Does Neptune ever
feel lonely, with a wall of
diamonds round it’s heart?
Diamonds might be precious but they’re composed of carbon, an element that’s common here on Earth and throughout the universe.
Under the right conditions (pressure and heat) carbon turns into diamonds. Marvin Ross predicted in 1981 that such conditions might be found in the mantels of the Solar System’s ice giants, Neptune and Uranus.
Recent research has supported this, with laser shock experiments on polystyrene performed by Kraus et al. (2017) replicating the conditions approximately 10,000km below the surfaces of Neptune and Uranus. Their experiments created nanodiamonds, supporting evidence that diamond precipitation occurs in the mantels of these planets.
Now further research has strengthened this evidence. The earlier studies used pure hyrdrocarbon systems (polystyrene is C8H8) but the interiors of Neptune and Uranus are more complex than that, consisting mainly of a dense fluid mixture of water (H2O), methane (CH4), and ammonia (NH3).
To understand diamond formation under more complex conditions similar to those found on Neptune and Uranus, Zhiyu et al. (2022) investigated diamond formation using polyethylene terephthalate (PET) plastics (C10H8O4). The researchers found that diamond formation is likely to be enhanced by the presence of oxygen, which in their research accelerated the splitting of the carbon and hydrogen.
Under the conditions found on Neptune and Uranus it’s likely that much larger diamonds would be formed, potentially millions of carats in weight. Over millennia these vast diamonds are predicted to sink slowly through the icy layers of the mantel before melting near the cores, creating an ever changing layer of diamonds around the cores of the planets.
The latest research may also explain another peculiarity about Neptune and Uranus: their unusual magnetic fields. Under the conditions that form diamonds in the mantel, the researchers also found evidence that superionic water might be created. Superionic water conducts electric current and is likely to impact the planets’ magnetic fields.
In addition to learning more about the Universe, there are practical implications for us on Earth resulting from the research too. Nanodiamonds have a range of important uses, including in medical sensors, non-invasive surgery, sustainable manufacturing, and quantum electronics. This latest research points the way towards a new way of fabricating nanodiamonds for such uses.
Further reading:
Ross, M. (1981) The ice layer in Uranus and Neptune—diamonds in the sky? https://doi.org/10.1038%2F292435a0
Kraus, D. et al. (2017) Formation of diamonds in laser-compressed hydrocarbons at planetary interior conditions https://doi.org/10.1038/s41550-017-0219-9
Zhiyu, H.E. et al. (2022) Diamond formation kinetics in shock-compressed C─H─O samples recorded by small-angle x-ray scattering and x-ray diffraction https://doi.org/10.1126/sciadv.abo0617