How much dust do you think comets and asteroids find in our fair planet and fall to the ground every year?
Go ahead, guess. A ton? Ten tons?
New research looking at micrometeorites – literally microscopic pieces of meteorites, particles of rocks and metals from space falling on Earth – shows that approx. 5 200 tons of this cosmic debris lays down on the ground annually. 5.2 million kilos per year, or about 14 tons per day. By the least. This is equivalent to the mass of a garbage truck every day.
Oof.
Note, it comes with some caveats, but the big picture of this is that we are constantly being hit by a lot of interplanetary damage.
It is also very small things, smaller than a millimeter in size and some smaller than the width of a human hair. There is no need to panic*. It slows down from interplanetary velocities to basically 0 in the atmosphere and then drifts down to earth. It’s no danger and is actually very welcome, because it tells us a lot about what’s going on in space.
You would think that most of this material that descends to the ground is in the form of meteorites of decent size, the centimeters or more of which are left. However, it appears that the stuff on the ground is actually dominated by particles smaller than a millimeter. In this case, the numbers count: there are just so many more small particles than large ones that they make up the bulk of the mass that gets on the ground.
The obvious way to do this study is to collect and count a sample of micrometeorites on the ground. Of course, nothing is that simple. The number you will find is small, so you need to make sure you look at many areas. You do not want the weather to interfere (like rainwear from one place to another, artificially increasing or decreasing the number you find). For the same reason, you do not want to be close to human activities. And it would help if you could easily sample the soil over a long but well-known time frame, allowing you to collect as many samples as possible and calculate a rate.
There is one place that meets all the criteria: Antarctica. On top of that is the Dome-C area where the Concordia research station is located. It is about 1,700 kilometers from the South Pole, at an altitude of 3,200 meters above sea level. It is almost ideal: it does not have much wind, so the snow is laid down evenly and the pace is well measured – about 2.7 grams of snow falls on every square centimeter per year (a depth of a few cm / year).
Over a period of years, scientists have used carefully cleaned equipment to dig several trenches, each more than 2 feet deep, to make sure they have snow that fell before people built the station in 1995, and they have the wind of the facility dug a few hundred yards. . They hauled the snow from depths of 3 to more than 8 meters, which was a time span of snow that fell from 1920 to 1980.
The snow is melted and the water is filtered to extract solids up to about 30 microns (0.03 mm, slightly smaller than the width of a human hair).
The micrometeorites they found are in two broad classes: unmelted micrometeorites and cosmic spheres. The former are irregularly shaped pieces that have survived atmospheric access unchanged, while the spherical shapes are pieces that are heated by our air through their hypersonic passage and form teenage balls.
In total, they found 1,280 unmelted micrometeorites and 808 cosmic spheres ranging from 30 to 350 microns. By measuring the masses and sizes and using the dates and amounts of snow in which they were found, they find that the velocity at which these things fall to the ground is about 3 and 5.6 micrograms per square meter per year. So, not much.
But if you multiply by the very large area of the earth (and extrapolate to a slightly larger size range of 12 – 700 microns), they get their number of 5 200 tons per year, or 14 tons per day.
I have to admit I was surprised when I read. The number I usually see around this material is that we are hit about 100 tons per day! Is one of these wrong?
They are actually compatible. I had a suspicion about it, but to make sure I contacted one of the authors, Jean Duprat, and kindly told me how it works (and confirmed my suspicion). Most materials that penetrate our atmosphere (called meteoroids) is small, smaller than a grain of sand. Much of the material ablates from the meteoroid (is heated and deflated by the rapid passage of air), creating ‘smoke’ particles that are even smaller than 30 microns. This material tends to stay in the upper atmosphere and creates a layer of meteoroid dust 90 – 100 km above the ground. Even if it fell to the ground, it would not be counted in this survey because the particles are too small.
So the total amount of material hit us per year is much higher. Interestingly, the amount of larger meteorite material – which you normally consider meteorites – is much lower on average, only about 10 tons per year. Bigger things are much rarer.
The scientists could get an idea of where this material comes from by looking at its composition and they find that about 80% of it comes from comets and the rest from asteroids. It ties in decently with what we know of dust in the inner solar system, though there are still questions.
The next time you leave on a dark, bright, moonless night, you may be lucky enough to see a bright meteor over the sky. As you do so, think for a moment about its fate. Most will eventually be part of our atmosphere, and a small portion will eventually run down to you … and now we have a better idea of how much it does.
*Well, given the past year or two, I can think of many reasons to panic, but it does not happen from astronomical sources.