A micrometeorite is a tiny piece of space dust which has somehow managed to survive passage through the atmosphere. They are no more than 1/8 inch across, and most micrometeorites are much smaller. They were usually once part of a larger meteor which disintegrated in Earth’s atmosphere.
The largest iron and iron-nickel micrometeorites may have a partly melted appearance similar to their larger cousins. Some have completely melted and resolidified as metallic spherules. They are also easiest to find. Any collection of airborne dust will contain some metallic particles, which can be separated from the rest by using a magnet. When examined under a microscope, some of those metallic particles will show signs of pitting and partial melting. These are the micrometeorites.
However, the smallest micrometeorites do not achieve atmospheric speeds sufficient to melt them. At most, they experience extreme temperatures for about a second before they are slowed down by the atmosphere. Their extraterrestrial origin can sometimes be identified by their chemical composition, although many micrometeorites may be nearly indistinguishable from other terrestrial matter. In fact, much of the extraterrestrial matter accumulated by the Earth may come from micrometeorites.
In the vacuum of space, there is no atmosphere to slow down a micrometeoroid. Micrometeoroids which strike a satellite or an astronaut’s spacesuit do so at their full velocity. Individually, they are much too small to do much damage, but there are a lot of micrometeoroids out there, and their damage can build up over time.
Micrometeorites are a major cause of space weathering. This was first identified in the agglutinates of lunar soil, which are created when a micrometeorite melts some of the material around its landing place and combines them. Agglutinates are more common in older lunar soil, and account for as much as 70% of mature lunar soils. This thin surface layer of high agglutinate content turns black because of all the iron. When the ferrous oxide of the micrometeorite combines with constant hydrogen enrichment from the solar wind, one of the products is water vapor.
There is no standard classification system for micrometeorites. However, they are usually grouped by composition and texture, the same as larger meteorites. As with larger meteorites, most micrometeorites which have been identified are chondritic. However, for many micrometeorites, the appropriate classification is not so clear.
It is likely that the types and sources of micrometeorites are similar to those of larger meteorites. These sources include asteroids, the moon, Mars, and comets, which in turn bring particles from the Oort Cloud.