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Watery Earth mystery solved by dust

Artist’s impression of a very young star surrounded by a disk of gas and dust
Artist’s impression of a very young star surrounded by a disk of gas and dust. Scientists suspect that rocky planets such as the Earth are formed from these materials.

Scientists at the University of St Andrews may have helped solve the mystery of why there is so much water on planet Earth.

An international team of researchers has discovered that small dust grains (no bigger than a millimetre) can accumulate substantial amounts of water from the surrounding gas and ice before they start to collide, stick and form the Earth with enough water to explain the Earth’s oceans.

Researchers from St Andrews, with colleagues from the Max Planck Institute for Extraterrestrial Physics in Germany and the University of Groningen, Delft University of Technology and Tilburg University in the Netherlands, concluded that this process takes only one million years, which is enough time, according to common evolutionary scenarios, for star and planet formation. The water-rich dust grains clump together to form first pebbles, then kilometre-sized boulders and, eventually, Earth. The research is published in the journal Astronomy and Astrophysics.

Dr Peter Woitke, of the Centre for Exoplanet Science at the University of St Andrews, said: “The mystery as to why Earth has so much water has previously baffled. One theory suggested that the water was delivered by icy comets and asteroids that hit the Earth. A second scenario suggests the Earth was born ‘wet’ with the water already present inside ten-kilometre-wide boulders from which the planet was built. However, the amount of water that these large boulders can contain is disputed.”

This latest research calculated a variant of the boulder-with-water scenario. The most common materials the dust grains and boulders are made of are silicates of various types, some of them similar to sand on the beach.

These silicates have been shown to be capable of hosting water molecules in their lattice structure, the co-called ‘wet silicates’. The new journal article shows that these wet silicates form naturally in space already below about 150°C, if time permits.


The paper, Equilibrium chemistry down to 100 K. Impact of silicates and phyllosilicates on the carbon to oxygen ration, by P Woitke, C Helling, G H Hunter, J D Millard, G E Turner, M Worters, J Blecic and J W Stock is published in Astronomy and Astrophysics, and is available via DOI 10.1051/0004-6361/201732193.

Additional reading: On water delivery in the inner solar nebula: Monte Carlo simulations of forsterite hydration.

Image: Artist’s impression of a very young star surrounded by a disk of gas and dust. Scientists suspect that rocky planets such as the Earth are formed from these materials.
Credit: NASA/JPL-Caltech

Issued by the University of St Andrews Communications Office.

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