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The frosty cubes we pull from our freezers are just one of 17 possible types of ice, and an 18th type isn’t far from being made real. A team of researchers has now discovered a type of porous, lightweight “aeroice” – the aerogel of ice, if you will – that can tell us more about how water works under extreme conditions.

In “normal ice”, water cools at ambient pressure and its molecules freeze into a solid crystal form arranged in hexagon shapes.

But not every kind of ice takes on this hexagonal structure. Pressure and temperature both affect the millions of configurations water can take on the molecular level. Hexagonal ice, and the occasional cubic ice in our upper atmosphere, are the only two forms that occur naturally on Earth. Other ices might be found on exoplanets or in the atmospheres of the outer planets.

Under atmospheric pressure or higher, water molecules get squeezed and freeze into a solid denser than normal ice.

But when the pressure drops below this, water molecules become a less-dense, lightweight crystal that’s more air than molecule – like an icy candy floss.

So far, we only know of two kinds of low-density ice: space fullerenes, which are 80 per cent of the density of normal ice; and zeolitic ices, which mimic the structure of mineral-based zeolites, but are built of blocks of water molecules like Lego towers, and are between 50 and 90 per cent the density of normal ice. But lighter ice structures hadn’t been spotted until now.

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Category: Science