How much dust from comets and asteroids do you think finds its way to our correct planet and falls to the ground every year?
Go ahead, guess what. A ton? Ten tons?
New research on micrometeorites – literally microscopic bits of meteorites, rock particles and metals from space falling on Earth – shows that approx. 5,200 tons of these cosmic remnants are placed on earth every year. 5.2 million kilograms per year, or about 14 tons per day. to the least. It’s the equivalent of eating a garbage truck every day.
Oof.
Beware, this comes with a few caveats, but the big picture here is that we are hit by a lot of interplanetary detritus all the time.
They are also very small things, smaller than a millimeter and some smaller than the width of a human hair. There is no reason to panic*. It slows from interplanetary speeds to virtually 0 up in the atmosphere and then heads for Earth. It’s not a danger, and it’s actually quite welcome, because it tells us a lot about what’s going on in space.
You might think that most of this material that leads to the ground would be in the form of meteorites of decent size, one centimeter or more. However, it seems that things on the ground are actually dominated by particles smaller than a millimeter. In this case, the numbers matter: there are only so many small particles than the large ones that they represent the majority of the mass that reaches the ground.
The obvious way to do this study is to collect a sample of micrometeorites in the field and count them. Of course, nothing is so simple. The number you will find is small, so you need to make sure you look at a lot of areas. You don’t want the weather to interfere (such as rain washing materials from one place to another, artificially increasing or decreasing the number you find). You do not want to be close to human activity for the same reason. And it would be useful if you could easily test the soil for a long but known period, so as to collect as many samples as possible and calculate a rate.
There is a place that meets all these criteria: Antarctica. Moreover, the Dome-C area where the Concordia Research Station is located. It is about 1,700 kilometers away from the South Pole, at an altitude of 3,200 meters above sea level. It is almost ideal: it does not have too much wind, so the snow is laid evenly, and the rate is well measured – about 2.7 grams of snow fall per square centimeter there per year (depth of a few cm / year).
Over the years, scientists have used carefully cleaned equipment to dig a few trenches, each more than 2 meters deep, to make sure they got to the snow that fell before people built the station in 1995 and dug them a few hundred meters to the wind of the installation. They extracted snow from depths between 3 and over 8 meters, representing a time period of snow that fell from 1920 to 1980.
The snow was melted and the water filtered to extract solids to a size of 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 first are irregularly shaped bits that have survived untouched by atmospheric entry, while the spheres come from bits that have been heated by their hypersonic passage through our air and formed small small balls.
In total, they found 1,280 unmelted micrometeorites and 808 cosmic spheres with sizes ranging from 30 to 350 microns. Measuring the masses and dimensions and using the data and quantities of snow in which they were found, he discovers that the rate at which these things fall to the ground is about 3 and 5.6 micrograms per square meter per year. So not much.
But when you multiply with the very large surface of the Earth (and extrapolate to a range of sizes slightly larger than 12 – 700 microns) you get their number of 5,200 tons per year, or 14 tons per day.
I have to admit, when I read that I was surprised. The number I usually see about this material is that we are hit by approx 100 tons a day! Is one of these wrong?
In fact, they are compatible. I had a suspicion in this regard, but to make sure I contacted one of the authors, Jean Duprat, he also kindly told me how this works (and confirmed my suspicion). Most of the material that enters our atmosphere (called meteoroids) is small, smaller than a grain of sand. A lot of that material ablates the meteoroid (it heats up and is blown by the rapid passage through the air), creating “smoke” particles that are even smaller than 30 microns. This material tends to remain in the upper atmosphere, creating a layer of meteoroid dust 90 to 100 km above the ground. Even if it fell to the ground, it would not have been counted in this study because the particles are too small.
So the total amount of material kicking we per year is much higher. Interestingly, the average amount of meteorite material – which you normally think are meteorites – that hits us is much smaller, only about 10 tons per year. Bigger things are much rarer.
Scientists have managed to get an idea of where this material comes from by looking at its composition and have found that about 80% of it comes from comets and the rest from asteroids. This feels decent with what we know about dust in the inner solar system, although a few questions remain.
Next time you go out on a dark, serene, moonless night, you may be lucky enough to see a bright meteor shower in the sky. If you do, think about his fate for a moment. Most of them will become part of our atmosphere, and a small portion will eventually reach you … and now we have a better idea of how much it does.
*Well, given the last year or two, I can think of a lot of reasons to panic, but it doesn’t happen from astronomical sources.