Ice of the bizarre kind: Researchers have gained new insights into the formation of so-called superionic ice under heat and high pressure. It is a crystalline form of water that is electrically conductive. It could occur naturally in the hydrous planets Uranus and Neptune. In addition to their importance for basic physical research, the findings therefore also shed light on the origin of the unusual magnetic fields of these two celestial bodies.
We know water as steam, liquid or ice. The last of these three physical states is known to be cold. However, this is only type I of the crystalline substance. In this version, under the earth’s pressure conditions and temperatures below zero degrees Celsius, the water molecules are arranged in a grid that is held together by hydrogen bonds between the hydrogen and oxygen atoms. But this is not the only possible version of water ice: under extreme temperature and pressure conditions, various other shapes with different characteristics can form.
A shape called superionic ice is of particular interest. In the case of particularly extreme environmental values, a special crystalline structure forms: semi-solid, semi-liquid and electrically conductive. The existence of superionic ice has been predicted on the basis of various models and its formation has even been observed under extreme laboratory conditions. However, some aspects of the formation of this literally exciting form of water ice as well as certain features have so far remained unclear. An international team of researchers has therefore continued to investigate the mysteries of the extreme state of water.
Hot “crushed ice” in the laboratory
To do this, the scientists generated pressures of up to 150 gigapascals – around 1.5 million times atmospheric pressure – in so-called diamond stamp cells the size of a computer mouse. Laser was used to bring water in the tiny sample chamber to temperatures of up to 6227 degrees Celsius. As the researchers explain, conditions prevailed such as those found in the interior of the planets Neptune or Uranus at a depth of several thousand kilometers. With the help of X-ray diffraction, the scientists have observed how the crystal structure of H₂O changes under these conditions. Optical spectroscopy methods were also used to determine the electrical conductivity.
As the researchers report, they were able to record how the structure of the water changed in the course of the experiments. At first, the oxygen and hydrogen atoms moved a little around their fixed positions. Then only the oxygen remained so stable and finally formed its own cubic crystal lattice. As the temperature continued to rise, the hydrogen ionized, releasing its electron to the oxygen lattice. The atomic nucleus – the positively charged proton of hydrogen – then became mobile within this solid. This created electrical conductivity, the scientists explain. In this way, the hybrid version of solid and liquid was formed – the superionic ice. The special thing about it was that the researchers were now able to identify two combinations of temperature and pressure at which superionic ice phases form.
Significance for planetary research
Based on these laboratory results, co-author Sergey Lobanov from the German Research Center for Geosciences in Potsdam now looks at Uranus and Neptune: “Our research brings together fundamental physical and geoscientific interests. Because water also plays an important role in the interior of many planets – in the case of the highly watery gas planets Uranus and Neptune also for the creation of their unusual planetary magnetic fields, ”says Lobanov. These are not parallel and symmetrical to the axis of rotation, as in the case of the earth, but are inclined and not centered. Nor are they created by the movement of liquid iron in the core. It is believed that the two planets’ magnetic fields are instead caused by a conductive, water-rich layer.
The new findings now provide further information, the scientists report: “In the phase diagram, we can plot the pressure and temperature inside Uranus and Neptune. The pressure can roughly be used as a measure of the depth inside the planets. On the basis of the phase boundaries, which we measured more precisely, we can now read that in both planets about the upper third is liquid and that there is superionic ice in the deeper interior. This confirms predictions about the origin of the observed magnetic fields, ”explains Lobanov.
But questions still remain unanswered. As Lobanov announced, the examinations in the high-pressure laboratory should therefore now continue. With the help of ever more precise insights into the formation and characteristics of the ice phases, the scientists want to elucidate the internal structure and the magnetic field of the two gas planets even better.
Source: Helmholtz Center Potsdam – GFZ German Research Center for Geosciences, Carnegie Institution for Science, specialist article: Nature Physics, doi: s41567-021-01351-8