Electric ice a shock to the solar system
ELECTRIC ice may pervade space. This strange form of water is more persistent than was previously thought, and the discovery could change our understanding of how the solar system formed. It might even give ice a new role in the emergence of the complex organic molecules needed for life.
In a single molecule of water – H2O – there is a charge separation. That’s because the two positively charged hydrogen atoms cluster at one end, away from the single negatively charged oxygen.
However, the charges get mixed up when ordinary ice, known as ice Ih, forms. While the oxygen atoms arrange themselves in a repeating pattern, the pairs of hydrogen atoms that extend from them don’t. Instead, they randomly take one of a number of positions (see graphic).
Cool ice to about 60 kelvin (-213.15 °C), though, and the hydrogens rearrange themselves so they are aligned. In the resulting, perfectly regular crystal,
called ice XI, there are distinct regions of positive and negative charge.
That polarisation makes ice XI clump together much more readily than ordinary ice 1h, in the same way that dust particles are drawn together by static electricity. If the early solar system contained a lot of the stuff, it could mean that planets formed much more quickly than current models assume. Electric ice could also attract organic compounds, possibly accelerating the emergence of complex molecules and eventually life.
While claims that ice XI may exist naturally in Antarctica have yet to be verified, astronomers have long suspected that it hangs out in the outer solar system. They haven’t directly detected it there, however.
In 2006, Masashi Arakawa, now at Kyushu University in Fukuoka, Japan, and colleagues produced ice XI in the lab between 57K and 66K. This is about the temperature found on Uranus and its moons, but too narrow a range for ice XI to be a major player in planet formation.
But ice XI turns out to be hard to kill. Tiny flecks of the stuff embedded in ordinary ice can help convert all of it to electric ice, a little like the ice dreamed up by Kurt Vonnegut in his novel Cat’s Cradle, a single crystal of which could freeze the world’s oceans. Crucially, the conversion of ice Ih to ice XI occurs at higher temperatures than if ice XI forms from scratch.
Arakawa’s team cooled disordered, run-of-the-mill snowflake ice until it transformed into ice XI at 60 K. Then they warmed it to 100 K so that it reverted to ordinary ice. When they turned the thermostat back down, ice XI returned at a higher temperature than before: 72 K. Yet cooling regular ice to 72 K did not change its structure (Geophysical Research Letters, DOI: 10.1029/2011GL048217).
The researchers think that nanoscale regions of ice XI survive the heat, providing a template for neighbouring regions, which then seem to “remember” how to become ice XI. “There are little seeds of XI inside the Ih,” says astrochemist Rachel Mastrapa of NASA’s Ames Research Center in California, who was not involved in the work. “Given the right conditions, those seeds can grow.”
Further experiments revealed that this memory effect happens even when ice XI is heated to 111 K, the sort of temperature found on the moons of Jupiter. That suggests ice XI may be far more common in space than thought.
There is at least one lingering doubt, though. Switching phases is a glacially slow process – pure water ice can take thousands of years to convert from the Ih form to ice XI.