A micrometeorite is a micrometeoroid that has survived entry through the Earth's atmosphere. Usually found on Earth's surface, micrometeorites differ from meteorites in that they are smaller in size, more abundant, and different in composition. The IAU officially defines meteoroids as 30 micrometers to 1 meter; micrometeorites are the small end of the range (~submillimeter).^[1] They are a subset of cosmic dust, which also includes the smaller interplanetary dust particles (IDPs).^[2]

Micrometeorites enter Earth's atmosphere at high velocities (at least 11 km/s) and undergo heating through atmospheric friction and compression. Micrometeorites individually weigh between 10⁻⁹ and 10⁻⁴ g and collectively comprise most of the extraterrestrial material that has come to the present-day Earth.^[3]

Fred Lawrence Whipple first coined the term "micro-meteorite" to describe dust-sized objects that fall to the Earth.^[4] Sometimes meteoroids and micrometeoroids entering the Earth's atmosphere are visible as meteors or "shooting stars", whether or not they reach the ground and survive as meteorites and micrometeorites.

Micrometeorite (MM) textures vary as their original structural and mineral compositions are modified by the degree of heating that they experience entering the atmosphere—a function of their initial speed and angle of entry. They range from unmelted particles that retain their original mineralogy (Fig. 1 a, b), to partially melted particles (Fig. 1 c, d) to round melted cosmic spherules (Fig. 1 e, f, g, h, Fig. 2) some of which have lost a large portion of their mass through vaporization (Fig. 1 i). Classification is based on composition and degree of heating.^[5][6]

The extraterrestrial origins of micrometeorites are determined by microanalyses that show that:

• The metal they contain is similar to that found in meteorites.^[7]
• Some have wüstite, a high-temperature iron oxide found in meteorite fusion crusts.^[8]
• Their silicate minerals have major and trace elements ratios similar to those in meteorites.^[9][10]
• The abundances of cosmogenic manganese (⁵³Mn) in iron spherules and of cosmogenic beryllium (¹⁰Be), aluminum (²⁶Al), and solar neon isotope in stony MMs are extraterrestrial^[11][12]
• The presence of pre-solar grains in some MMs^[13] and deuterium excesses in ultra-carbonaceous MMs^[14] indicates that they are not only extraterrestrial but that some of their components formed before the Solar System.

An estimated 40,000 ± 20,000 tonnes per year (t/yr)^[3] of cosmic dust enters the upper atmosphere each year of which less than 10% (2700 ± 1400 t/yr) is estimated to reach the surface as particles.^[15] Therefore the mass of micrometeorites deposited is roughly 50 times higher than that estimated for meteorites, which represent approximately 50 t/yr,^[16] and the huge number of particles entering the atmosphere each year (~10¹⁷ > 10 µm) suggests that large MM collections contain particles from all dust-producing objects in the Solar System including asteroids, comets, and fragments from the Moon and Mars. Large MM collections provide information on the size, composition, atmospheric heating effects and types of materials accreting on Earth while detailed studies of individual MMs give insights into their origin, the nature of the carbon, amino acids and pre-solar grains they contain.^[17]

Chemical analysis of the microscopic chromite crystals, or chrome-spinels, retrieved from micrometeorites in acid baths has shown that primitive achondrites, which represent less than half a percent of the MM reaching Earth today, were common among MMs accreting more than 466 million years ago.^[18]

Micrometeorites have been collected from deep-sea sediments, sedimentary rocks and polar sediments. They were previously collected primarily from polar snow and ice because of their low concentrations on the Earth's surface, but in 2016 a method to extract micrometeorites in urban environments^[19] was discovered.^[20]

The influx of micrometeoroids also contributes to the composition of regolith (planetary/lunar soil) on other bodies in the Solar System. Mars has an estimated annual micrometeoroid influx of between 2,700 and 59,000 t/yr. This contributes to about 1 m of micrometeoritic content to the depth of the Martian regolith every billion years. Measurements from the Viking program indicate that the Martian regolith is composed of 60% basaltic rock and 40% rock of meteoritic origin. The lower-density Martian atmosphere allows much larger particles than on Earth to survive the passage through to the surface, largely unaltered until impact. While on Earth particles that survive entry typically have undergone significant transformation, a significant fraction of particles entering the Martian atmosphere throughout the 60 to 1200-μm diameter range probably survive unmelted.^[59]

• Carbonaceous chondrite, a class of chondritic meteorites comprising at least seven known groups and many ungrouped
• Center for Meteorite Studies at Arizona State University
• Cosmic dust
• Glossary of meteoritics
• List of Martian meteorites
• List of meteorite minerals
• List of meteorites on Mars
• Meteorite classification
• Meteoritical Society
• Solar System
• The British and Irish Meteorite Society