A micrometeorite is an extraterrestrial particle, ranging in size from 50 µm to 2 mm, collected on the Earth’s surface. Micrometeorites are micrometeoroids which have survived entry through the Earth’s atmosphere. They differ from meteorites in being smaller, more plentiful and different in composition and are a subset of cosmic dust, which also includes the smaller interplanetary dust particles (IDPs). Micrometeorites enter the Earth’s atmosphere with high velocities (at least 11 km/s) and undergo heating through atmospheric friction and compression. Individual micrometeorites weigh between 10−9 and 10−4 g and collectively contribute most of the extraterrestrial material that has come to the present day Earth. Fred Lawrence Whipple first coined the term “micro-meteorite” to describe dust-sized objects that fall to the Earth. 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 micrometorites.
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.
The extraterrestrial origins of micrometeorites are determined by microanalyses that show that:
- The metal they contain is similar to that found in meteorites.
- Some have wüstite, a high-temperature iron oxide found in meteorite fusion crusts.
- Their silicate minerals have major and trace elements ratios similar to those in meteorites.
- 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
- The presence of pre-solar grains in some MMs
and deuterium excesses in ultra-carbonaceous MMs indicates that they are not only extraterrestrial but that some of their components formed before our solar system.
An estimated 30,000 ± 20,000 tonnes per year (t/yr) 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. Therefore, the mass of micrometeorites deposited is roughly 50 times higher than that estimated for meteorites, which represent approximately 50 t/yr, 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 our 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.
Micrometeorites have been collected from deep-sea sediments, sedimentary rocks and polar sediments; they are currently collected primarily from polar snow and ice. Because of their low concentrations on the Earth’s surface, MMs are sought in environments that concentrate these materials relative to terrestrial particles.
Classification and origins of micrometeorites
Modern classification of meteorites and micrometeorites is complex; the 2007 review paper of Krot et al.summarizes modern meteorite taxonomy. Linking individual micrometeorites to meteorite classification groups requires a comparison of their elemental, isotopic and textural characteristics.
Comet vs asteroid origin of micrometeorites
Whereas most meteorites likely originate from asteroids, the contrasting makeup of micrometeorites suggests that most originate from comets.
Fewer than 1% of MMs are achondritic and are similar to HED meteorites, which are thought to be from the asteroid, 4 Vesta. Most MMs are compositionally similar to carbonaceous chondrites, whereas approximately 3% of meteorites are of this type. The dominance of carbonaceous chondrite-like MMs and their low abundance in meteorite collections suggests that most MMs derive from sources different than those for most meteorites. Since most meteorites probably derive from asteroids, an alternative source for MMs might be comets. The idea that MMs might originate from comets originated in 1950.
Until recently the greater-than-25-km/s entry velocities of micrometeoroids, measured for particles from comet streams, cast doubts against their survival as MMs. However, recent dynamical simulations suggest that 85% of cosmic dust could be cometary. Furthermore, analyses of particles returned from the comet, Wild 2, by the Stardust spacecraft show that these particles have compositions that are consistent with many micrometeorites. Nonetheless, some parent bodies of micrometeorites appear to be asteroids with chondrule-bearing carbonaceous chondrites.
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 about 1 m of micrometeoritic content to the depth of the martian regolith each billion years. Measurements from the Viking program indicate that the martian regolith is a mixture of 60% basaltic rock and 40% 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. Whereas 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.