Stardust

Early this August there were some clear nights without moonlight and I went for some little midnight walks, especially to see the sky, which is slowly disappearing due to the light pollution from the growing metropolis in the center of Johnson County. I saw a few meteors streak across the heavens, leaving a momentary trail of blue-white fire, although one was the Luna moth green of burning copper and it might have been a bit of space junk containing copper wiring reentering the atmosphere. Different elements produce different flame colors, both in the lab and in the sky.

You probably already know that an ordinary yellow star like ours runs its nuclear furnace on hydrogen fuel and produces the lightweight elements like carbon, silicon, oxygen, and phosphorus, up to the weight of iron. Midweight elements such as nickel, cobalt, and copper have been observed to be produced by the explosion of a star called a supernova. But the production of the very heavy elements such as gold and platinum remained a theoretical concept, not confirmed by observation, until August 17, 2017.

On that day, the message finally arrived from the constellation Hydra, a mere 130 million light years away, that two neutron stars had collided. These things are not big, but are incredibly dense, a handful of the stuff is calculated to weight billions of tons. Two of them colliding become the pressure cooker of the universe, creating the very heavy elements.

Everything in our solar system appears to be built from debris from earlier stars, which gravity has gathered together and organized, and is still doing so. There is a saying amongst hydrologists that “all water is used water.” And in a deeper sense, all atoms are used atoms. Shooting stars, also called meteors, are samples of those recycled stars coming to us at this very moment. And as they burn up from the friction of their hypervelocity meeting the resistance of our atmosphere, some remnants make it all the way to the earth’s solid surface, often greatly modified by their fiery entry. Others approach more slowly and catch the spin of our atmosphere going away from them and only melt. All these remnants are stardust, also technically called micrometeorites. Most of the particles that survive are in the 0.2 – 0.4 millimeter size range, which in our Department of Agriculture’s soil classification is medium sand.

But up until 2008, there seemed to be two realities regarding their abundance. When captured in traps specifically made for them, they seemed to be rather common, and should be on every rooftop and in every road dust sample. But when actually searched for in those places, that grain size range seemed to be all a mix of earthbound dust plus manmade particles, and nothing from elsewhere.

However, in 2008, Matthew Genge and colleagues published Classification of Micrometeorites, and made it available on the internet. This made it clear that the identity of an individual particle depends heavily upon its entry aerodynamics and thermodynamics, and that there are actually a limited number of variations on the theme, allowing a classification system to group those with similar histories.

A sample of one type of micrometeorite. These became molten glass for a moment upon entry. The leading side ablated away, leaving an iron-rich residual core to take the abuse, much like the tiles of the front of a manned re-entry space vehicle.

Knowing what to look for inspired Jon Larsen to search on ordinary surfaces in Norway, such as roads, roofs, and parking lots. And successfully finding them, he expanded the search to other countries, and included more-or-less natural environments like beaches, mountains, and deserts. And they in fact are everywhere, as predicted by the trap measurements.

Larsen published his results in a two-part book titled In Search of Stardust. Following a brief introduction, the first part illustrates photos of groups of micrometeorites, which have had a similar aerodynamic history entering our atmosphere and therefore look rather alike.

The second part portrays natural earthbound grains, which might be confused with micrometeorites – fulgurites, tiny crystals, microtectites, ooids, volcanic droplets, and ones I never knew about before such as iberulites, plus a variety of man-made particles from roof shingles, welding spatter, smelting metals, fireworks, making mineral wool, using grinding wheels, etc.

This book uses occasional terminology familiar to mineralogists and astronomers, and if that doesn’t describe you, be prepared to do occasional homework while reading it.

So here is another dimension of nature, right in our own backyards, which we never recognized. Some stardust comes from beyond our solar system, manufactured by neutron star collisions and containing the heaviest elements like gold and platinum. And they get delivered directly to your garden, fertilizing your tomatoes and broccoli with the best the universe has to offer.