Well this is really quite fascinating. NASA Astronaut Don Pettit, currently aboard the ISS a few hundred kilometres above Earth’s surface, does what any curious human being would do in his place. He does cool things that you can only do in microgravity, and writes about them on the internet, in his blog Letters to Earth.
This one post in particular caught my eye. This is an image of ice crystals grown in space from a thin film of water, taken using a makeshift crossed polariser to show the individual ice crystals. Space frost, if you will. No gravity to cause segregation as ice crystals start to form. No crystallites forming first and floating to the surface due to their lower density. The result is this, which is both pretty and fascinating.
So a few things here are really quite interesting. Firstly, note the shapes. All the ice crystals at the edge are long and thin – termed “columnar crystals”. They’re shaped like that because the crystals begin to grow at the edges of whatever vessel they’re contained in. In this case, the film of water was suspended in a metal hoop, so the places where water was in contact with metal were the first places where ice crystals formed. They’re also bloody huge! With no gravity to affect the way the crystals form, they can just keep on growing, making for much larger crystals than you’d find on Earth.
Secondly, those speckles you can see are water bubbles. You can find those in the ice crystals you’d make in your freezer. All water contains some amount of dissolved air, and as the water freezes into ice, the air is dropped out of solution, forming bubbles. In the ice cubes you make in your freezer, the bubbles again would float to the surface. You usually end up with a cluster of them near the top of the ice cube. In space though, the lack of gravity means that many those bubbles are more or less uniformly distributed throughout the ice. Just like in Earthbound ice though, the majority of bubbles are centred in what was obviously the last part of the ice to freeze.
Finally, those big patches in the image, near the centre? Those are single crystals. And they’re also huge! Here on Earth, single crystal materials are used in a number of high performance applications, because they have some unique engineering properties. For example, the turbine blades in jet engines are made from single crystals of metal – the only way for them to survive the unreal stresses and strains which proliferate inside jet engines. But this sheet of water has effortlessly formed one huge single crystal.
If companies like SpaceX and Planetary Resources truly are heralding a new era of space industry, I think it’s only a matter of time until orbital factories start to be constructed to make use of the unique manufacturing conditions of microgravity!
You can see more sparkly pictures of Don’s space ice on Letters to Earth.