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(meteorobs) AMS FAQ on Fireballs and Meteorite Dropping Fireballs
Hello again,
My friend Lew forgot to mention that the American Meteor Society is also
interested in receiving fireball reports from throughout North America
(including Canada), the Caribbean, and the eastern Pacific. You can get to
our fireball form by following the links off of the main page:
http://www.serve.com/meteors/
or more directly from the opening fireball page at:
http://www.serve.com/meteors/genfb.html
or you can get to the fireball form directly at:
http://www.serve.com/meteors/form_1.html
THE AMS has an information exchange policy set up with the IMO FIDAC, such
that all reports we receive will be forwarded on to Europe as well.
An additional attraction of the AMS fireball pages is the Bright Meteor
Diary, located at:
http://www.serve.com/meteors/bmd.html
The Bright Meteor Diary is a current, running list of the fireball events
that have occurred over our coverage region throughout the year, and allows
investigators to see if others are reporting the same event that they
witnessed. In addition to 1998, there is also a BMD for 1997 posted.
Below, I am also posting the AMS FAQ on Fireballs and Meteorite Dropping
Firegballs, which might help to answer some of the questions that are
appearing on this topic:
The American Meteor Society, Ltd.
Frequently Asked Questions (FAQ)
About Fireballs and Meteorite Dropping Fireballs
Version 1.2
Question List:
1. What is a fireball? What is the difference between a fireball and a
bolide?
2. How frequently do fireballs occur?
3. Can you see fireballs in daylight, and will a fireball leave a trail?
4. I saw a very bright meteor. Did anyone else see it, and to whom should
I report it?
5. Can fireballs appear in different colors?
6. Can a fireball create a sound? Will the sound occur right away, as you
watch the fireball, or is their some delay?
7. How bright does a meteor have to be before there is a chance of it
reaching the ground as a meteorite?
8. Can a meteorite dropping fireball be observed all the way to impact
with the ground?
9. Are meteorites "glowing" hot when they reach the ground?
10. How frequently do meteorite falls occur?
11. How big are most meteorites? Do they fall as single objects or
clusters of objects?
12. How fast are meteorites traveling when they reach the ground?
13. How can I recognize a meteorite, and where should I hunt for them?
14. Where can I get a potential meteorite authenticated?
15. What do fireballs and meteorites tell us about their origins?
16. Author's note on fireball / meteorite statistics.
Below are some relatively concise answers to the above questions. If you
need further clarification or have further questions, please feel free to
contact us via electronic mail.
1. What is a fireball? What is the difference between a fireball and a
bolide?
A fireball is another term for a very bright meteor, generally brighter
than magnitude -3 or -4, which is about the same magnitude of the planet
Venus in the morning or evening sky. A bolide is a special type of fireball
which explodes in a bright terminal flash at its end, often with visible
fragmentation.
If you happen to see one of these memorable events, we would ask that you
report it to the American Meteor Society, remembering as many details as
possible. This will include things such as brightness, length across the
sky, color, and duration (how long did it last), it is most helpful of the
observer will mentally note the beginning and end points of the fireball
with regard to background star constellations, or compass direction and
angular elevation above the horizon.
The table below will aid observers in gaging the brightness of fireballs:
Object magnitude
----------------------------
Polaris +2.1
Vega +0.14
Sirius -1.6
Bright Jupiter -2.5
Bright Mars -2.8
Bright Venus -4.4
1st Quarter Moon -10.4
Full Moon -12.6
Sun -26.7
2. How frequently do fireballs occur?
Several thousand meteors of fireball magnitude occur in the Earth's
atmosphere each day. The vast majority of these, however, occur over the
oceans and uninhabited regions, and a good many are masked by daylight.
Those that occur at night also stand little chance of being detected due to
the relatively low numbers of persons out to notice them.
