Geodynamica

Hydrocarbons are observed on the surfaces of many icy moons and planets. At high-pressure high-temperature conditions, such as occur in larger planets, they are known to dissociate to form diamond and hydrogen. Within our solar system Uranus and Neptune easily reach the requisite 10 GPa and 2000 K for this process, while icy moons do not.  Densities indicative of icy compositions are commonly observed for exoplanets, many of which have radii intermediate between the local icy moons and icy planets. These so-called 'mini-neptunes' are a common class of exoplanet and, where hydrocarbons are incorporated within their ices, are candidates for diamond formation. Here we simulate model icy exoplanets to investigate the size required to induce diamond formation.  Where the conditions are met, the denser diamond will sink through the ices deeper into the planet under gravity. This provides a source of internal heating, and will sequester the carbon deep within the planet. As a consequence, exoplanets with deep ice layers likely have shallow regions depleted in carbon.