Comets, Asteroids, Meteroids, Meteors, Meteorites Some Facts about COMETS 1. Comets are usually named after their discoverers. 2. Comets are wanderers who visit our solar system. 3. Their home ground is the Oort Cloud Region (9,300,000,000 miles from the sun). 4. In ancient times it was believed that comets were the souls of heroes or kings on their way to heaven, or messengers of disasters, etc. 5. Comets may be the left over rock dust and icy matter from the formation of the solar system. 6. Some comets are sun-grazing (sweep close to the sun). 7. Comet Kohoutex was seen in 1974. It may not be back for a million years or more. TABLE OF RETURNING COMETS COMET PERIOD SEEN RETURNED Encke 3.3 years 1980 Always within range of telescope Tempel 2 5.3 years 1978 1983 Holmes 7.1 years 1979 1986 Faye 7.4 years 1977 1984 Halley 76 years 1910 1986 Some Facts about ASTEROIDS 1. An asteroid is a rocky object, smaller than a planet, that orbits the sun. 2. Asteroids orbit the sun between the planets Mars and Jupiter. 3. During the early age of our solar system thousands of asteroids crashed into Mercury, Venus, Earth, Mars and Jupiter. 4. There are tens of thousands of asteroids in the asteroid belt region. 5. Ceres, discovered in 1801, is the largest asteroid known to man. 6. All asteroids lumped together would be smaller than Earth's moon. 7. Pioneer 10 is the first man-made spacecraft to travel through the Asteroid Belt. 8. Small chunks are called meteoroids. METEOROIDS, METEORS, METEORITES If you have gazed at the night sky for any length of time you have no doubt seen "falling stars", "shooting stars" or meteors. These are bits of material which have been heated to incandescence by the friction of the air. Those pieces which are sufficiently large to not vaporize completely and reach the surface of the Earth are termed meteorites. The incandescent trails as they are coming through the Earth's atmosphere are termed meteors, and these chunks as they are hurtling through space are called meteoroids. Meteorites may look very much like Earth rocks, or they may have a burned appearance. They may be dense metallic chunks or more rocky. Some may have thumbprint-like depressions, roughened or smooth exteriors. Upon closer examination most meteoritic samples fall into predominately nickel-iron alloy called siderites or more commonly iron meteorites; the predominately rocky-silicates called aerolites or stony meteorites. In between these extremes are those which are generally an even mixture of silicate minerals and nickel-iron alloy which are called siderolites, or stony-iron meteorites. There are other sub-classes which will be discussed later. Meteoritic samples vary from tiny dust grains to giant boulders having masses of several thousand kilograms. It is difficult to distinguish meteorites from terrestrial rocks for the samples found on the ground i. e. "finds". Generally, the finds are of the iron variety because they do appear different. However, many meteorites have been seen to fall, thus the "falls" are excellent sources of meteoritic samples (of course, it takes two simultaneous viewings from different points to pinpoint the location of the fall). Estimates indicate that perhaps as little as 1,000 to more than 10,000 tons of meteoritic material falls on the Earth each day. However, most of this material is very tiny -- in the form of micrometeoroids or dust-like grains a few micrometers in size. (These particles are so tiny that the air resistance is enough to slow them sufficiently that they do not burn up, but rather fall gently to Earth. Analysis of ocean sediments show the presence of large quantities of micro-meteorites. Considering the proportion of ocean to land, the majority of meteorites should fall into the water. Meteorites vary in size from micrometer size grains to large individual boulders. The largest individual iron is the Hoba meteorite from southwest Africa which has a mass of about 54,000 kg. The stones are much smaller, the largest falling in Norton County, Kansas having a mass of about 1,000 kg. Considering the vast infall of meteorites, one cannot help but wonder if anyone has been hurt or killed by meteorites. There are only two documented cases on record. The only incident of a fatality was a shower of stones which fell upon Nakhla, near Alexandria, Egypt on June 28, l911. One of these stones killed a dog. On November 30, 1954, Mrs. Hewlett Hodges of Sylacauga, Alabama was severely bruised by an 8 pound stony meteorite that crashed through her roof. This is the first known human injury. If one compares a sampling of meteorites discovered as "falls" or "finds", one notices a discrepancy. *FINDS % FALLS % Irons 409 66.0 29 5.0 Stony-Iron 46 7.5 8 1.5 Stony 165 26.0 547 93.5 (*From Watson, Fletcher G. (1956) BETWEEN THE PLANETS, Harvard University Press. Although this is an old reference, the proportions remain about the same today.) Why the difference? For the finds the iron meteorites are visibly different in mass (density) and appearance from the surrounding terrestrial rocks. The stony meteorites are very similar to the Earth rocks -- the major difference being perhaps a charred or melted appearance on the surface. Thus the proportion of meteorite types for the falls is perhaps a more valid proportion -- a preponderance of stones, a very tiny