Scoriaceous, unmelted and ultracarbonaceous micrometeorites
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.
Partly melted micrometeorites look like tiny cinders full of bubbles. Unmelted ones look like nothing special: dark, irregular crumbs. But under powerful microscopes they turn out to be pristine pieces of asteroids and comets. The rarest, called ultracarbonaceous micrometeorites, are mostly made of carbon-rich organic matter and probably come from comets far out in the Solar System.
Key facts
- Scoriaceous
- Partly melted, frothy, with many bubbles
- Fine-grained unmelted
- Clumps of tiny grains, often like the matrix of carbonaceous chondrites
- Coarse-grained unmelted
- Visible crystals of olivine and pyroxene, often from chondrules
- Ultracarbonaceous
- Typically half or more organic matter; first described in 2010
Scoriaceous micrometeorites
Scoria is the name for frothy, bubble-filled volcanic rock. Scoriaceous micrometeorites look like microscopic versions of it. They started to melt during entry, and gases trapped in their minerals, such as water from clay minerals and sulphur from sulphides, boiled out and inflated the half-molten grain with bubbles called vesicles.
Most scoriaceous particles started as fine-grained material similar to the matrix of carbonaceous chondrites, which is rich in water and other volatile substances. They often have a thin rim of magnetite and may still contain unmelted mineral grains. They are a step on the way to becoming a cosmic spherule: a little hotter, and the bubbles would have escaped and the droplet pulled itself round.
Fine-grained unmelted micrometeorites
These are compact or porous clumps of grains mostly less than a micrometre across, bound together with carbon-rich material. Many are very like the dark matrix of CM and CI carbonaceous chondrites, the most primitive meteorites known, and some still contain the clay minerals that formed when water flowed through their parent asteroid. Others contain carbonates, sulphides and tiny crystals of magnetite, all minerals made by water.
Entry heating usually changes them a little. Their clays may lose water and their surfaces often carry a magnetite rim a few micrometres thick, a little like the fusion crust on a meteorite. But inside, their chemistry, isotopes and organic matter are largely intact.
Coarse-grained unmelted micrometeorites
Coarse-grained micrometeorites are made mainly of crystals of olivine and pyroxene large enough to see clearly under a microscope, sometimes with glass and metal. Many are fragments of chondrules, the once-molten droplets found in chondrite meteorites, and some appear to come from rocks that crystallised from melts. Studies of their oxygen isotopes link some to ordinary chondrites and others to carbonaceous chondrites.
Ultracarbonaceous micrometeorites
In 2010, a French team studying micrometeorites from snow near a research station on the high Antarctic plateau described particles unlike any meteorite. About 0.1 mm across, they were made of between about half and four-fifths organic matter, with only small mineral grains scattered through it. They became known as ultracarbonaceous Antarctic micrometeorites, or UCAMMs.
Their hydrogen holds a striking clue. Some contain up to about ten times as much deuterium, the heavy form of hydrogen, compared with ordinary hydrogen, as the water in Earth's oceans. Such strong enrichment forms in extremely cold places, at temperatures only a few tens of degrees above absolute zero, such as the edge of the Solar System or the cloud of gas and dust in which the Sun was born. Their organic matter is also unusually rich in nitrogen. One explanation is that they formed on icy bodies far from the Sun, where cosmic rays turned frozen nitrogen and methane into complex organic solids. Researchers think they come from comets.
UCAMMs are extremely rare and fragile. They have been recognised almost entirely in carefully melted Antarctic snow, the gentlest collecting method there is, because they would not survive long in sea water, soil or a gutter.
Refractory and achondritic particles
A few unmelted micrometeorites contain minerals rich in calcium and aluminium, like the calcium–aluminium-rich inclusions found in chondrites, which formed close to the young Sun about 4,567 million years ago. Under one per cent of micrometeorites resemble achondrites, especially the basalt-like rocks of the HED group thought to come from the asteroid Vesta.
Why scientists value them
Unmelted micrometeorites are fragments of asteroids and comets that never reached Earth as meteorites. They sample many more parent bodies than the meteorite collections do, including fragile, carbon-rich and icy ones. They contain presolar grains, tiny crystals made in other stars before the Sun existed. And they let researchers compare dust from the sky with samples brought back by spacecraft from the asteroids Ryugu and Bennu and the comet Wild 2. See what they reveal about the early Solar System.
Did you know?
Unmelted micrometeorites are usually indistinguishable from ordinary dust under a hand lens. They are found mostly by researchers using electron microscopes on very clean Antarctic samples.
Researchers have found close similarities between some fine-grained micrometeorites and the samples returned from the dark, water-rich asteroid Ryugu, hinting that bodies like it are an important source of space dust.
The first ultracarbonaceous micrometeorites came from snow only a few metres deep, which had fallen within roughly the previous half-century.