What are micrometeorites?
Micrometeorites are tiny pieces of rock and metal from space, smaller than about 2 mm, that have survived the fall through Earth's atmosphere and come to rest on the ground.
Imagine a meteorite shrunk until it is thinner than a few human hairs laid side by side. That is a micrometeorite. They are far too small to see falling, but so many arrive that they land on every square metre of the planet. Most are a tenth to a few tenths of a millimetre across.
Key facts
- Size
- About 0.01 mm to 2 mm; most between 0.05 and 0.5 mm
- Mass of one particle
- Roughly a billionth of a gram to a tenth of a milligram
- Made of
- Mostly rock rich in olivine and pyroxene, iron oxides and metal; some are full of carbon
- Where they land
- Everywhere on Earth, all the time
Meteoroid, meteor, meteorite: the small version
Space rocks get different names depending on where they are. A solid body travelling through space is a meteoroid; the streak of light it makes in the air is a meteor; any piece that reaches the ground is a meteorite. The same pattern works at the small end. A grain drifting through the Solar System is a micrometeoroid, and once it has landed on Earth it becomes a micrometeorite.
There is no sharp natural boundary between a micrometeorite and an ordinary meteorite. Scientists commonly draw the line at 2 mm: anything smaller that came from space and was recovered on Earth counts as a micrometeorite. At the lower end, particles smaller than about a hundredth of a millimetre (10 micrometres) are usually treated simply as dust.
Cosmic dust, interplanetary dust particles and micrometeorites
Cosmic dust is the umbrella term for all the tiny solid grains in space, from the specks between the planets to the dust between the stars. Only a small part of it ever meets the Earth.
Interplanetary dust particles, often shortened to IDPs, are the smallest grains that scientists collect from the Earth's own air. They are typically a few to a few tens of micrometres across and are caught high in the stratosphere, around 20 km up, by research aircraft before they can mix with dust from the ground. Read more on the page about dust from the stratosphere.
Micrometeorites are generally larger, mostly 0.05 mm and up, and are picked up from the ground, the ice or the seabed after landing. Because they are bigger, many of them were heated more strongly on the way in, and many melted completely. IDPs and micrometeorites overlap in size and origin; the main difference is where and how they were collected.
How small is small?
A human hair is roughly 0.05 to 0.1 mm thick. A typical micrometeorite of 0.2 mm is about the size of a grain of fine sand or the full stop at the end of a printed sentence. A ball of rock that size weighs about fifteen millionths of a gram, so you would need tens of thousands of them to make up the weight of a paper clip.
Particles this small are too faint to make a meteor you could see with your eyes. The naked-eye meteors on a clear night are usually made by grains from about a millimetre to a centimetre across. The far more numerous micrometeoroids make trails that only radar and sensitive cameras pick up.
Different from the meteorites in museums
Micrometeorites are not just small meteorites. The two groups sample different parts of the Solar System. Most meteorites that fall are ordinary chondrites from a handful of stony asteroids, while carbon-rich carbonaceous chondrites make up only a few per cent of falls. Among micrometeorites the balance is reversed: most of the ones that kept their original texture resemble carbonaceous chondrites, and computer models suggest that most of the dust Earth sweeps up was shed by comets. See where they come from.
Their small size also changes what survives. Fragile, crumbly material that would break apart as a large boulder can arrive intact as a single grain. Some micrometeorites contain organic matter and minerals not seen in any known meteorite, which is why researchers treat them as a separate and valuable collection of space samples.
Three main kinds
Micrometeorites are sorted mainly by how hot they became on the way down. Cosmic spherules melted completely and froze into tiny balls. Scoriaceous micrometeorites partly melted and became frothy. Unmelted micrometeorites kept the minerals and textures they had in space. The page on types of micrometeorite explains each one.
Did you know?
The word micrometeorite was coined by the American astronomer Fred Whipple, who also worked out in 1950 why the smallest grains can reach the ground without burning up.
Micrometeorites are the most common extraterrestrial material on Earth's surface. By mass, they outweigh all the larger meteorites that land in a typical year many times over.
Because they are so small, micrometeorites usually fall far more slowly than larger stones once the air has braked them, drifting down over hours or days.