amount of stony-iron and modest amount of irons. What does this mean? Where do they come from? Calculations indicate that near the Earth meteoroids have speeds in the range of 65 km/sec. The Earth's orbital speed of 30 km/sec means that they could have a relative speed between 30 and 95 km/sec. Although these speeds are not high enough to suggest interstellar origin, another fact is that plots show their trajectories to be elliptical and not hyperbolic, thus they must come from within our own solar system. Perhaps they share a common origin with the asteroids. The relative composition of the meteoroids provides a clue for a hypothesis that they or some originated from a planet (or planet like) body which exploded at some time in the distant past. The composition of stony meteorites is so similar to the Earth that it does suggest a rocky mantle. The iron is similar to what we believe the Earth's core is like. Thus, in 1943 R. A. Daly (in Meteorites and an Earth Model, Bull. Geol. Soc. of Amer. 54,401-456) proposed the following hypothesis for the ancestral body for the meteorites: 1. It had an iron (nickel-iron) core and a stony mantle. 2. The volume of the core is much less than the mantle. The planet must have been relatively small. 3. The force of gravity on this celestial body was much less than that of the Earth. What evidence do we have to support this hypothesis? 1. We find both iron and stony meteorites. 2. There is a much larger proportion of stony to iron meteorites. 3. The presence of stony-iron meteorites shows a mixing of types. We shall return to meteoritic origin later. Looking out at the night sky one can generally see several "shooting stars" or meteors per hour. The incandescent light comes from the resistance of the air, rapidly heating the meteoroid as well as ionizing the surrounding air. Thus we see a trail of light falling toward Earth. Usually the meteoroid is completely vaporized, in a second or less, but the larger meteors may have trails several kilometers long and last sufficiently for remnants to be found, i.e. meteorite falls. Some of the meteors are so bright that enough light is emitted to cast shadows. These are called fire balls (some can even be seen during day light.) The rate at which meteors appear is not constant and varies with the time of the year as well as time during the night. Sometimes during the year the number of meteors seen increases dramatically, these are termed "meteor showers". In fact some meteor showers occur annually or at rather regular intervals. The number is greater in autumn and winter. The number always increases after midnight and is usually greatest just before dawn. This is understandable when one considers the motion of the Earth. The Earth is heading more into its orbital path, also the night side of the Earth is turning toward this direction. When a meteor shower occurs, they appear to emanate or radiate from a point called the radiant. These meteor showers may vary in the number of meteors but regularly occur annually, and last for a few days. Perhaps the most famous are the Perseids which have maximum about August 12 but begin to occur about July 25 and last until about August 18, which is much longer than most. Meteor showers are usually named after a star or constellation which is close to the radiant. Many of the meteor showers are associated with comets. This deduction is because the more spectacular displays are coincident with the period of the comet -- as well as spectrographic data matter from the comet is spread out along its orbit, thus when the Earth's orbital path intersects the orbital path of the comet, a meteor shower occurs. Occasionally this occurs twice annually. Often the material is unevenly distributed which accounts for the variation of intensity. The leonids are associated with comet Tempel-Tuttle; Aquarids and Orionids with Halley and the Taurids with Encke. The majority of meteors burn up before reaching the Earth's surface, however, examination of some meteorites indicates a loose structure or vapor grown crystal aggregates which indicate a fluffy nature. This gives rise to theories that some meteoroid material was aggregated, some subjected to heating-vaporization-condensation. This contrasts with the idea that meteoroids originated from an exploded planet or planetoid or asteroid. Thus many meteoroids break apart in the upper atmosphere, and become "fluffy meteors". These are some of the pieces of the jigsaw puzzle. Other than spectrographic analysis of the light of the meteoric trails, the only chemical evidence lies in those extant samples; the meteorites. The mineral content is on the whole similar to Earth, however, there are some particular minerals which are unique to the meteorites. DATES OF MAJOR METEOR SHOWERS NUMBER DATE OF APPROXIMATE PER HOUR NAME MAXIMUM LIMITS AT MAXIMUM Quadrantids Jan 4 Jan 1-6 110 Lyrids Apr 22 Apr 19-24 12 Eta Aquarids May 5 May 1-8 20 Delta Aquarids Jul 27-28 Jul 15-Aug 15 35 Perseids Aug 12 Jul 25-Aug 18 68 Orionids Oct 21 Oct 16-26 30 Taurids Nov 8 Oct 20-Nov 30 12 Leonids Nov 17 Nov 15-19 10 Geminids Dec 14 Dec 7-15 58 METEORITES Meteorites generally fall into three major classes, the iron, which are primarily iron and nickel at one end of the scale, and the stones which are primarily silicates at the other, with the stony-irons