"Every day some 200 tons of extraterrestrial material enter the Earth’s porous atmosphere. The largest of these objects, meteors, become giant fireballs with the ability to light up the daytime sky and can cause local, regional, or global destruction upon impact. Others become shooting stars, neither large enough to survive their fiery trip through the atmosphere, nor small enough to escape their fate. The smallest of these materials, however, make it to the surface of the Earth as micrometeorites without much in the way of fanfare. No fiery explosions in the sky. No damage or destruction. Just a silent fall to Earth." Bold added.
"Micrometeorite is often abbreviated as MM."
"By definition, these micrometeorites are generally less than 1 mm in diameter—literally dust-sized."
Microspherules that look like micrometeoritic microspherules are usually of terrestrial origin. They are products of human activity.
"Even if the characteristics of the collected spherical particles were absolutely the same, attributed to the micrometeorites, the further investigations, directed to the statistical study of their distribution in the environment, have revealed several observation, listed below, making very questionable the extra-terrestrial origin of these spherical particles:"
- "The distribution of these spherical particles is very inhomogeneous: their concentration is very high in areas where people live (and even higher in the industrial zones) and they are practically absent in areas located fare from the urbane areas (few present are very small in sizes)."
- "In areas closed to the zones of human activity, fibers and granules [...] can be frequently found."
- "There is inhomogeneous distribution of these spherical particles also in height: they can be frequently found on roofs of houses of 3-4 floors height, and, even in industrial zones, very few spherical particles with reduced diameter can be found on roofs of buildings higher than 20 floors."
"Why we can find such spherical particles rather far from the industrial activity zones but close to the people living areas? The answer is very simple. Lighters can be considered as small grinding wheels." The sparks recovered from lighters consist of similar microspherules.
There "are certain post-industrial processes, the flicking of a lighter or the use of a grinding wheel, among others, that have the ability to create morphologically similar objects. In fact, in any successful rainwater or road dust sample one will also find scores of false positives".
"Most MMs are broadly chondritic in composition, meaning "that major elemental abundance ratios are within about 50% of those observed in carbonaceous chondrites."
Some MMs are chondrites, (basaltic) howardite, eucrite, and diogenite (HED) meteorites or Martian basalts, but not lunar samples.
"[T]he comparative mechanical weakness of carbonaceous precursor materials tends to encourage spherule formation."
From the number of different asteroidal precursors, the approximate fraction in MMs is 70 % carbonaceous.
"[T]he carbonaceous material [is] known from observation to dominate the terrestrial MM flux."
The "H, L, and E chondritic compositions" are "dominant among meteorites but rare among micrometeorites."
"Ureilites occur about half as often as eucrites (Krot et al. 2003), are relatively friable, have less a wide range of cosmic-ray exposure ages including two less than 1 Myr, and, like the dominant group of MM precursors, contain carbon."
"The carbon-rich areas [in the backscattered scanning electron micrograph of MM particle 119] appear dark (arrows); the bright inclusions are dominated by Fe-Ni sulfides and silicates."
MM 119 contains "extremely large amounts of carbon as well as excesses of deuterium. While this high organic content usually comes from distant interstellar space where molecular clouds gather to form new stars, other clues say these space rocks likely formed in our own solar system. This contradicts long-held notions that all organic matter with extreme deuterium excesses have interstellar origins."
MM 119 was recovered "from 40 to 55 year-old snow [and contained] crystalline materials ... that indicate [it] formed close to our sun, and much more recently than predicted."
The micrometeorite on the left is ultra-rich in carbon with unusually high concentrations of deuterium. It is from the same location as MM 119.
To reduce contamination from human activity and industry, micrometeorites are collected from remote locations such as Antarctica shown in the image on the right.
"In the years and months leading up to the first manned mission to the Moon, NASA scientists worried whether or not the Lunar Lander would find steady footing upon touchdown. The debate centered on just how much space dust was covering the surface of the Moon. With no atmosphere and little erosion, some hypothesized that the Moon had been collecting the very same micrometeorite materials as the Earth — but in far greater quantities. Enough, in fact, to create a layer of space dust 20 feet thick across the entire surface of the Moon. Concerns for the spacecraft and crew were eventually allayed, and upon touchdown the Lander settled in no more than a few inches of loose space dust."
- Most micrometeorites are likely to be other than iron meteorites.
- Ryan Thompson (4 December 2012). "The Dark Flight of Micrometeorites". Retrieved 2015-01-07.
- Susan Taylor, Gregory F. Herzog, Gregory, Jeremy S. Delaney, (2007). "Crumbs from the crust of Vesta: Achondritic cosmic spherules from the South Pole water well". Meteoritics & Planetary Science 42 (2): 223-33. doi:10.1111/j.1945-5100.2007.tb00229.x.
- Attilio Anselmo (2007). "Observation of False Spherical Micrometeorites" (PDF). Cogoleto (GE) 16016 Italy: Stoppani S.P.A. Retrieved 2015-01-07.
- Nancy Atkinson (6 May 2010). "Antarctic Micrometeorites Provide Clues to Solar System Formation". AAAS, UniverseToday.com. Retrieved 2015-01-08.