The space between stars in galaxies – the so-called interstellar medium – is filled with dust and gas, and the interactions between these particles are crucial in the formation of stars. To better understand the events that lead to star birth, Martin McCoustra and colleagues investigated the key role that ice plays in these interactions, which typically take place at temperatures of around 10 kelvin.

Astronomers believe that most interstellar dust grains are coated with ice, which is composed of water, carbon monoxide and other compounds. When these particles cluster together, gravitational energy is thought to be converted into heat, causing the ice to evaporate. Using a technique known as temperature-programmed desorption, McCoustra and colleagues measured the rate that different molecules evaporated from an icy surface as it was heated up.

The researchers had expected to find that the water ice and the carbon monoxide ice formed distinct layers. But they discovered that these ices become mixed when the grains are heated slightly. “When you grow water films at very low temperatures, they act like a sponge, and carbon monoxide deposited on top of these films can flow into the cavities,” explains McCoustra. The team will now incorporate their results into models of star formation.

Geophysicists know from our planet’s magnetic field that iron exists at the centre of the Earth, but the exact composition of the core is unknown. Other elements are thought to be present because seismic waves – which move at well-defined speeds through different materials – travel through the Earth more slowly than they travel through iron.

By studying the abundance of various elements in the solar system, Mike Gillan and Dario Alfe chose likely candidates for these additional elements – silicon, sulphur and oxygen. Taking into account the quantum mechanical properties of these elements, Gillan and Alfe used Monte Carlo simulations to compute the density of the core – which has a solid centre and a liquid outer layer – for a wide range of compositions. They also estimated the temperature of the core for the different compositions and pressures to within 400 kelvin – an improvement on the accuracy of earlier experiments.

“We found that there must be oxygen in the core, otherwise you can’t reproduce the seismic observations at all,” says Gillan. He and Alfe calculated that at least 8% of the core must consist of oxygen, and about 8% of it consists of a mixture of sulphur and silicon.

Both groups were speaking at the Institute of Physics Condensed Matter and Materials Physics Conference, which was held jointly with the 19th General Conference of the Condensed Matter Division of the European Physical Society.