A thick layer of diamonds may be hidden hundreds of kilometres below the surface of Mercury, according to an experiment recreating early conditions on the solar system鈥檚 smallest planet.
We already know that Mercury is rich in carbon, the element that makes up diamonds, thanks to observations by NASA鈥檚 Messenger orbiter, which revealed a surface covered in graphite. 聽This carbon is likely to be spread throughout the planet鈥檚 interior, where its exact form will depend on the temperature and pressure at each location.
To investigate, at the University of Liege in Belgium and his colleagues recreated the conditions of Mercury鈥檚 interior as it existed 4 billion years ago, when the planet consisted of a vast ocean of magma around a small metallic core.
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The team put a mixture of compounds known to be present on Mercury, including sulphur, titanium and aluminium dioxide, inside a carbon-rich graphite anvil press, crushing it under 7 gigapascals of pressure and heating it up to nearly 2000掳C. The temperature at which the sample fully melted was around 1900掳C, which Charlier and his team say would have been the temperature of Mercury鈥檚 magma ocean when it formed, and the right conditions to form diamonds.
While this would be warm for humans, the researchers were surprised to find the melting point was lower than predicted, as not much higher would see graphite rather than diamonds form.
鈥淲e found out it鈥檚 pretty cold 鈥� colder than we expected,鈥� says Charlier. 鈥淐older means a greater chance to get stable diamonds.鈥�
The researchers think that once these diamonds formed in the core, they would have floated up through Mercury鈥檚 interior and eventually settled between its dense metallic core and less dense silicate mantle, growing in thickness over millions of years.
Today, the diamond layer could be as thick as 200 metres and make up around 1 per cent of the planet鈥檚 total mass. Individual diamonds in the layer could be much larger than ones found on Earth, says Charlier, but it is hard to know for sure. 鈥淟et鈥檚 be honest,鈥� he says, 鈥淲e have no idea about the potential size of those diamonds.鈥�
Nature Communications
