Cosmic Dust N98-18554 D. E. Brownlee and S. A. Sandford galaxy and the solar system. Its average spatial density is less than one particle per i ustcubicis akilometerubiquitous but componentthe total numberof our of particles is large. The particles are so prevalent that both interstellar and interplanetary dust can readily be seen with the naked eye under dark sky conditions. Zodiacal light, a glow usually seen in the east before dawn twilight and in the west following sunset, is sunlight reflected off dust particles orbiting the Sun. The dark band marking the central plane of the Milky Way is caused by absorption of background starlight by dust concentrated in the plane of our galaxy. In fact, dust is so prominent that light typically travels only a thousand years in the plane of our galaxy before it is absorbed by dust. The distance traversed in this time is only 1% of the diameter of the galaxy. Interstellar dust is the predominant form of the condens- able elements in the galaxy that are not in stars. The grains form in gas outflows from stars and they are processed, perhaps even destroyed and reformed, in the interstellar medium and molecular clouds. Interplanetary dust is debris recently liberated from comets and aster- oids within the solar system. There is a strong link between interplanetary and interstellar dust. Prior to COLOR F'HO-iOSi_;A_H 145 ] formation of the solar nebula, tant roles in the origin and experience melting from most of the atoms heavier development of life. Inter- atmospheric drag but, because than helium that found their planetary dust particles (IDPs) of their smaller size, they tend way into our solar system that are collected in the to have isothermal interiors were contained in interstellar stratosphere and analyzed in and they melt completely, grains. Some of these grains the laboratory are preserved resulting in the loss of were incorporated into fragments from asteroids and volatiles such as sulfur, comets and asteroids. Comets comets, bodies which formed sodium, and organic matter. forming in the outer fringes in the outer regions of the Such particles form melt of the nebula presumably solar nebula, are relatively spheres (cosmic spherules) contain relatively higher rich in volatiles, and which that have been collected in abundances of presolar grains. are a source of abiotic organic abundance from the ocean Many of the grains preserved molecules that have fallen to floor and polar ice deposits. in comets may then be older Earth throughout its history. than the Sun and planets. Particles smaller than 0.1 mm, Comets are thus vehicles Interplanetary dust particles however, enjoy a special potentially capable of carry- accreted by the Earth must advantage over conventional ing the products of interstellar first enter the atmosphere. meteorites and larger dust chemical processes directly to They enter the atmosphere at particles. These grains can planets. They are believed to velocities ranging from usually enter the atmosphere contain substantial organic 11.2 km/s, the Earth's escape without melting. This is components produced in velocity, to a maximum of possible because the particles space by nonequilibrium 72 km/s, the velocity of a only travel at high velocity at reactions such as catalysis head-on impact with a par- altitudes above 90 km where and radiation processing of ticle in a retrograde parabolic the air is very tenuous. For a condensed volatiles. orbit. The kinetic energies given velocity, the frictional corresponding to these power density generated on The collection and analysis of velocities are capable of the face of a particle depends extraterrestrial dust particles is melting or vaporizing the only on the local air density. important to exobiology incoming particles if the At 90 kin, the frictional because it provides informa- energies are converted to energy is generated slowly tion about the sources of internal heat. In the case of enough that it can be ther- biogenically significant conventional meteorites, mally reradiated without the elements and compounds the surfaces are heated to particle being heated to its that accumulated in distant vaporization temperatures, melting point. Smaller par- regions of the solar nebula although the short time scale ticles decelerate at higher and that were later accreted of heating and the combined altitudes and for a given onto the planets. Both inter- effects of thermal inertia and initial velocity there is a stellar and interplanetary dust conductivity confine melting limiting size below which no seeded the early Earth with and strong heating to the melting will occur. Particles elements and compounds outer millimeter or so of the that do not melt are called that may have played impor- body. The interiors of the micrometeorites and are meteorites remain essentially unheated. Dust particles in the 0.1 mm to several mm size range typically also 146 genuine samples of interplan- observed to fragment imply tors worldwide. Most of