User:Marshallsumter/Radiation astronomy1/Micrometeorites

"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.

Notations
"Micrometeorite is often abbreviated as MM."

Theoretical micrometeorites
"By definition, these micrometeorites are generally less than 1 mm in diameter—literally dust-sized."

The International Astronomical Union (IAU) officially defines meteorites as 30 micrometers to 1 meter; micrometeorites are the small end of the range (~submillimeter). They are a subset of cosmic dust, which also includes the smaller interplanetary dust particles (IDPs).

Planetary sciences
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:"
 * 1) "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)."
 * 2) "In areas closed to the zones of human activity, fibers and granules [...] can be frequently found."
 * 3) "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.

Typologies
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.[5] 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."

Textures
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 ranging from unmelted particles that retain their original mineralogy, in the image on the right, to partially melted particles, to round melted cosmic spherules. Some of which have lost a large portion of their mass through vaporization, image on the left.

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 in iron spherules and of cosmogenic beryllium, aluminum , 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 the Solar System.

Carbons
"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.

Chromites
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.

Earth
To reduce contamination from human activity and industry, micrometeorites are collected from remote locations such as Antarctica shown in the image on the right.

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.

An estimated 40,000 ± 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.

MMs have been 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 was discovered.

Sediments
MMs have been extracted in urban environments The "urban" cosmic spherules have a shorter terrestrial age and are less altered than the previous findings.

Melted micrometeorites (cosmic spherules) were first collected from deep-sea sediments during the 1873 to 1876 expedition of HMS Challenger, consisting of "two groups [of micrometeorites]: first, black magnetic spherules, with or without a metallic nucleus; second, brown-coloured spherules resembling chondr(ul)es, with a crystalline structure". Spherules were extraterrestrial because they were found far from terrestrial particle sources, they did not resemble magnetic spheres produced in furnaces of the time, and their nickel-iron (Fe-Ni) metal cores did not resemble metallic iron found in volcanic rocks and the spherules were most abundant in slowly accumulating sediments, particularly red clays deposited below the carbonate compensation depth, a finding that supported a meteoritic origin. In addition to those spheres with Fe-Ni metal cores, some spherules larger than 300 µm contain a core of elements from the platinum group.

Since the first collection of HMS Challenger, cosmic spherules have been recovered from ocean sediments using cores, box cores, clamshell grabbers, and magnetic sleds. Among these a magnetic sled, called the "Cosmic Muck Rake", retrieved thousands of cosmic spherules from the top 10 cm of red clays on the Pacific Ocean floor.

Terrestrial sediments contain micrometeorites that have been found in samples with The oldest MMs are totally altered iron spherules found in 140- to 180-million-year-old hardgrounds.
 * low sedimentation rates such as claystones and hardgrounds
 * easily dissolved such as salt deposits and limestones
 * mass sorted such as heavy mineral concentrates found in deserts and beach sands.

Moon
"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."

Mars
The influx of micrometeoroids 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 which contributes to about 1 m of micrometeoritic content to the depth of the Martian regolith every billion years that is composed of 60% basaltic rock and 40% rock of meteoritic origin, where 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.

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 from those of most meteorites. Since most meteorites 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.

Cosmic dusts
In the image on the right, the dots particle is highly rough (chondritic porous: "CP"). CP types are usually aggregates of large numbers of sub-micrometer grains, clustered in a random open order.

Cosmic dust, also called extraterrestrial dust or space dust, is dust which exists in outer space, or has fallen on Earth.

Thousands of tons of cosmic dust are estimated to reach the Earth's surface every year, with most grains having a mass between 10−16 kg (0.1 pg) and 10−4 kg (0.1 g). The density of the dust cloud through which the Earth is traveling is approximately 10−6 dust grains/m3.

Cosmic dust contains some complex organic compounds (amorphous organic solids with a mixed aromatic–aliphatic structure) that could be created naturally, and rapidly, by stars.

Interstellar dust particles were collected by the Stardust spacecraft and samples were returned to Earth.

Zodiacal light shown in the image on the left is caused by cosmic dust.

The faint light extending up from the horizon just below centre of this photo is known as zodiacal light, caused by sunlight scattering from cosmic dust in the plane of our Earth’s orbit.

A second band of light can be seen at the horizon on the lower left. This red light is airglow, produced by the Earth’s atmosphere. Airglow is caused by processes taking place in the upper atmosphere, including cosmic rays, recombining photoionized atoms, and various chemical reactions between oxygen, nitrogen, hydroxyl, sodium, and lithium atoms.

The third and final band is the Milky Way, our home galaxy, high in the sky. This band consists of billions of stars of all kinds. Many of them are hidden to the human eye behind large layers of interstellar dust, giving the Milky Way its characteristically mottled look.

Cosmic dust particles evolve cyclically.

Cosmic dust can also be detected directly (in-situ) using a variety of collection methods and from a variety of collection locations, where estimates of the daily influx of extraterrestrial material entering the Earth's atmosphere range between 5 and 300 tonnes.

