Stardust in the snow

As I write this in mid-December there is no snow here – probably a trend – but people around me are singing “Let it Snow” and dreaming of a white Christmas. Snow is wonderful stuff, considering that it is just frozen water, but when you examine snowflakes you see beautiful crystalline forms usually with six-fold symmetry, and each one looks different. It is said that no two snowflakes are alike, and one may respond, “How do you know?” Since typical snowflakes have about 10 quintillion (1019) water molecules, they cannot be identical. But each water molecule is identical to every other.

Underlying that six-fold symmetry is the shape of the water molecule: H2O. The chemical bonds between the central oxygen atom and the two hydrogen atoms form an angle of 105 degrees, close enough to 120 degrees to form tiny hexagonal ice crystals around dust particles in supercooled air. How the six branches grow from the corners depends on the temperature and humidity, always changing as it is blown around, making every snowflake different.

The shape of the water molecule underlies snowflakes’ six-fold symmetry. These snowflakes were spotted on the author’s cold car roof. Photo: Mike Albrow

Here I want to talk about the nonsnow in the snow – those tiny dust particles that start them off. While most are wind-blown from the ground or come from volcanoes, thousands of tons of cosmic dust rain onto Earth from space each year.

Let’s go as far from civilization as we can, to Antarctica, and collect snow where no people or dogs have been. Clean old snow! This is a good place to find meteorites, meteors that have survived burning up in the atmosphere. You may have one in your backyard and not know it, but on a pristine snowfield any rocks must have dropped from the sky.

Scientists melted half a ton of Antarctic snow and separated the dust sediment. They found a few atoms of a rare isotope of iron, 60Fe, which has four more neutrons than normal iron 56Fe, and is radioactive. Half of those nuclei decay every 100,000 years – that’s called the half-life. That may seem a long time to you, but 60Fe cannot be several millions of years old.

The scientists conclude that it was must have been made in a supernova explosion in our part of the galaxy, perhaps 40,000 years ago. That made a vast cloud of all kinds of radioactive debris. The solar system is still passing through that cloud and sweeping up dust. You have some in your backyard, but Antarctic snow is where to find it.

This is a version of an article that originally appeared in Positively Naperville.