| The cubes that chill iced teas and Aperol spritzes on a hot summer day may not seem particularly mysterious. You learned the basics in school: Ice is made of H2O molecules, each consisting of a pair of hydrogen atoms flanking an oxygen. Below zero degrees Celsius (32 degrees Fahrenheit), these molecules freeze into a repeating formation — usually hexagons, which are related to the sixfold symmetry of snowflakes. At higher temperatures this grid dissolves, as the molecules gain enough energy to either slosh around as water or fly free as vapor. What else is there to know? A lot, as it turns out. Deep research into the properties and behaviors of ice has sparked discoveries in multiple areas of physics. Materials researchers have found that nature can produce a bewildering variety of crystalline forms of water beyond the familiar type that forms in your freezer. Even if we stick to freezer ice, the varying speeds at which it can form have led physicists to rethink how systems evolve in general. And climate scientists are only beginning to understand the complicated interactions between Arctic ice, ocean and sunlight. Ice fascinates physicists because it's a familiar and easily available substance with a rich range of behaviors. In recent years, that fascination has produced an array of strange and wonderful research highlighting that the cubes in your drink are but the tip of an iceberg — forgive me — of ice-related phenomena.
What's New and Noteworthy One of the most basic questions physicists ask about ice is: What forms of ice can exist? The hexagonal grid of H2O molecules forms naturally on ponds and in the sky, but under more extreme conditions the same molecules can assume other arrangements — currently 19 and counting. In 2019, physicists confirmed the existence of "superionic" ice, or ice XVIII, by blasting apart water droplets with high-intensity lasers and imaging their structure with X-rays. Ice XVIII is weird. It's black, hot and metallic — the oxygen atoms form a cubic lattice that the positively charged hydrogen atoms flow through, much as electrons flow through a copper wire. The pressure-cooker conditions needed to make this metallic ice exist inside icy planets like Neptune and Uranus. These planets' abundant supplies of H2O molecules could make ice XVIII the most common form of water in the universe. Evidence has also accrued for ice XIX, a slow-flowing variant of VI with a distinct arrangement of hydrogens, and ice XX, another superionic phase. Researchers also have a lot to learn about what makes water molecules freeze into icy lattices. Subzero temperatures are essential, but additional ingredients — such as bacteria or specks of dust — are also needed to coax the molecules into alignment. Physicists have been using computer simulations to explore how different materials enable or hinder ice formation. Ice-inspired research has led physicists to uncover new principles of changing systems in general. In the 1960s, a hasty Tanzanian student named Erasto Mpemba was surprised to notice that hot milk seemed to freeze into ice cream faster than cool milk — a phenomenon that came to be called the Mpemba effect following his description of it in a publication. Physicists are divided as to whether hot water really freezes faster than cold water, but Mpemba's effect has been confirmed in other substances. The counterintuitive phenomenon drove physicists to consider, in the abstract, how a system in a state of change (such as warm water in a freezer) reaches an unchanging equilibrium (such as ice). In modern experiments using microscopic glass beads controlled by lasers, researchers showed that energetic "hot" particles sometimes found shortcuts to low-energy "cold" states simply because their energy helped them explore their options more quickly than their more sluggish counterparts. For all ice's strangeness, it's the substance's most familiar behavior — melting — that is proving the most consequential. Earth's temperature depends on its ability to reflect sunlight, and the polar ice caps are the shiniest parts of the planet. As the climate warms, white ice melts into dark ocean, which absorbs more heat and causes more melting. It's a simple process that depends on complicated details — the thickness of the ice, how many dark puddles form on its surface, how fissures fracture and re-fracture the shifting frozen landscape. In 2020, Shannon Hall reported on a journey to the Arctic with the scientists who observe and analyze these details. Their research illuminates how the physics of ice may decide our fate. | 