NASA's Spitzer Space Telescope has captured stunning infrared views of the famous Andromeda galaxy to reveal insights that were only hinted at in visible light, in the second image down on the right.

This Spitzer's 24-micron mosaic is the sharpest image ever taken of the dust in another spiral galaxy. This is possible because Andromeda is a close neighbor to the Milky Way at a mere 2.5 million light-years away.

The Spitzer multiband imaging photometer's 24-micron detector recorded 11,000 separate snapshots to create this new comprehensive picture. Asymmetrical features are seen in the prominent ring of star formation. The ring appears to be split into two pieces, forming the hole to the lower right. These features may have been caused by interactions with satellite galaxies around Andromeda as they plunge through its disk.

Spitzer also reveals delicate tracings of spiral arms within this ring that reach into the very center of the galaxy. One sees a scattering of stars within Andromeda, but only select stars that are wrapped in envelopes of dust light up at infrared wavelengths.

This is a dramatic contrast to the traditional view at visible wavelengths, which shows the starlight instead of the dust. The center of the galaxy in this view is dominated by a large bulge that overwhelms the inner spirals seen in dust. The dust lanes are faintly visible in places, but only where they can be seen in silhouette against background stars.

Cosmic dust may be formed near a supermassive black hole.

Dust grains are not spherical and tend to align to interstellar magnetic fields, preferentially polarizing starlight that passes through dust clouds, where interstellar reddening is not intense enough to be detected, high precision optical polarimetry has been used to glean the structure of dust within the Local Bubble.

The detection of interstellar dust in Antarctica was done by the measurement of the radionuclides Fe-60 and Mn-53 by highly sensitive accelerator mass spectrometry.

HH 151 in the image third down on the right is a bright jet of glowing material trailed by an intricate, orange-hued plume of gas and dust.

HH 151 is a bright jet of glowing material trailed by an intricate, orange-hued plume of gas and dust. It is located some 460 light-years away in the constellation of Taurus (The Bull), near to the young, tumultuous star HL Tau.

In the first few hundred thousand years of life, new stars like HL Tau pull in material that falls towards them from the surrounding space. This material forms a hot disc that swirls around the coalescing body, launching narrow streams of material from its poles. These jets are shot out at speeds of several hundred kilometres per second and collide violently with nearby clumps of dust and gas, creating wispy, billowing structures known as Herbig-Haro objects — like HH 151 seen in the image above.

Such objects are very common in star-forming regions. They are short-lived, and their motion and evolution can actually be seen over very short timescales, on the order of years. They quickly race away from the newly-forming star that emitted them, colliding with new clumps of material and glowing brightly before fading away.

The scattering of light from dust grains in long exposure visible photographs is quite noticeable in reflection nebulae, and gives clues about the individual particle's light-scattering properties, investigated by the scattering of X-rays by interstellar dust, as diffuse haloes, due to the dust.

Stardust grains (also called presolar grains by meteoriticists ) are contained within meteorites, from which they are extracted.

Stardust is a scientific term referring to refractory dust grains that condensed from cooling ejected gases from individual presolar stars and incorporated into the cloud from which the Solar System condensed.

Many new aspects of nucleosynthesis have been discovered from the isotopic ratios within the stardust grains.

Supernova condensates, usually shortened by acronym to SUNOCON (from SUperNOva CONdensate ) to distinguish them from other stardust condensed within stellar atmospheres. SUNOCONs contain in their calcium an excessively large abundance of 44Ca, demonstrating that they condensed containing abundant radioactive 44Ti, which has a 65-year half-life. The outflowing 44Ti nuclei were thus still "alive" (radioactive) when the SUNOCON condensed near one year within the expanding supernova interior, but would have become an extinct radionuclide (specifically 44Ca) after the time required for mixing with the interstellar gas. Its discovery proved the prediction from 1975 that it might be possible to identify SUNOCONs in this way.

Planetary solids have been found that are older than the Earth.

The second image down on the left is a scanning electron microscope image of an interplanetary dust particle that has roughly chondritic elemental composition and is highly smooth (chondritic smooth: "CS"). CS types are usually aggregates of large numbers of sub-micrometer grains, clustered in a random open order.

The last image on the right shows the major elements of 200 stratospheric interplanetary dust particles.

The total influx rate of meteoritic sites of most IDPs captured in the Earth's stratosphere range between 1 and 3 g/cm3, with an average density at about 2.0 g/cm3.

Solid-state water in the interstellar medium, and particularly, of water ice mixed with silicate grains in cosmic dust grains has been reported.

Micrometeoroids
While the tiny sizes of most micrometeoroids limits the damage incurred, the high velocity impacts will constantly degrade the outer casing of spacecraft in a manner analogous to sandblasting. Long term exposure can threaten the functionality of spacecraft systems.

The risk is especially high for objects in space for long periods of time, such as satellites. They also pose major engineering challenges in theoretical low-cost lift systems such as rotovators, space elevators, and orbital airships.

Hypotheses

 * 1) Most micrometeorites are likely to be other than iron meteorites.