“Impossible” Compounds Found on Super, Earth-like Planets

December 31, 2015 | Joanne Kennell

Earth-like planet
Photo credit: Ph03nix1986/Wikimedia (CC BY-SA 4.0)

These compounds break the rules of chemistry.

With the use of mathematical models, scientists have “looked” into the interior of super-Earths and discovered that some of the compounds break the classical rules of chemistry.

The authors of the paper are a team of researchers from MIPT, led by Artem Oganov, a professor of the Skolkovo Institute of Science and Technology and the head of the MIPT Laboratory of Computer Design.  They used an algorithm called USPEX, created by Oganov, to find out which compounds may form by silicon (Si), oxygen (O), and magnesium (Mg) at high pressures on super-Earths (planets with a solid surface and a mass several times greater than Earth).

“Earth-like planets consist of a thin silicate crust, a silicate-oxide mantle -- which makes up approximately 7/8 of the Earth's volume and consists more than 90% of silicates and magnesium oxide -- and an iron core. We can say that magnesium, oxygen, and silicon form the basis of chemistry on Earth and on Earth-like planets,” said Oganov.

The researchers studied several compositions of Mg-Si-O that might occur at pressures ranging from 5 to 30 million atmospheres (1 atmosphere is equal to the average atmospheric pressure on Earth) — pressures that probably exist in the interior of super-Earths.  For example, recently discovered Gliese 832c is five times heavier than Earth, and Kepler-10c is 17 times the mass of Earth.

Based on the results of the computer model, the interior of these planets may contain “exotic” compounds of MgSi3O12 and MgSiO6 — meaning they have many more oxygen atoms than the MgSiO3 here on Earth.  It turns out the MgSi3O12 is a metal oxide and a conductor, whereas other substances containing Mg-Si-O are semiconductors.  

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“Their properties are very different to normal compounds of magnesium, oxygen, and silicon - many of them are metals or semiconductors. This is important for generating magnetic fields on these planets. As magnetic fields produce electrical currents in the interiors of a planet, high conductivity could mean a significantly more powerful magnetic field,” said Oganov.

A stronger magnetic field means the planet will have more protection from cosmic radiation, and therefore may have more favorable conditions for life to form.  The team also predicted new magnesium and silicon oxides that do not fit with the classical rules of chemistry — SiO, SiO3 and MgO3, in addition to oxides MgO2 and Mg3O2, previously predicted by Oganov at lower pressures.

The model was able to determine the decomposition reaction that MgSiO3 undergoes at ultrahigh pressures on super-Earths — which increases the heat transfer rate within the mantle.  “This affects the boundaries of the layers in the mantle and their dynamics. For example, an exothermic phase change speeds up the convection of the mantle and the heat transfer within the planet, and an endothermic phase change slows them down. This means that the speed of motion of lithospheric plates on the planet may be higher,” said Oganov.

Convection, which results in the mixing of the mantle and subsequent movement of the tectonic plates, can be either fast or slow.  The fact that Earth’s continents are in constant motion, “floating” on the surface of the mantle, is what gives volcanism and air for us to breathe.

So these super-Earths may not only have a protective magnetic field, but they may also have breathable air.  That is pretty exciting.

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