How much space dust falls on Earth each year?
Thousands of tonnes of micrometeorites reach the ground every year. The exact figure is surprisingly hard to pin down, and different methods have given answers that differ by more than ten times.
The best recent measurement, from very clean Antarctic snow, suggests about 5,200 tonnes of micrometeorites reach the ground each year, give or take about 1,200 tonnes. More dust than that enters the top of the atmosphere, but much of it is vaporised on the way down. Larger meteorites add only a few tonnes to a few tens of tonnes a year.
Key facts
- Reaching the ground
- About 5,200 tonnes a year, uncertain by roughly a quarter (Antarctic snow study, 2021)
- Entering the atmosphere
- Estimates from about 2,000 to about 100,000 tonnes a year; many recent ones fall between 10,000 and 40,000
- Per square metre
- Around a hundred-thousandth of a gram a year, or roughly one sizeable particle
- Most of the mass
- In grains about 0.1 to 0.3 mm across
Why the number is hard to measure
No single instrument sees all the dust. Particles arrive at different speeds and sizes, some burn up completely, and those that land are mixed into the soil, the sea and the ice, where they are diluted by enormous amounts of Earthly dust. Each method sees only part of the picture, so each gives a different answer, and the honest summary is a range rather than one number.
There is also a difference between two questions that are easily confused: how much material enters the top of the atmosphere, and how much reaches the ground as solid particles. The first is larger, because a large share of the incoming mass is turned to vapour.
Counting craters on a satellite
One of the most quoted figures comes from a bus-sized satellite called the Long Duration Exposure Facility, which a space shuttle placed in orbit in 1984 and another retrieved in 1990. During nearly six years in space, its panels were peppered with microscopic craters. By counting the craters on the side that faced away from Earth and working out what size of grain made each one, researchers estimated in 1993 that about 40,000 tonnes of dust, give or take 20,000 tonnes, meets the Earth each year. They also found that most of the mass arrives in grains around 0.2 mm across.
Watching the upper atmosphere
Other estimates come from what the dust leaves behind high in the air. Vaporised grains create thin layers of metal atoms, such as sodium and iron, between about 80 and 105 km up, and these layers can be measured with lasers from the ground. Radar sees the faint trails of tiny meteors. Estimates based on the atmosphere and on radar have often come out much lower than the satellite figure, some as low as a few tonnes a day. Reviews of all the methods have quoted inputs anywhere from about 5 to about 300 tonnes a day, which is roughly 2,000 to 100,000 tonnes a year.
Collecting what lands
The most direct method is to collect the particles themselves from a place where nothing else accumulates. Near a research station on the high Antarctic plateau, a French-led team spent almost 20 years and six expeditions digging snow that had fallen in known years, melting it in clean conditions and filtering out every extraterrestrial grain between 0.03 and 0.35 mm. Their result, published in 2021, was that about 5,200 tonnes of micrometeorites reach the Earth's surface each year: about 3,600 tonnes as melted cosmic spherules and about 1,600 tonnes as unmelted particles. They estimated that around 15,000 tonnes a year enters the top of the atmosphere, so roughly a third survives to the ground as solid grains.
Earlier collections at the South Pole, filtered from the bottom of the station's drinking-water well, suggested that only about a tenth of the particles in the 0.05 to 0.7 mm range survive the journey. The size of the surviving share depends strongly on how fast the grains arrive, which is one reason studies disagree.
What it means for your garden
Spread 5,200 tonnes a year over the whole planet and each square metre receives about a hundred-thousandth of a gram a year. In practice that means roughly one cosmic spherule larger than 0.1 mm per square metre per year, plus many smaller grains. A large flat roof of 1,000 square metres might therefore collect hundreds of genuine micrometeorites over a decade, mixed with far more industrial and natural dust. That is why rooftop sediment is worth searching; see micrometeorites in towns and cities.
Over geological time the total is still tiny. Even at today's rate, all the micrometeorites that have landed since the dinosaurs died out, 66 million years ago, would make a layer only about a fifth of a millimetre thick if spread evenly over the Earth.
The rate changes over time
The dust supply is not constant. When an asteroid shatters, its debris adds a pulse of dust that can last for millions of years. Sediments on the ocean floor record such pulses through helium-3, a rare form of helium carried in by space dust. They show a rise beginning about 8.2 million years ago, matched to the break-up of a large asteroid called Veritas, and another around 36 million years ago, at about the same time as two large impact craters formed. Much earlier, about 466 million years ago, the break-up of the parent asteroid of the L chondrites flooded the inner Solar System with dust and rocks; see fossil micrometeorites.
Did you know?
One of the scientists behind the satellite crater estimate later became a NASA astronaut and flew on the space shuttle in 2008.
In a typical year, micrometeorites deliver a hundred times or more extraterrestrial material to the ground than all the meteorites big enough to pick up by hand.
A single large impact can outweigh many years of dust: the Chelyabinsk asteroid of 2013 weighed around 10,000 tonnes before it broke apart in the air.