Interplanetary dust particles particle size

Figure 10. Electron microscope picture of an interplanetary dust particle. The size of the particles carrying the extraterrestrial Me that are accumulated in marine sediments typically is between 3 and 35 pm (e.g., Farley et al. 1997) [photograph courtesy of Scott Messenger, http //stardust.wustl.edu].

The number of scientific articles published on meteorites has increased dramatically in the last few years few of these, however, concern themselves with small meteorites, the size of which lies between that of the normal meteorites (from centimetres to metres in size) and that of interplanetary dust particles. In the course of an Antarctic expedition, scientists (mainly from French institutions) collected micrometeorites from 100 tons of Antarctic blue ice (Maurette et al 1991). These micrometeorites were only 100 400 pm in size five samples, each consisting of 30-35 particles, were studied to determine the amount of the extraterrestrial amino acids a-aminoisobutyric acid (AIBS) and isovaline—both of which are extremely rare on Earth—which they contained. The analysis was carried out using a well-tested and extremely sensitive HPLC system at the Scripps Institute, La Jolla. Although the micrometeorites came from an extremely clean environment, the samples must have been contaminated, as they all showed traces of L-amino acids. Only one sample showed a significantly higher concentration of AIBS (about 280 ppm). The AIBS/isovaline ratio in the samples also lay considerably above that previously found in CM-chondrites. 

Many of the micron-sized interplanetary dust particles (IDPs) have approximately chondritic bulk composition (see Chapter 1.26 for details). Porous IDPs match the Cl composition better than nonporous (smooth) IDPs. On an average, IDPs show some enhancement of moderately volatile and volatile elements (see Palme, 2000). Arndt et al. (1996) found similar enrichments in their suite of 44 chondritic particles (average size 17.2 1.2 p.m). The elements chlorine, copper, zinc, gallium, selenium, and rubidium were enriched by factors of 2.2-2.7. In addition, these... 

Solid bodies of extraterrestrial material that penetrate the atmosphere and reach the Earth s surface are called meteorites. Other extraterrestrial materials include micrometer-sized interplanetary dust particles (IDEs) collected in the lower stratosphere and polar ices. Most meteorites and IDEs are fragments of asteroids, but some IDEs may represent cometary material and some meteorites are fragments of the planets Mars and Earth s moon. Meteorites recovered following observed falls are called/a// those which cannot definitely be associated with observed falls are called finds. Meteorites are given names based on the location where they were recovered (e.g., the Allende meteorite fell in Allende, Mexico). Meteorites recovered in Antarctica and the deserts of Australia and northern Africa are given names and numbers, because numerous samples are found in the same locations. Fragments thought to be of the same meteorite fall, which, in Antarctica or hot deserts, may have different numbers or even names because they were found in different locations, are called... 

Two characteristics of the light from noctilucent clouds may be observed with no more than one s eyes and a polarizing filter its color and whether or not it is strongly polarized. This enabled Ludlum (1957) to estimate the size range of noctilucent cloud particles. Because of the observed strong polarization he set 0.16 jum as their upper size limit on the basis of the observed color—white, silvery, sometimes bluish, but not sufficiently so as to indicate very small particles—he set 0.008 jam as their lower size limit. From other than optical evidence he also concluded that the particles were not ice, but were more likely to be volcanic, meteoric, or interplanetary dust. 

The focus of the chapter is on the smallest range of particle sizes (< 1 cm) and thus the initial phases of dust coagulation in young gas-rich disks, since these are the ones that can be observed in extrasolar systems and have preserved some very early record in chondritic meteorites and interplanetary dust particles (IDPs). 

The sizes of the cometary dust grains vary from less than a micron to probably several centimeters. Infrared observations near 10 pm show the silicate spectral features. In addition, there seems to be a black ingredient presumed to be carbon. Due to different accelerations from the solar radiation pressure, the larger particles follow the comet close in its orbit and are more concentrated to the orbital plane. They become sometimes visible in the anti-tails , narrow spikes which point towards the Sun by an effect of projection when the Earth crosses the comet s orbital plane. Non of the meteorites found so far on Earth seem to be of cometary origin. However, very fluffy micron sized interplanetary dust grains (Brownlee particles) which have been collected by high flying aircraft are possibly cometary debris. 

Typical visual meteors are associated with particles with representative diameters ranging from 1 mm to 20 cm. ° The size distribution of interplanetary dust, however, peaks at substantially smaller diameters. Meteoroids... 

Meteoritic materials that enter the atmosphere as small particles do not have the atmospheric strength filter that hampers the survival of cometary rocks. Small cometary dust particles survive atmospheric entry, and studies of interplanetary dust collected in the stratosphere have provided significant insight into the nature of cometary materials. Particles of 10 xm size decelerate at altitudes near 100 km where the ram pressures are only 0.02 kPa (Brownlee, 1985), many orders of magnitude lower than that experienced by conventional meteorites. Dust samples are also the largest source, by mass, of meteoritic materials. Over 3 X 10" t of interplanetary particles smaller than 300 xm diameter impact the Earth each year (Love and Brownlee, 1995). 

Meteorites from the asteroid belt are a potential source of some of the purine bases present in RNA. As asteroids travel in their orbits between Mars and Jupiter they collide with each other and pieces are broken off that vary in size from large bodies (potential meteorites) to dust particles. If the energy of the collision is great enough, this material is propelled out of its orbit into the interplanetary medium and some of the material eventually reaches the Earth. Approximately 10 kg of asteroidal material reached the primitive Earth s surface (6). This corresponds to a layer of material weighing 2x10 kg/m if spread uniformly over the surface of the Earth. The carbon content of the soluble organics present (1%) is equivalent to a layer of carbon compounds 25 m thick on the primitive Earth. Since meteorites contain about 1 ppm of purines and pyrimidines, then about lO kg of these compounds were on the primitive Earth. 

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