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Crystallography of ice

Why is it interesting to study ice? The book reviewed in this issue 4 of Volume 28 of Crystallography Reviews boosts the reader’s amazement, enthusiasm and curiosity about the wonder and science of ice and snowflakes. While the review article gives a comprehensive overview of all the known twenty polymorphic forms of ice with an outlook to further predictable forms. Hydrogen is the most common element in the Universe, around 75% of all atoms in our galaxy is hydrogen. Oxygen is the third most common element in space, making up about 1% of all the atoms. Water, made of these two elements, plays a key role in the formation and evolution of our planetary system and it is essential in the life on Earth. The planet Earth is formed in the warm part of the sun’s protoplanetary disk, at a location well within the ‘snow line’. The presence of water is vital in the search for extraterrestrial life. Brighter regions observed by an optical telescope could indicate reflections of frozen water. Landers and rovers can collect samples from the surface of a planet to be placed in an analysis chamber. There is water-ice on the surface of the moon near the poles [1]. A subglacial lake on Mars, 1.5 km below the southern polar ice cap was detected [2]. Asteroids in the asteroid belt also contain large amounts of water-ice that could be harvested if humans ever regularly travel beyond the inner Solar System. Europa, a moon of Jupiter, and Enceladus, a moon of Saturn, have huge subsurface oceans with a layer of tens or hundreds of kilometres of ice covering their surfaces. Other moons of Jupiter and Saturn such as Ganymede and Titan may have subsurface oceans as well. There are very likely still millions of other icy bodies out there, just waiting to be explored. The varying conditions on the myriads of planets, moons, asteroids in space provide the opportunity of the probable formation of different ice polymorphs. Although ice may be one of the most studied crystalline solids in human history, new discoveries on ice are still being reported on a regular basis. The full review article ‘Neutrons meet ice polymorphs’ by Kazuki Komatsu from the Geochemical Research Center, Graduate School of Science, The University of Tokyo, Japan, gives an extensive review of the discovery and crystallographic characterization of ice polymorphs formed in different conditions. The presented historical background elucidates the experimental difficulties in ice research. The current epoch described by the author is the ‘age of ice-rush’, as the rate of discovery of ice polymorphs has accelerated in the last two decades owing to the advances in neutron diffraction studies of ice under pressure. The most extreme conditions, of both high-temperature and high-pressure, led to a new ice polymorph being created (namely XVIII) at 100GPa and 2000K. The transition between hydrogen-ordered and hydrogen-disordered phases is a common problem for many ice polymorphs. Ice polymorphs may exist in fully ordered and fully disordered states, but also they can be in partially-ordered states in between. The review by Kazuki Komatsu consists of three