Where do micrometeorites 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.
Comets leave trails of dust as their ice turns to gas near the Sun. Asteroids crash into each other and throw off clouds of fragments. Over thousands to millions of years, the push of sunlight makes these grains spiral slowly towards the Sun, and the Earth sweeps some of them up. Computer models suggest that most of the dust we collect came from comets.
Key facts
- Largest source
- Comets whose orbits are controlled by Jupiter, according to most models
- Second source
- Asteroids, especially the debris of recent collisions
- Minor sources
- Fast long-period comets, dust from beyond Neptune, and a trickle of dust from between the stars
- Travel time
- Very roughly a million years for a 0.1 mm grain to spiral in from the asteroid belt
The dust cloud we live in
The inner Solar System is filled with a thin, flattened cloud of dust called the zodiacal cloud. It lies roughly in the plane in which the planets orbit. From a truly dark site you can see it as the zodiacal light: a faint cone of glow rising from the horizon after the end of twilight in spring evenings or before dawn in autumn mornings, in the northern hemisphere. That glow is sunlight scattered by the same kind of dust that ends up on our roofs.
The cloud cannot last on its own. Its grains are constantly lost, falling into the Sun, being blown away or colliding with planets, so it must be topped up all the time by fresh dust from comets and asteroids.
Comets
A comet is a lump of ice and dust. When it comes near the Sun, its ice turns to gas and carries dust off its surface, building the tail and leaving a trail of grains along its orbit. Meteor showers, such as the Perseids, happen when the Earth crosses one of these trails.
Most of the dust that becomes micrometeorites, however, is not shower dust. Shower grains hit the Earth fast and most burn up completely. Instead, models of the zodiacal cloud, tested against the speeds of radar meteors and the infrared glow of the cloud, point to Jupiter-family comets: comets on short orbits, taking less than about 20 years to go round the Sun, whose paths have been bent by Jupiter's gravity. Different models give them roughly 70 to more than 85 per cent of the dust entering the atmosphere. The 2021 Antarctic snow study found its counts fitted best with about 80 per cent of the micrometeorites coming from comets and the rest from asteroids.
Comets that fall in from the distant Oort cloud, and comets like Halley's that take many decades per orbit, strike the Earth at up to 72 km/s. Their dust is mostly vaporised, so they add little to the micrometeorites on the ground.
Asteroids and dust bands
The asteroid belt is a slow grinding mill. Collisions chip and shatter asteroids, producing fragments of every size, from boulders down to dust. In 1983 an infrared satellite discovered faint bands of warm dust circling the Sun at particular tilts. Later work linked them to asteroid families, groups of fragments from a single catastrophic collision, especially young ones such as the Karin family, which formed about 5.8 million years ago, and the Veritas family, about 8.3 million years old.
Some micrometeorites can be matched to asteroid material: coarse-grained unmelted particles that look like broken chondrules, and particles whose oxygen isotopes match ordinary or carbonaceous chondrites. A very small share, under one per cent, look like the basaltic rocks of the asteroid Vesta.
The slow spiral inwards
Dust does not fly straight from an asteroid to the Earth. A grain orbiting the Sun absorbs sunlight on one side and re-emits the energy in all directions. Because the grain is moving, that re-emitted light gives it a tiny backward push, which acts like a brake. This effect, called Poynting–Robertson drag, makes small grains spiral slowly towards the Sun. For a grain 0.1 mm across starting in the asteroid belt, the journey takes very roughly a million years; smaller grains drift in faster.
On the way, a grain's orbit becomes more circular and its speed relative to the Earth drops. That is important: dust that arrives slowly is far more likely to survive entry, which is one reason asteroid and Jupiter-family comet dust is over-represented among micrometeorites compared with the fast dust of meteor showers. See entry and survival.
Dust from further away
Beyond Neptune, collisions in the Kuiper belt make dust too, and a spacecraft carrying a dust counter built by students has measured it far out in the Solar System. Some of this distant dust may reach the Earth. Ultracarbonaceous micrometeorites, packed with organic matter rich in nitrogen and heavy hydrogen, are thought to come from very cold, distant bodies.
Dust from between the stars also blows through the Solar System as the Sun moves through the galaxy. Spacecraft detected this stream in the 1990s, and a few candidate grains were captured by a sample-return mission and reported in 2014. These interstellar grains are typically less than a thousandth of a millimetre across, far smaller than the micrometeorites people collect.
From the Moon and Mars?
Impacts blast material off the Moon and Mars, and some must end up as dust in orbit around the Sun. A few lunar or Martian micrometeorites may exist in collections, but they are hard to recognise and none is yet well established. They would be a very small share of the total.
Did you know?
The zodiacal light was described long before anyone knew what caused it. It is easier to see from dark tropical skies, where the cone of light can look as bright as the Milky Way.
The light from the Sun itself pushes on dust. For the very smallest grains, the push outwards is stronger than the Sun's gravity, so they are blown right out of the Solar System.
Earth is not the only target. Micrometeoroids also rain on the Moon and Mars, and the soils of both contain a small share of meteoritic material.