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Adding a new dimension to the early chemistry of the solar system

What was our solar system composed of at the beginning of its formation? Using sophisticated computer simulations, researchers from France and Australia have obtained new insights into the chemical composition of the dust grains that formed in the early solar system which went on to form the building blocks of the terrestrial planets.

The formation of our solar system started from the collapse of a self-gravitating and slowly rotating cloud of gas and dust. As the radius of the cloud decreased, the rotational velocity increased to preserve angular momentum. The increasing rotational speed created large centrifugal forces, and the material which was not already accreted into the new born protostar formed a large thin disk. Meteorites, comets, and all planets in our solar system, including the Earth, formed from rocky material that condensed out from this thin disc, called the solar nebula, which surrounded our young, newly formed sun 4.5 billion years ago.

As regulated by the laws of thermodynamics, the local temperature and pressure in the solar nebula determined the chemical composition of the dust grains that condensed out of the cooling disc. This dust then accreted to form bigger objects like meteorites, asteroids, and planets. Intuitively, we would expect to find hot, high pressure environments closer to the young sun and cold, rarefied environments far from the sun. This is at basis of the one-dimensional thermodynamic condensation sequence for a thin disc, which shows that high temperature materials called refractories (such as ceramic-like dust and silicates) should be located close to the sun while volatile materials (such as ices and sulfur compounds) should form far from the Sun where temperatures are cooler.

However, meteorites, in particular carbonaceous chondrites, one of the oldest objects of our solar system (about 4.5 billion years ago), contain a mix of both refractory and volatile material. Mercury, the closest planet to the sun, also shows mix of both refractories and, surprisingly, volatiles which should not be present according to the traditional one-dimensional condensation sequence.

In order to investigate the problem, Francesco Pignatale from the Centre de Recherche Astrophysique de Lyon, in France and co-authors Sarah Maddison, Kurt Liffman, and Geoff Brooks from Swinburne University of Technology in Australia, computed two-dimensional maps of the distribution of condensates using a 2D model of the solar nebula available, which accounts not only for the distance from the sun but also for the thickness of the disc.

The resulting maps have revealed a complex chemical distribution of the dust, with refractory materials present at large distances from the sun on the surface of the disc, and volatile materials present in the inner disc close to the young sun.

Comparison between the temperature distribution in the 1D disk model (top) and 2D disk model (bottom) with the temperature distribution from D’Alessio et al., 1998 and 1999 and rendering from Pignatale et al., 2016. R is the distance from the forming star in AU (astronomical unit, the average distance between the Earth and the Sun, ~149*106 Km), Z is the altitude from the midplane of the disc and T is the temperature in Kelvin).

The reason of this counter-intuitive behaviour is due to the complex temperature and pressure distribution of disc. According to the 1D disk model, regions close to the sun are characterized by hot temperatures while regions far from the star experience very cold environments.

On the other hand, in the 2D disk model, it is possible to find high temperature regions on the disc surface and at relatively high distances from the forming sun, which are directly heated by the sun. Very cold regions close to the sun in the inner disc are also present. Here, the high concentration of dust prevents the stellar radiation from efficiently going through the disc, thus failing to heat the gas and dust.

These new 2D calculations provide a clearer view of the pristine chemistry of our solar system soon after its formation.

Featured image credit: Artist’s concept of a protoplanetary disk, where particles of dust and grit collide and accrete forming planets or asteroids by NASA. Public Domain via Wikimedia Commons.

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