consisting of an even mixture of silicate and nickel-iron. IRON The IRON or SIDERITES consist generally of 98% nickel-iron (the mineral name being plessite). The nickel-iron generally exists in two forms, the kamacite, which is a low nickel (4-7%) and the high nickel content taenite (30-60% nickel). Some cobalt is also usually present. These two minerals are not present on the surface of the Earth. Along with the metal, the accessory minerals generally consist of schreibersite with some troilite, cohenite and graphite. Also occasionally some daubreelite, although it is rather rare. STONY-IRON The STONY-IRON, either PALLISITES or MESOSIDERITES, generally are a fairly even mixture of nickel-iron in an open sponge-like matrix with the silicate minerals consisting of small rounded grains or bodies. The composition of the stony-iron meteorites is about 50% nickel-iron and 50% of the silicates olivine or pyroxene with occasional troilite (ferrous sulfide). STONY The STONY or AEROLITES may be classified into two or three subdivisions. These are the CHONDRITES, ACHONDRITES, and CARBONACEOUS CHONDRITES. CHONDRITES are silicate meteorites having chondri or small rounded bodies of olivine or pyroxene. About 90% of the stony meteorites are chondrites. Chondrites generally consist of 46% olivine, 25% pyroxene, 11% plagioclase and about 12% nickel-iron. ACHONDRITES are those stony meteorites not having chondri. Only about 10% of the stones are achondrites. Achondrites generally consist of 72% pyroxene, 25% plagioclase, 9% olivine, and only about 1% nickel-iron. CARBONACEOUS CHONDRITES are a fairly rare meteorite, having organic molecules, water, the mineral serpentine, and usually having chondri. Meteorites vary in size from small grains to large chunks of several thousand pounds. The iron meteorites or siderites generally consist of 98% of a nickel-iron alloy called plessite. Due to physical processes of melting and recrystallizing the nickel-iron can be separated into two types, nickle rich (30-60% Ni) taenite and low nickel (4-7%) kamacite. These separate into laminar or layered bands which are not visible even in a polished section. However, if the polished section is etched slightly with acid, the layers, called the Widmanstatten pattern, becomes visible. Although nickel-iron is found on the Earth, the particular minerals kamacite and taenite are so far only found in meteorites. Usually some cobalt is also present (less than one percent). Along with the metals, the accessory minerals (about 2%) generally consist of schreibersite (an iron nickel phosphide;) with some troilite (a ferrous sulfide; FeS) cohenite,an iron-nickel carbide; and graphite (carbon/C). Also occasionally some daubreelite (an iron chromium sulfide); although it is rather rare and not found in terrestrial rocks. The iron meteorites can be divided into four basic groups. The hexahedrites have a nickel content less than 7%. The accessory mineral is generally only troilite. The hexahedrite nickel-iron is only the kamacite variety. The octahedrites having an overall nickel content between 6.5 and 18%. The octahedral crystal structure (confirmed by x-ray analysis) may be coarse, medium or fine grained. The octahedrites which are far more prevalent contain both kamacite and taenite. The accessory minerals are usually troilite and graphite. The nickel-rich axtites have a nickel content generally between 12 and 25% but sometimes greater. these contain modular or laminar inclusions of graphite, schreibersite, cohenite and troilite. The nickel-iron present is only taenite. The final and unusual group may be referred to as irons-with-silicate-inclusions (IWSI) in which about one-fourth of the volume (still only a few percent of the mass) is made up of irregular masses of silicates such as olivine, plagioclase, pyroxene along with graphite, troilite and schreibersite. The stony-iron, siderolites, or pallisites or mesodiderites generally lie intermediate between the stones and irons with a 50-50 mixture of nickel-iron alloy and silicate minerals. Generally the silicate minerals exist as small rounded bodies or grains in a sponge-like matrix of the alloy. The minerals are generally olivine or pyroxene with occasional troilite. The stony-irons generally fall into two groups: 1. The pallisites which have the silicate (predominately olivine) in a continuous matrix of nickel-iron. 2. The mesosiderites which have pyroxene and troilite along with the olivine in a discontinuous matrix of metal. The stony or aerolites are predominately silicate minerals with small amounts of nickel-iron alloy. They may be classed several ways, the major division being the presence or absence of small rounded bodies of silicates called chondri or chondrules. Thus those which have these chondrules are called chondrites, those which do not are achondrites. The vast majority (or about 90%) of stony meteorites are chondrites. These may be classified and sub-classified as follows: The chondrites, or ordinary chondrites consist generally of 46% olivine, 25% pyroxene, 11% plagioclase and about 12% nickel-iron with traces of troilite and chromite. However the specific analysis may provide further subdivisions based upon the specific amount of metal (i.e H=High, L=Low and LL=Very Low.) In the ordinary chondrites, the iron classification difference lies in