Water is weird.
And I mean that in a good way.
Its amazing chemical properties can – and have – filled books, and it’s no exaggeration
to say the properties of water make life possible.
But it really is super weird.
Most chemicals have one solid form, or at most a couple.
Depending on who you ask and how you count, water has seventeen or more.
And while there’s only the one on Earth, we expect to find these exotic forms of ice
… in space.
Temperature and pressure have a big influence on whether a chemical will exist in a solid,
liquid, or gaseous state at any given moment.
They both affect how molecules arrange themselves in a stable way.
And there are so many kinds of ice because of the unique chemistry of water molecules.
The oxygen atom in a water molecule has two hydrogens sticking off it, and it also has
two lone pairs of electrons.
Electrons are tiny, but the negative charges repel each other.
So those pairs of electrons actually take up space.
Effectively, there are four things sticking off the oxygen.
They shuffle around to be as far away from each other as possible, and that takes the
form of a tetrahedron with oxygen in the center.
The hydrogen atoms and electron pairs on different water molecules can form hydrogen bonds with
one another – one hydrogen with one electron pair.
As long as every water molecule is neatly hydrogen bonded with its neighbors in a crystalline
form, that’s solid ice.
But tetrahedrons can fit together in more than one way.
Plus, the electron pairs on nearby molecules repel each other and can push the molecules
a bit farther apart.
The result is that water molecules are constantly jostling each other around to find a stable
configuration.
Change the temperature or the pressure just a bit, and the molecules will shift to a different
crystalline form.
There are about seventeen of these crystalline forms.
Each one gets a Roman numeral, named in order of their discovery, from good ol’ ice I
on up.
The reason we say "about" 17 is because it’s really hard to achieve the extreme temperatures
and pressures needed to make all of them in a lab.
Some of the ones that have been observed are metastable, or stable-ish.
Another form of ice would theoretically dominate at that temperature and pressure, but the
metastable form is the one the molecules have settled into for a moment.
There are also forms of ice that have been predicted to exist in computer models and
simulations, but we’ve never actually created.
Then, there are forms of ice that exist outside the Roman numeral system, because they’re
not crystalline forms.
Amorphous ice, for example, doesn’t have a very orderly, repeated crystalline structure,
so it doesn’t get a fancy number.
But it’s still a solid, like glass, or butter.
And there’s even wilder forms, called superionic ices, where the oxygen atoms are locked into
a crystal lattice but the hydrogen atoms are free to move around.
So how many of these ices have you unknowingly run into, or made snowmen out of?
Probably just one.
All of the ice that falls out of the sky and piles up on the ground on home sweet planet
Earth is ice I. Specifically, it’s the hexagonal form of ice I.
There’s also a cubic form of ice I, which is found in clouds.
The crystals grow a little differently, but on a molecular level, they’re indistinguishable.
So they both get the same number.
But ice I isn’t the most common form of ice in the universe.
That title probably goes to amorphous ice, which space is chock full of.
It coats lots of particles of interstellar dust.
Amorphous structures, like amorphous ice, form when substances cool too quickly to settle
into an orderly crystal structure.
And interstellar space can get pretty gosh-darn cold.
Superionic ice probably exists on Uranus and Neptune, the ice giant planets.
Some scientists think this bizarre ice phase could account for the strange magnetic field
properties that have been observed on those planets.
And beyond our solar system, an exoplanet called Gliese 436b is thought to host a whole
bunch of super-hot ice X.
Now, that’s not a verbal typo.
We don’t normally think of ice as being hot, but the surface temperature of this exoplanet
is estimated to be a toasty 439 degrees Celsius.
This is where that temperature-pressure relationship comes into play.
The pressure on this exoplanet is huge too – so high that even warm liquid water basically
gets squashed solid.
There are probably even more weird ices lurking out there in space, ready for us to discover.
Water is something so common and so essential that we don’t tend to think about how peculiar
its chemistry really is.
But when you take a close look at it, these molecules can surprise you!
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