What micrometeorites reveal about 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.
Each micrometeorite is a sample of a small body that formed 4.6 billion years ago. Because so many different parents contribute dust, a large collection is a much broader survey of the Solar System's building blocks than meteorites alone. Some grains contain crystals made in other stars before the Sun existed.
Key facts
- Main parent material
- Mostly like carbonaceous chondrites, unlike most meteorite falls
- Oldest components
- Presolar grains, made around other stars before the Sun formed
- Key tools
- Oxygen isotopes, hydrogen and nitrogen isotopes, electron microscopes
- Big finding
- Material was mixed from the hot inner disc to the cold outer one
A wider sample than meteorites
The meteorites that fall to Earth come from relatively few parent bodies. About four out of five falls are ordinary chondrites, from a small number of stony asteroids whose fragments happen to be steered towards us by the gravity of Jupiter and Saturn. Fragile, crumbly bodies are under-represented because their rocks break up in space and in the air.
Dust is different. Every comet and asteroid that sheds or collides makes it, and it reaches the Earth by the slow drift described on the page about where micrometeorites come from. So micrometeorites sample the population of small bodies far more evenly. The striking result is that most unmelted micrometeorites resemble carbonaceous chondrites, especially the water-altered CM and CI types, which make up only a few per cent of meteorite falls.
Fingerprints in oxygen
Oxygen has three stable isotopes, and the ratios between them vary from one part of the early Solar System to another. They act as a fingerprint that links a rock to its family. Measurements of oxygen isotopes in hundreds of micrometeorites, including cosmic spherules that have melted, show that most match carbonaceous chondrites, a smaller share match ordinary chondrites, and some fall into groups not matched by any known meteorite. This is direct evidence that micrometeorites include samples of parent bodies missing from the world's meteorite collections.
Grains older than the Sun
The most primitive micrometeorites and interplanetary dust particles contain presolar grains: crystals of silicate, silicon carbide, graphite and oxides that condensed in the winds of dying stars or in exploding stars, long before the Sun formed about 4.6 billion years ago. They are recognised by isotope ratios wildly unlike anything made in the Solar System. Some fluffy cometary dust particles hold presolar grains at concentrations far higher than in any meteorite, showing that the material they formed from was barely processed.
Mixing across the disc
The young Sun was surrounded by a disc of gas and dust, hot near the centre and frozen far out. One might expect comets, which formed in the cold outer disc, to contain only low-temperature material. Instead, comet dust collected by the Stardust mission and some micrometeorites contain grains that formed at well over 1,000 °C, close to the Sun, such as chondrule fragments and refractory minerals. Something carried them outwards, probably flows in the disc and outbursts from the young Sun. Micrometeorites thus record a Solar System in which material travelled far from where it formed.
Ice and cold chemistry
Ultracarbonaceous micrometeorites, with their strong excesses of deuterium and nitrogen-rich organic matter, preserve chemistry that can only happen in extreme cold, a few tens of degrees above absolute zero. They are among the best evidence that complex organic solids formed on icy bodies at the frontier of the Solar System, as described on the page about unmelted micrometeorites.
Records of collisions
Because the dust supply jumps when a large asteroid shatters, micrometeorites and the minerals they leave in sediments record the collisional history of the asteroid belt. Chromite crystals extracted from ancient limestone, for example, show that the mix of dust falling about 466 million years ago, just before a huge break-up, included many grains from rare types of meteorite that are scarce today. See fossil micrometeorites.
Did you know?
Some presolar grains found in space dust formed around stars that died before the Sun was born, and may be more than 5 billion years old.
A single cosmic spherule can be analysed for its oxygen isotopes using a beam of ions only a few thousandths of a millimetre wide.
Scientists sometimes find unmelted relict crystals inside spherules whose chemistry still matches the chondrules of a particular meteorite group.