the etary dust. Most particles typical crushing strengths of particles are easily identified below 50 _m in size do not 105 dynes/cm 2 and in the as extraterrestrial on the basis melt upon atmospheric entry extreme case of the Draconid of their elemental composi- and the survival of solar flare meteors (the youngest meteor tion which is similar to that cosmic ray tracks and miner- stream) the pressure is only of primitive meteorites but als with low thermal stabili- 103 dynes/cm 2. Micrometeor- quite different from most ties in the particles indicate ites this fragile can enter the terrestrial particles. Typical that typical 10 _m particles atmosphere without fragmen- particles are composed of Mg, are not heated above 600°C. tation whereas larger objects Si, Fe, S, Ca, Na, A1, Ni, Mn, cannot. Cr, and Ti in ratios that An additional factor favoring match their relative abun- the survival of interplanetary Thus, typical cometary rocks dance in the Sun. Oxygen and dust during atmospheric entry cannot enter the atmosphere carbon are also major is the fact that they are not without being crushed. Once constituents but, as in mete- subjected to large ram pres- crushed, most fragments orites, their relative abun- sures during atmospheric should either melt or vaporize dances are less than solar. An deceleration. The ram pres- because fragmentation occurs extraterrestrial origin can be sure experienced by dust at lower altitudes where proven for any given particle particles is orders of magni- frictional heating of small by detection of He implanted tude smaller than must be hypervelocity particles is by the solar wind or tracks in survived by larger objects severe. If comets do not mineral grains produced by such as conventional meteor- contain any strong centimeter solar flare cosmic rays. As ites. This allows fragile materi- and larger components, then previously mentioned, larger als to enter as dust but pre- dust would be the only particles are also recovered vents them from surviving as meteoroid type that could from deep sea sediments and millimeter or larger sized carry cometary molecules to from polar ice (Greenland and objects. At a given velocity the Earth's surface. Antarctica). Particles larger the ram pressure is propor- than 0.1 mm are usually tional to the ambient air Micrometeorites ranging in melted, but a few particles up density. The maximum size from 5 to 50 _m are to 1 mm survive without any ram pressure experienced routinely collected in the melting. These rare, giant by a micrometeorite that stratosphere using high- micrometeorites are presum- decelerates near 90 km is altitude aircraft. The particles ably particles that entered at 100,000 times smaller than are characterized and curated relatively low velocity and that experienced by a 10-kg for distribution to investiga- very low incident angles. In rock that has retained its low-angle impacts the hyper- cosmic velocity down to an velocity interaction with the altitude of 40 kin. Even the atmosphere occurs at higher most fragile conventional altitudes and results in pro- meteorite is a factor of longed but less severe heating. 100 times stronger than the materials observed in cometary meteor showers. The altitudes and velocities where cometary meteors are 147 General Properties of Microns 2.5 3 4 5 7 10 15 20 Interplanetary I I I I I I Dust r21-M4-3A he most common (Olivine) IDPs collected from the stratosphere are r21 -M3-SA black, fine-grained (Layer-lattice silicate) materials that have major and minor element compositions similar to those of carbon- aceous chondrites, i.e., primi- tive meteorites. Particles with this composition are referred to as being "chondritic" even though they may have no u23-M4-4A (Pyroxene) actual relation to chondritic meteorites. Some micromete- I I I orites do not have chondritic 4000 3200 2400 1600 800 compositions, but many of Wavenumbers these are large single-mineral grains that have surficial debris indicating that they were previously embedded in Figure 8-1. The three major infrared spectral IDP classes. From top to larger samples of fine-grained bottom, these spectra art" representative of lDPs dominated by the minerals chondritic material. The olivine, layer-lattice silicates, and pyroxenes, respectively. The spectra can be "chondritic" particles are easily separated by the profiles of their characteristic I0 pm (I000 cm -1) small (typically less than Si-O stretching features. The spectra also show longer wavelength features 20 _m in diameter) and due to Si-O-Si bending vibrations in the silicates, hi addition to the Si-O classifying them into mean- bands, spectra from the particles dominated by layer-lattice silicates also ingful groups is much more show bands centered near 3.0 and 6.012m (3330 and 1670 cm -1) due to difficult than with con- O-H stretching and H-O-H bending vibrations in adsorbed and absorbed ventional kilogram-sized water, and bands near 6.8 and 11.4 pm ('1470 and 875 cm -]) due to C-O meteorites.