Panning for Starstuff
An 1833 depiction of the Leonid Meteor ShowerWhenever a sizeable meteor shower comes around—such as the annual Perseid show, peaking on the night of August 12/13—my attention is drawn to the notion of space dust, and star stuff, and Earthly versus cosmic provenance for what may otherwise be considered, mundanely, dust.
Each year at this time, Earth plows through a belt of dust left behind by the periodic passage of comet Swift-Tuttle, causing a significant bump in the number of meteors that might be seen. We usually think of shooting stars—meteors—as bits of dust or small pebbles that fly into Earth's atmosphere at high speed and are vaporized in a blaze of glory by their flight of friction.
In reality, it's the Earth running into the dust specks, not the other way around. It's a bit like a car on a freeway speeding through a cloud of insects, if you think of the windshield as the Earth's atmosphere and the smears and streaks of "bug guts" as the flashy demise of meteors. Earth's orbital speed is about 18 miles per second, which explains the fast and fiery trajectory of shooting stars.
But what happens to the material in the meteor after its luminous tail fades? Well, what do you think? It's a trail of dust high in the atmosphere: eventually, it is pulled to Earth, like any other dust particles, and settles down around us, invisibly, to become part of the landscape, and part of the air we breathe.
How much meteor dust, as well as larger lumps of meteorite that strike the Earth's surface before completely burning up, is there?
It's not known absolutely how much meteoritic dust bathes our world every day, and estimates range widely depending on how the calculation is done, but one (maybe conservative) figure I found is 40,000 metric tonnes per year—about 110 metric tonnes per day. This estimate is for the total amount of material falling on Earth from space, be it in the form of the largest meteorites that occasionally collide with the ground or the continual "rain" of the tiniest micrometeorites that aren't even large enough to register a tail visible to our eyes. All of it. (And, as I said, only one estimate.)
Now let the slicing and dicing of that number begin!
40,000 metric tonnes is about the weight of one hundred fully loaded Boeing 747-400 jumbo jets, or about an eighth the weight of the Empire State Building, or about 48,000 1967-model Volkswagen Beetles.
So, if that amount of material were distributed evenly around the world, how much of it would fall on, say, one typical city block? (I'll assume a city block is a tenth of a mile square—or 1/100 of a square mile.) Given Earth's total surface area of about 197 million square miles, and 100 city blocks in each square mile, that gives 19.7 billion city blocks. Dividing 40,000 metric tonnes by 19.7 billion city blocks yields an astounding…2 grams of material per city block…per year…. Conveniently, that's the weight of half a sugar cube, so is easy to visualize…but not so impressive a figure after all. Guess I won't be panning for meteor dust anytime soon.
But what about over time? Assuming the same rate of meteor-dust fall, since the dawn of civilization my city block will have received 14 kilograms of the stuff—enough to fill a bucket or two….
Of course, ultimately, ALL of the material that the Earth is composed of (all 5.97 billion trillion metric tonnes of it) originally "fell to Earth" from the protosolar nebula, which the entire Solar System formed from. (I had to throw that in to end this blog with something a little more impressive than a couple bucketfuls of dust.)