destroyed in the nebula. How this primitive, low temperature matrix survived, and how it came to be in the same rock as chondrules, with their high temperature origins, is a fundamental unanswered problem.

Some models of the early Solar System propose that chondrules, and CAIs, formed close to the early Sun where they were heated in flares. They were then picked up in magnetic winds streaming from the early Sun and the hot inner Solar Nebula and scattered out to great distances where they could mix with unheated matrix. Another view suggests that chondrules were present in the same place in the nebula, and the matrix simply survived because its smaller grain-size protected it against heating in shocks.

The answer to this problem lies in the compositions of chondrules and matrix. Analyses have shown that together matrix and chondrules appear to have a composition closer to that of the Solar System as a whole. This complementarity may have arisen if elements lost by chondrules during heating were gained by matrix.

To determine whether chondrules and matrix were formed together or seperately high precision chemical analyses of volatile and moderately volatile element, that are most easily lost during heating, will be conducted.