Beyond Earth: study confirms the existence of superionic water, predicted 20 years ago
Liquid and solid at the same time: this is the strange form of water under extreme conditions, which do not exist on Earth but which can be found on other planets.
Experimental confirmation of the existence of this particular configuration of water – called superionic – comes from a recent study published in Nature Physics and carried out by researchers at the US Lawrence Livermore Laboratory and the University of California, Berkeley, who recreated in the laboratory the very high pressures and temperatures at which the phenomenon occurs.
The existence of superionic water, which could be found on planets relatively ‘close’ to us, such as Uranus and Neptune, or on several extrasolar planets, is nothing new.
Hypothesized in the 1980s, this particular ‘phase’ of water had been accurately described in a study, published in 1999 in the journal Science, conducted by a group of scientists at SISSA and the Abdus Salam International Centre for Theoretical Physics, in collaboration with researchers at the Max-Planck Institute in Stuttgart.
Almost 20 years ago, that research group demonstrated for the first time the existence of a superionic phase of water using advanced models of realistic computer simulation.
“The new experimental result is a further evidence that the theoretical model we defined many years ago was able, despite the then-limited computational resources, to predict very well physical situations which were difficult to reproduce in a laboratory”, commented Erio Tosatti, one of the authors of that study.
But what exactly does this superionic phase of water consist of? “It is a very interesting state because it is neither solid nor liquid: the oxygen remains fixed, while the protons are free to flow”, added Sandro Scandolo, another co-author of the 1999 publication.
In the presence of very high temperatures and pressures, the bonds between oxygen and hydrogen of water molecules are weakened by the heat. At the same time, the high pressure keeps the oxygen blocked in a solid alignment, while the hydrogen atoms, in a liquid form, can flow in between them. Under these conditions, superionic water becomes a conductor where electricity is carried by positively charged atoms rather than electrons, as is normally the case.
This is a phenomenon with potentially significant implications. “If we succeeded in recreating this configuration under less prohibitive conditions, we would have an ideal battery, with charges that move freely inside a solid mechanical structure”, explained Scandolo.
The next step will be to understand whether on planets with abundant water, such as Uranus and Neptune, water is in a liquid, solid, or superionic state.