We’re used to the idea that ice is a good sign on exoplanets – despite the myriad of weird and wonderful forms extraterrestrial species could take, it’s pretty well accepted that alien life would need water to evolve.
But a paper published this month in Physical Review Letters suggests that, under certain conditions, the opposite might be true. A team of physicists has finally solved a mystery surrounding a bizarre form of ice – and it could have implications for the search for extraterrestrial life.
Now, what you think of as “ice” is actually just one of many different phases. Virtually all ice on Earth, from the cubes in your drink to the glaciers in Antarctica, is technically ice Ih – the “h” stands for “hexagon”, the shape the oxygen atoms take as the water freezes. Very occasionally, high in the atmosphere, Earth sees a second form of ice: ice Ic, pronounced “ice icy” because, well, scientists love a good pun.
But under extreme pressure, ice can form in any of 17 other phases. There’s ice II, III, and so on, all the way up to ice XVI, each categorized by the conditions required for it to form. Finally, there’s square ice, discovered in 2014, and amorphous ice, a phase so weird and exotic that it’s technically categorized as glass.
Hiding in the middle of this list is ice VII. To form at room temperature, ice VII requires pressures 30,000 times higher than at sea level, which is why it’s only been found in pretty weird places – trapped inside diamonds deep within the Earth’s mantle or in a lab housing the world’s most powerful X-ray laser.
The problem is that various lab experiments on how ice VII forms have ended up with different results. One experiment, for instance, found ice VII forms first on a nearby surface, then spreads inwards. Others found water samples freeze evenly throughout in astonishingly fast times – some as quick as 10 nanoseconds.
To solve this puzzle, the team used a mathematical model to investigate nucleation – the first stage of freezing, when the water molecules begin to arrange themselves into the structure of a solid. By analyzing this model under various conditions, they made a breakthrough that explains dozens of previous experiments, according to a statement.
It turns out that, below a certain pressure threshold, ice VII does indeed form from the outside and spread inwards. But above this threshold, the ice spreads rapidly throughout the sample – within billionths of a second.
“Our work shows that ice VII forms in a very unusual way – by popping into existence in tiny clusters of about 100 molecules and then growing extremely fast, at over 1,000 miles per hour [1,610 kilometers per hour]!,” lead author Jonathan Belof explained to Physics Central.
But this discovery could be bad news for alien life.
Ice VII may be rare on Earth, but exogeologists think it exists on icy moons and planets such as Europa – or the slew of “ocean world” planets outside our Solar System. Recently, these were suggested as possible, if unexpected, candidates for life – but the team’s discovery throws all that into question once again.
“Water on these ocean worlds, under bombardment from other planetary bodies such as meteors or comets, undergoes intense changes for which life might not survive,” explained Belof.
“The shock waves launched by the explosions from these planetary impact events can compress water to a pressure over 10,000 times that found on the Earth’s surface and cause the water to freeze into… ice VII.”
So could planets support any kind of life under these extreme conditions? The team hope their research will help find the answer.