Why Ice Is Slippery: A 200-Year Mystery Gets a New Answer

Every winter, countless people slip on icy sidewalks. Meanwhile, figure skaters glide gracefully across frozen rinks. Both experiences stem from the same phenomenon: ice is slippery. Yet after more than 200 years of scientific inquiry, researchers still debate exactly why.

Last year, scientists at Germany's Saarland University proposed a fourth hypothesis that challenges all previous explanations. Instead of ice melting to create slipperiness, they argue that the crystal structure itself breaks down under pressure and motion.

The proposal adds a new chapter to one of science's most enduring everyday mysteries—and highlights how even the most familiar phenomena can harbor unexpected complexity.

Three Theories, Two Centuries of Debate

The scientific quest to understand ice's slipperiness began in the mid-1800s with three competing theories.

The pressure theory emerged first. English engineer James Thomson suggested that when we step on ice, our weight creates enough pressure to lower the melting point, forming a thin water layer. His brother William (Lord Kelvin) experimentally confirmed that pressure does indeed affect melting points.

But in the 1930s, Frank Bowden and T.P. Hughes at Cambridge calculated a problem: an average skier doesn't exert nearly enough pressure to significantly alter ice's melting point. You'd need to weigh thousands of kilograms to make a meaningful difference.

The friction theory became their alternative explanation. Testing in an artificial ice cave in the Swiss Alps, they found that materials with high heat conductivity (like brass) created more friction than poor conductors (like ebonite). Their conclusion: frictional heating melts the ice surface, creating the slippery layer.

Yet this theory faces a timing problem. As University of Amsterdam physicist Daniel Bonn points out, "You only melt the ice behind you, not the ice you're actually skating on." Ice can be slippery the moment we step on it, before any motion occurs.

The premelting theory offers a third explanation. In 1842, Michael Faraday observed that ice surfaces naturally host a thin, quasi-liquid layer even before contact. Surface molecules in ice crystals have fewer neighbors to bond with than interior molecules, giving them more freedom of movement. This "premelted layer" could explain the slipperiness.

While scientists generally accept that such layers exist near ice's melting point, they disagree about their role in creating slipperiness.

The Fourth Answer: Crystal Destruction

The German researchers found all three theories insufficient. Ice remains slippery even at extremely cold temperatures where no premelted layer should exist, pressure effects are minimal, and frictional heating is negligible.

Materials scientist Achraf Atila and colleagues drew inspiration from an unexpected source: diamond polishing. Gem polishers have long known that some faces of a diamond are "softer" than others. In 2011, German researchers explained this through computer simulations showing that sliding diamonds create a disordered, "amorphous" layer where atoms break free from their crystal bonds.

The Saarland team argues that ice behaves similarly. When ice surfaces slide against each other, water molecules form tiny "welds" through hydrogen bonding. As these welds break and reform during sliding, they gradually destroy ice's ordered crystal structure, creating an amorphous layer that behaves more like a liquid than a solid.

Crucially, this happens through mechanical destruction of the crystal lattice, not thermal melting. Their simulations showed this amorphization occurring even at temperatures too low for traditional melting.

The Debate Continues

Not everyone's convinced by the amorphization theory. Luis MacDowell at Madrid's Complutense University maintains that "all three controversial hypotheses operate simultaneously to one or another degree." His simulations suggest different mechanisms dominate under different conditions—pressure near the melting point, friction at moderate speeds, and premelting at higher temperatures.

This ongoing disagreement reflects a deeper challenge in materials science: the same phenomenon can have multiple contributing causes that vary with conditions. Ice's slipperiness might not have a single explanation but rather a complex interplay of factors.

The practical implications extend far beyond academic curiosity. Understanding ice's true slipperiness mechanism could revolutionize winter sports equipment design, improve road de-icing strategies, and enhance frozen food storage technologies.

Bonn's team continues investigating with their "microscopic ice-skating rink"—rotating metal pieces at various speeds while measuring the forces involved. Each experiment adds data points to a puzzle that's proven remarkably resistant to simple solutions.

The ice slipperiness debate embodies science at its most honest—acknowledging uncertainty while methodically building understanding. Sometimes the most profound discoveries come not from exotic frontiers, but from finally grasping what's been slipping beneath our feet all along.