Two weeks ago Brandon Dunovant, who lives in a Chicago suburb, got the idea to collect the grit that had accumulated on his roof in hopes of finding micrometeorites. Maybe you’ve heard of this experiment or even tried it yourself.
As you might guess, micrometeorites are tiny extraterrestrial particles, most under than 2 millimeters in size. They derive from comets, asteroids and even the moon and Mars, entering the atmosphere gently enough to avoid vaporization.
Some remain unmelted, others partially melt. Micrometeorites deliver an astonishing 20,000 to 30,000 tons of extraterrestrial material to Earth every year. Enough to inspire some to search their rooftops for asteroid dust. Dunovant got out the hose, sprayed down his shingles several times over and collected the washed-off debris by tying a fine-mesh on the ends of his downspouts. Since many meteorites are attracted to a magnet, he swept through his gritty pile with a powerful rare-earth magnet to separate potential meteorites from twigs and other detritus.
After an entire day tweezing apart the more interesting specimens from shingle grit, what was left made Brandon’s jaw drop.
“What I was looking at were aerodynamically-shaped black metallic pieces, some perfectly round, some pancake shaped, some bars, a couple buttons” and more. Dunovant wondered if they might be melted bits of the Orionid meteor shower which had peaked that weekend.
Despite their wonderful shapes and magnetic attraction, Brandon’s debris – like the materials found by many others who’ve tried the same experiment – contain few if any meteoric particles. According to Dr. Michael Zolensky, who curates cosmic dust for NASA, a typical Orionid meteor particle strikes the atmosphere at around 50 miles per second “pretty much guaranteeing that all the comet dust gets oxidized, melted, and
vaporized.” Like a leaf taking forever to fall, even a surviving Orionid needs a few days to make the journey from 60 miles up to the ground. That makes it even less likely that Brandon captured one.
Then there’s the size issue. Most micrometeorites are between 1 and 10 microns (a mircon’s a millionth of a meter or 1/1000 of a millimeter) across with bigger ones up to 1/10 to 3/4 millimeter in size. The largest piece, an elongated bar, pictured in Brandon’s gem jar photos above is 3 mm across; most of the others are 1.5mm or smaller.
Zolensky and others who researched micrometeorites especially in populated areas have discovered that much of what’s collected by amateurs is a combination of particles released from power plants, smokestacks, commercial boilers as well as tiny balls of clay from distant dust storms. The power plant emissions contain magnetite which responds well to a magnet.
Collection experiments in much cleaner places relatively free of industrial pollution like the ices of the South Pole and Greenland have yielded fine samples of extraterrestrial dust in some though not in all cases. When chemically analyzed, scientists find the same minerals as our found in larger meteorites though weighted toward the carbon-richer carbonaceous types.
Zolensky looked for micrometeorites at the South Pole years back and essentially found none. More recently, he and a team collected possible cosmic dust on a remote Pacific atoll and expect to have to wade through millions of terrestrial grains to find even a few tantalizing spheres from the asteroid belt.
So Brandon and others who’ve dragged a fine-toothed comb through their roof debris may possibly have something in their cache of curious particles, but the odds of real micrometeorites are slim indeed. Questions still remain. What causes the apparent aerodynamic shapes? I’ve searched fly ash photos online and can’t find any that quite match those in Brandon’s images. If you’d like to try the experiment on your own to see what’s drifting down on your house, click HERE for detailed instructions.
Both today’s blog and yesterday’s Part I are tinged with irony because Dr. Peter Jenniskens realized he was too hasty in evaluating the “rock” found outside Lisa Webber’s house after the recent California meteorite fall. On closer inspection, he determined it was indeed a meteorite. Read more about his change of mind HERE.