Types of micrometeorite
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.
Think of a ladder with cool at the bottom and hot at the top. At the bottom are unmelted grains that look much as they did in space. Next come partly melted, bubbly grains. At the top are cosmic spherules, which melted completely into tiny balls. Each step tells scientists how fast and how steeply the grain arrived.
Key facts
- Classified by
- Degree of heating, then composition and texture
- Most common on the ground
- Cosmic spherules: about two-thirds of the mass in the 2021 Antarctic study
- Most valuable to science
- Unmelted particles, which keep their original minerals
- Standard scheme
- Published in 2008 and widely used since
The heating ladder
The classification most researchers use was set out in 2008 by a group of micrometeorite specialists. It places every particle on a sequence of increasing heating. From least to most heated, the main classes and subtypes are:
- Fine-grained unmelted micrometeorites: compact or porous clumps of minerals too small to see individually with an ordinary microscope, often resembling the dark matrix of carbonaceous chondrites.
- Coarse-grained unmelted micrometeorites: made mostly of visible crystals of olivine and pyroxene, often fragments of chondrules or of once-molten rock.
- Scoriaceous micrometeorites: partly melted and full of bubbles, like a microscopic cinder.
- Cosmic spherules, fully melted, which are divided by composition into S-type (stony), G-type (stony with lots of iron oxide) and I-type (iron). S-type spherules are further divided by texture, from cooler to hotter: porphyritic olivine, barred olivine, cryptocrystalline, glass, and the rare CAT spherules rich in calcium, aluminium and titanium.
Carbon-rich and other special kinds
Some unmelted particles fall outside the main groups. Ultracarbonaceous micrometeorites, first described from Antarctic snow in 2010, are made mostly of organic matter, with only scattered mineral grains. Refractory particles contain minerals like those in the calcium–aluminium-rich inclusions of chondrites, the oldest solids in the Solar System. A tiny fraction are achondritic, resembling the basalt of the asteroid Vesta. These are all described on the page about scoriaceous and unmelted micrometeorites.
How common is each type?
The answer depends heavily on where the particles were collected and at what size. In clean Antarctic snow, unmelted particles are common among the smallest grains, while cosmic spherules dominate among the larger ones. The 2021 Antarctic snow study found that by mass about two-thirds of the micrometeorites reaching the ground were cosmic spherules and one-third unmelted or partly melted.
Among cosmic spherules from Antarctica, S-types make up the great majority, I-types only a few per cent and G-types fewer still. In deep-sea sediments, I-types appear far more common, because they are collected with magnets and resist sea water better than glass and olivine. In city roof dust, almost every confirmed micrometeorite so far is a cosmic spherule, because fragile unmelted grains are lost among the dirt and destroyed by weathering.
Why the type matters
A particle's type is a record of its journey. An unmelted grain probably arrived slowly or at a grazing angle, which hints at asteroid dust or dust from a Jupiter-family comet. A glass spherule or a CAT spherule was heated so strongly that it may have arrived fast. Scientists can model the heating and work back from a large collection of particles to the mix of speeds and sources in the dust cloud around the Sun.
The type also decides what can be learned. Unmelted particles keep minerals, organic matter and isotopes from their parent body almost untouched. Cosmic spherules have lost their original texture, but their chemistry, oxygen isotopes and any surviving unmelted grains, called relict grains, still reveal what they started as.
Not every space sphere is a micrometeorite
Other processes also make tiny spheres of melted rock and metal. Large meteoroids shed molten droplets as they burn through the air; these ablation spherules are found around some meteorite falls and craters. Big impacts splash out molten rock that freezes into impact spherules and glassy microtektites, sometimes spread across whole continents. Industry, volcanoes and lightning make spheres too. Telling these apart is the subject of recognising a micrometeorite.
Did you know?
The textures of cosmic spherules are named after the textures of chondrules in meteorites, because both formed as molten droplets that cooled very quickly.
The whole journey from cold space dust to a frozen droplet takes only a few seconds, yet the particle may then lie in Antarctic ice for hundreds of thousands of years before anyone finds it.
Some spherules contain a relict grain: a crystal that never melted, sitting inside a sphere that did. It is a clue to the mineral the particle started with.