the relative total iron content, and iron present in the metal. The amount of iron in the silicates olivine and pyroxene run counter to this. Iron Total Iron in Iron in Iron in Classification Iron Metal Olivine Pyroxene High H type 27% 16% 13% 8% Low L type 22% 6% 17% 10% Very Low LL type 21% 2% 20% 12% (These percentages are averages.) The texture and crystal structure (degree of metamorphism) are given number 3 for the least metamorphosed to 6 for the most highly metamorphosed. The extraordinary or unusual chondrites are the enstatite chondrites and the carbonaceous chondrites. The Enstatite chondrites consist of minerals in a highly reduced state including the minerals - enstatite (a variety of orthorhombic pyroxene), plagiocalse and troilite and sinoite, oldhamite (CaS), osbornite (TiN) and others. Enstatite chondrites cannot conveniently be classified by iron content. The carbonaceous chondrites, a fairly rare type, have little or no free metal, contain some iron compounds but contain serpentine, organic compounds and water of hydration. The carbon and water are used to sub-classify the carbonaceous chondrites. Those having the most water and carbon (water about 20.1%, C about 3.5%) are termed C1 or type I. They also contain magnetite and ferric chamosite. Those having an intermediate amount of water and carbon (water = 13.4%, C = 2.5%) are termed C3 or Type II. They also contain larger amounts of olivine and troilite along with some enstatite and ferric chamosite. Those carbonaceous chondrites having the least amount of carbon and water (water = 1.0%, C = 0.5%) are termed C3 or type III. They contain olivine, enstatite, troilite, pentlandlite but no ferric chamosite. The type I carbonaceous chondrites do not, as a rule, contain chondrules, but are classified as chondrites because of their chemical composition and similarity to type II and III. The achondrites are comparatively rare and are more similar in texture to the lunar and terrestrial igneous rocks. Out of a sample of 2,000 meteorites reported, only 64 are achondrites. They fall into two major categories, the low calcium or calcium poor variety and the high calcium or calcium rich variety. However they possess such a diversity that they fall into eight distinct classes. The classification names are either name of the predominate mineral or the location name of examples. The Calcium Poor Achondrites: Aubrite/Enstatite achondrites. Primarily enstatite, a magnesium silicate, variety of orthorhombic pyroxene, 9 examples are known. Diogenite/Hypersthene or Bronzite achondrites. Primarily the variety of orthorhombic pyroxene; hypersthene, an iron magnesium silicate, 8 examples are known. Chassignite/Olivine achondrites. Primarily olivine. An iron magnesium silicate, one example is known. Urelite/Olivine-Pigeonite achondrites. Primarily olivine, and a monoclinic variety of pyroxene; pigeonite, with some nickel-iron and carbon. This is the only variety of meteorites in which diamonds have been found, 3 examples are known. The Calcium Rich Achondrites: Eucrites and Howardites/Basaltic achondrites or orthopyroxene-pigeonite-plagioclase achondrites and pigeonite plagioclase achondrites. These are made up of monoclinic and orthorhombic pyroxenes and plagioclase. Over 40 specimens fall into this category Nakhlite/Diopside achondrites. These are predominately monoclinic pyroxenes, especially diopside, with some olivine, 2 known specimens. Angrite/Augite. This variety is primarily monoclinic pyroxene augite, one specimen is known. There is another variety of object of suspected extra-terrestrial origin which does not fit into the classification scheme of meteorites. These are the tektites. Tektites are a separate class of silica-rich glass which resemble obsidian but are quite distinct from any terrestrial obsidian. As yet none have been observed to fall, thus there is some dispute as to their origin, whether meteoric, volcanic, or cosmic. While meteorites are found widely distributed over the entire earth, tektites have only been found in widely scattered locations. Their size is generally under 200 grams, are rounded masses and have been found in regions which rule out volcanic origins. They may be fragments of a thin glassy surface of a comet or perhaps the result of condensation of cometary material. One theory indicates lunar origin as they resemble some of the lunar glass -- although the tektites are much larger. Thus we see that some material is similar to the Earth and moon and some is quite different. Some evidence indicates cometary origin. To accept a planetary explosion origin, one needs to consider that impact upon a body of diameter greater than 1,000 km will probably not result in an explosion or fragmentation. Thus, perhaps some meteoroids have their origin with the asteroids, some with comets, and some....? The book is not closed. There is a great deal of careful study and experimentation left to be accomplished. Prepared by: Dr. Harry B. Herzer, III NASA Aerospace Education Services Project Oklahoma State University, Stillwater, Oklahoma ----------------------------------------------------------------- This file was collected from NASA Spacelink (205) 895-0028. If you have any questions, call the NSS BBS (412) 366-5208 or send netmail to 129/104. 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