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
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