They believe that the molecules near the surface behave differently from those inside the ice. Ice is crystalline, which means that each water molecule is locked in a periodic lattice. However, at the surface, water molecules have fewer neighbors to interact with and therefore have more freedom of movement than in solid ice. In the so-called dissolved layer, the molecules are easily removed by a skate, ski or shoe.
Today, scientists generally agree that the melt layer exists, at least near the melting point, but they disagree on its role in the glaciation.
A few years ago, Luis MacDowell, a physicist at the Complutense University of Madrid, and his collaborators ran a series of simulations to find out which of the three theories—pressure, friction or pre-melting—best explains ice sliding. “In computer simulations, you can see atoms moving,” he said—something impossible in real experiments. “And you can actually look at the neighbors of those atoms” to see if they are periodically separated, as in a solid, or disordered, as in a liquid.
They realized that their simulated ice was actually covered by a liquid-like layer just a few molecules thick, as the melting theory predicts. When they simulated a heavy object sliding over the ice, the layer thickened, according to the compression theory. Finally, they tested frictional heating. Near the melting point, the melted layer was already thick, so frictional heating did not have much effect on it. However, at lower temperatures, the slippery material produced heat that melted the ice and thickened the layer.
“Our message is: All three opposing beliefs are at work at one time or another,” MacDowell said.
Hypothesis 4: Amorphization
Or maybe local melting isn’t the main cause of the ice slide.
Recently, a team of researchers from Saarland University in Germany identified arguments against all three existing theories. First, for the pressure to be high enough to melt the surface of the snow, the contact area between (say) the ski and the snow must be “absurdly small,” they wrote. Second, if the ski is traveling at realistic speeds, experiments show that the heat generated by friction is not sufficient to cause melting. Third, they found that at very cold temperatures, ice still slides even though there is no melted layer ahead. (The surface molecules still have neighbors, but at low temperatures they don’t have enough energy to overcome the strong bonds with the solid ice molecules.) “So whether the sliding of the ice comes from the fusion of all or a few of them, or there is something else we don’t know yet,” said Achraf Atila, a material scientist in the group.
Scientists are looking for other explanations in research on other materials, such as diamonds. Gem polishers have long known from experience that some sides of a diamond are easier to polish, or “softer,” than others. In 2011, another group of German researchers published a paper explaining this phenomenon. They created a computer simulation of two smooth diamonds facing each other. Surface atoms were mechanically dislodged from their bonds, allowing them to move, form new bonds, and so on. This slip forms a structureless, “amorphous” layer. Contrary to the crystalline nature of diamond, this layer is disordered and behaves more like a liquid than a solid. This amorphization effect depends on the orientation of the molecules on the surface, so some sides of the crystal are softer than others.
Atila and his colleagues argue that the same process occurs in ice. They simulate smooth ice surfaces, keeping the temperature of the simulated system low enough to ensure no melting. (So any smoothness can have a different meaning.) Originally, surfaces attracted each other, like magnets. This was because water molecules are dipoles, with unequal concentrations of positive and negative charge. The positive end of one molecule attracts the negative end of another. Ice traction creates small welds between slippery surfaces. As the areas slid past each other, the seams broke and formed new ones, gradually changing the composition of the ice.
