President Bush exercised the first veto of his presidency on Wednesday, turning down a bill that would have increased the scope of human embryonic stem-cell research. Several dozen “Snowflake” babies—born from frozen embryos that had been donated rather than destroyed—were on hand to help celebrate the announcement. The Snowflake Embryo Adoption Program took its name “because embryos are unique and fragile.” Is each snowflake that falls from the sky really unique?
Yep, although it may not always look that way. As snowflake expert Kenneth Libbrecht has pointed out, any two ice crystals of a reasonable size are going to be different on the atomic level. That’s because ice crystals are made up of water molecules, which aren’t always exactly the same. Some water molecules have an atom of deuterium in place of one of the hydrogens, and others have an atom of O18 in place of the regular O16. Even a smallish crystal might comprise a quintillion water molecules—so the chance that any two would have the same pattern of deuterium and O18 atoms is effectively zero.
On the gross level, though, snowflakes can look very much alike. In 1951, the International Commission on Snow and Ice divided ice crystals into seven basic shapes, ranging from “stellar crystals” to “irregular forms.” More recent schemes have subdivided those categories into 35, 41, or even 80 distinct types. The simplest among these is the hexagonal prism—which serves as the foundation for all of the more complex crystal forms. A pair of these basic shapes could look identical under the microscope. (Like ice crystals, not all “Snowflake” children appear unique to the naked eye. Among the 110 babies who have been born through the program are two sets of identical twins.)
Most ice crystals that fall from the sky are “irregular,” which means they can have any shape at all. To say that no two irregular snowflakes are alike is akin to saying that no two grains of sand are alike, or no two specks of dust. It’s more interesting to point out that the more intricate, symmetrical snowflake forms (like the stellar crystals) never repeat themselves. That’s because a complex snowflake grows in different ways depending on the weather conditions it experiences in the sky. A single ice crystal might get blown from one temperature and humidity zone into another many times before it falls to the ground. The exact timing of these movements determines its eventual shape. (For example, a flake that starts growing as a cylinder might end up in conditions that are more conducive to the growth of disklike crystals. The end result would be a “capped column,” or something that looks a bit like a Star Wars tie fighter.)
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Explainer thanks Kenneth Libbrecht of the California Institute of Technology and Ron Stoddart of the Snowflake Embryo Adoption Program.