Micrometeorites
Every day, Earth sweeps up tonnes of dust from space. Most of it arrives as grains smaller than a grain of sand, and the ones that reach the ground are called micrometeorites. They are the most common extraterrestrial material on our planet, they land everywhere, and with patience you can even find them yourself.
Micrometeorites are space rocks less than about 2 mm across, most of them a tenth to a few tenths of a millimetre. Thousands of tonnes reach the ground each year, far more than all the larger meteorites put together. Most began as dust shed by comets or knocked off asteroids in collisions. Some melted into tiny balls on the way down; others survived almost untouched and carry material older than the Earth.
- Size
- About 0.01 mm to 2 mm; most 0.05–0.5 mm
- Reaching the ground
- Roughly 5,000 tonnes a year (best recent estimate)
- Main sources
- Comets, especially those steered by Jupiter, and asteroids
- Arrival speed
- At least 11 km/s; commonly about 12–20 km/s
- Best hunting grounds
- Antarctic snow and ice, deep-sea clay, deserts, roofs
- Oldest found
- Fossil particles in rocks about 2.7 billion years old
Start with the basics Search for them yourself
The basics
What micrometeorites are, how many arrive and where they come from.
What they are
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.
How much falls
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.
Where they come from
Micrometeorites begin as dust shed by comets and ground off asteroids in collisions. Sunlight slowly drags that dust inwards until some of it crosses the Earth's path.
Entry and survival
A micrometeoroid hits the top of the atmosphere at more than 11 km/s. Within a few seconds it is braked, heated and either vaporised, melted or, if it is lucky, left almost untouched.
Kinds of micrometeorite
Melted, partly melted and unmelted particles, and how each is recognised.
Types
Micrometeorites are classified first by how much they were heated on the way down, and then by what they are made of. The result is a ladder that runs from untouched dust to glass beads.
Cosmic spherules
Cosmic spherules are micrometeorites that melted completely as they entered the air and froze again as tiny balls. They are the most common micrometeorites in most collections and the kind amateur hunters are most likely to find.
Unmelted and partly melted
Not every micrometeorite melts. Some arrive gently enough to keep the minerals, textures and even the organic matter they had in space, making them some of the most precious samples in planetary science.
Recognising them
Almost every tiny magnetic ball you find will be man-made or natural Earth material. Knowing the look-alikes is the most important skill a micrometeorite hunter can learn.
Where they are collected
From Antarctic snow and the deep-sea floor to the stratosphere, city roofs and spacecraft.
Antarctic ice and snow
The cleanest, best-preserved micrometeorites come from the polar ice sheets, where there is almost no Earthly dust to hide them and the cold stops them from weathering.
Deep sea and deserts
The first micrometeorites ever recognised came from the bottom of the ocean. The deep-sea floor and the world's driest deserts are still two of the best places to find them, because both gather dust over very long periods.
The stratosphere
The smallest space dust can be caught before it ever touches the ground. Since the 1970s, high-flying aircraft have collected interplanetary dust particles about 20 km above the Earth.
Towns and cities
For decades, scientists believed micrometeorites could never be picked out of city dirt. In 2016 that idea was overturned, when hundreds of genuine space particles were identified in the grime from roof gutters in three European capitals.
Space missions
To be sure exactly where a grain of dust came from, you have to fetch it yourself. Spacecraft have caught comet dust in flight, counted impacts from dust far beyond Neptune and brought back grains from asteroids.
What they tell us
The early Solar System, the delivery of water and carbon, and the air of the ancient Earth.
The early Solar System
Micrometeorites carry material from many more comets and asteroids than the meteorites in our museums, including fragile, carbon-rich bodies whose rocks would never survive the fall as a large stone.
Water and organics
Every year, micrometeorites drop tonnes of carbon and water-bearing minerals onto the Earth. On the young Earth the rain of dust was far heavier, and it may have been one of the ways the raw materials for life arrived.
Fossil micrometeorites
Micrometeorites trapped in ancient rocks are tiny probes of the past. Iron spherules 2.7 billion years old have been used to test what the upper atmosphere was like long before animals existed.
Search for them yourself
A safe, ground-level method, the kit you need and how to photograph your finds.
Search yourself
You do not need to go to Antarctica. With permission, a bucket of sediment from the foot of a downpipe, some water, a magnet, sieves and a microscope, you can search for real space dust at home, without ever climbing a ladder or going onto a roof.
Equipment and photography
A micrometeorite hunt needs only modest equipment to begin, but good optics and lighting make the difference between seeing a black dot and seeing the barred crystals that prove what it is.
People and questions
The people who opened up the field, and answers to the questions people ask.
Key researchers
From Victorian oceanographers to a jazz guitarist, a small number of people turned space dust from a curiosity into a major field of planetary science.
Questions answered
Short, clear answers to the questions people ask most about micrometeorites, from whether they are dangerous to whether you can sell one.
Related elsewhere on Howardites: the glossary entries for micrometeorite and interplanetary dust particle, and comets.