Introduction

The upper layers Uranus and Neptune, as well as many exoplanets are composed primarily of small molecules like water, ammonia, and methane. These are referred to as ‘icy planets’ with the ‘ices’ consisting of these small molecules.  Within planets material is subject to extreme pressure and temperature, up to millions of atmospheres and thousands of degrees. Such extreme conditions have remarkable effects on the properties of matter, which can affect the overall characteristics of the planet.

Examples of this include water ices becoming superionic at high pressure and temperature. In these ices the hydrogen ions can move freely within a crystal lattice of oxygen ions, so they are electrically conductive. Large convections within conductive ice layers have been proposed as an origin for the unusually complex magnetic fields of Uranus and Neptune, giving an example of planet scale effects arising from the properties of material within them.  The methane also undergoes changes at extreme conditions, first forming more complex hydrocarbons which then decompose into diamond and hydrogen deeper within the planets.

Diamond Rain Controversy

Decomposition of hydrocarbons and subsequent ‘diamond rain’, in which the denser diamond sinks through the surrounding ices, have previously been suggested. However, depending on the technique used, there was a huge disagreement in the pressure and temperature required for diamond formation from hydrocarbons. Static compression, in which the sample is squeezed between two anvils made from diamond, found diamond to form above 10 GPa (100 kbar) and 2500 K. Samples subject to dynamic compression, in which shockwaves are driven into the sample, does not observe it below 140 GPa and 6000 K.

To resolve this discrepancy, and so determine where in icy planets diamond forms, and in what size of icy exoplanet this process can occur, we performed time resolved X-ray pump-probe experiments on statically compressed polystyrene at the European X-ray Free Electron Laser (XFEL).  The chemical structure precursor hydrocarbon does not matter, as carbon hydrogen bonds become short lived and memory of the original structure lost prior to diamond formation. The polystyrene was compressed with a perforated gold foil which absorbs the extremely high intensity X-ray pulses (~100 μJ over <50 fs) delivered by EuXFEL with 4.5 MHz repetition rate (220 ns between pulses). By aligning the beam on the small perforations, the bulk of the X-rays pass though and probe the heated polystyrene within, while the edges of the beam are absorbed by the gold and heat the system for the next pulse.  In this way the response of dense hydrocarbons to temperature can be followed in time and diamond formation observed in situ.

The figure shows the time evolution of the X-ray diffraction intensity over a run at 20 GPa and 2500 K. The pink arrow shows the onset of the diffraction peak from diamond after 30 μs. Indeed, for every run between 19 and 27 GPa, the range of pressures studied, diamond formed above 2500 K, but always after 30 to 40 μs. Shorter times or lower temperatures did not produce diamond. This explains the discrepancy in previous results – dynamic compression typically only lasts a few nanoseconds – orders of magnitude shorter timescale than diamond formation at these milder conditions. Indeed, the conditions at which dynamically compressed hydrocarbons form diamond coincide with those at which hydrogen becomes metallic, which may influence the carbon-hydrogen demixing kinetics.

Lower Diamond Formation Pressure

The lower pressure required to form diamond impacts our understanding of the processes within icy planets. The diamond rainfall provides a source of internal energy from the sinking diamond, and this will originate from a shallower depth changing the region which is heated. In contrast to dynamic compression studies, our results place the depth for diamond formation above the superionic ice layers. Subduction plumes of diamond rich material will then fall through these layers which will stir them.  This stirring might play a role in driving or initiating the large-scale convections within the conductive superionic ices which are proposed as an origin of icy planets’ magnetic fields.

Beyond the solar system the results also change our understanding of icy exoplanets. The lower pressure requirement means that diamond formation, and the subsequent heating and convective effects, can occur in much smaller planets. It also points to, possibly abrupt, changes in their composition based on size. During accretion when pressure within the icy mantle exceeds that required for diamond formation the diamond rain process will begin. The denser diamond then sinks into the planet pointing to a deep carbon rich, possibly diamond, layer.  Formation of this layer would deplete the carbon in the mantle of icy planets large enough for diamond rain, with profound effects on their internal composition, chemistry, and potential astrobiology.