Additionally, the brighter the fireball, the more rare is the event. As a
general thumbrule, there are only about 1/3 as many fireballs present for
each successively brighter magnitude class, following an exponential
decrease. Experienced observers can expect to see only about 1 fireball of
magnitude -6 or better for every 200 hours of meteor observing, while a
fireball of magnitude -4 can be expected about once every 20 hours or so.
3. Can you see fireballs in daylight, and will a fireball leave a trail?
Yes, but the meteor must be brighter than about magnitude -6 to be noticed
in a portion of the sky away from the sun, and must be even brighter when
it occurs closer to the sun.
Fireballs can develop two types of trails behind them: trains and smoke
trails. A train is a glowing trail of ionized and excited air molecules
left behind after the passage of the meteor. Most trains last only a few
seconds, but on rare occasions a train may last up to several minutes. A
train of this duration can often be seen to change shape over time as it is
blown by upper atmospheric winds. Trains generally occur very high in the
meteoric region of the atmosphere, generally greater than 80 km (65 miles)
altitude, and are most often associated with fast meteors. Fireball trains
are often visible at night, and very rarely by day.
The second type of trail is called a smoke trail, and is more often seen in
daylight fireballs than at night. Generally occurring below 80 km of
altitude, smoke trails are a non-luminous trail of particulate stripped
away during the ablation process. These appear similar to contrails left
behind by aircraft, and can have either a light or dark appearance.
4. I saw a very bright meteor. Did anyone else see it, and to whom should I
report it?
The American Meteor Society (AMS) collects fireball reports from throughout
North America, the Caribbean, and the Pacific islands for use by our
organization and other meteor organizations. Persons who have seen a bright
meteor event are encouraged to report their sighting to us. If multiple
sightings of a single event can be grouped together, it is sometimes
possible to determine the actual trajectory of the object in question.
The easiest way to report a fireball to us is to utilize our on-line form,
located at our Internet Web site. This site is located at
http//www.serve.com/meteors.
Another feature of this Web site is the "Bright Meteor Diary," which
permits on-line browsing of our electronic fireball database.
Implemented in March, 1997, this database permits visitors to search for
reports about a particular fireball event, as sorted by date and location.
Even if others are reporting the same fireball event that you saw, you are
still encouraged to add your own sighting, in order to improve our
information.
5. Can fireballs appear in different colors?
Vivid colors are more often reported by fireball observers because the
brightness is great enough to fall well within the range of human color
vision. These must be treated with some caution, however, because of
well-known effects associated with the persistence of vision. Reported
colors range across the spectrum, from red to bright blue, and (rarely)
violet. The dominant composition of a meteoroid can play an important part
in the observed colors of a fireball, with certain elements displaying
signature colors when vaporized. For example, sodium produces a bright
yellow color, nickel shows as green, and magnesium as blue-white. The
velocity of the meteor also plays an important role, since a higher level
of kinetic energy will intensify certain colors compared to others. Among
fainter objects, it seems to be reported that slow meteors are red or
orange, while fast meteors frequently have a blue color, but for fireballs
the situation seems more complex than that, but perhaps only because of the
curiousities of color vision as mentioned above.
The difficulties of specifying meteor color arise because meteor light is
dominated by an emission, rather than a continuous, spectrum. The majority
of light from a fireball radiates from a compact cloud of material
immediately surrounding the meteoroid or closely trailing it. 95% of this
cloud consists of atoms from the surrounding atmosphere; the balance
consists of atoms of vaporized elements from the meteoroid itself. These
excited particles will emit light at wavelengths characteristic for each
element. The most common emission lines observed in the visual portion of
the spectrum from ablated material in the fireball head originate from iron
(Fe),magnesium (Mg), and sodium (Na). Silicon (Si) may be under-represented
due to incomplete dissociation of SiO2 molecules. Manganese (Mn), Chromium
(Cr), Copper (Cu) have been observed infireball spectra, along with rarer
elements. The refractory elements Aluminum (Al), Calcium (Ca), and Titanium
(Ti) tend to be incompletely vaporized and thus also under-represented in
fireball spectra.
6. Can a fireball create a sound? Will the sound occur right away, as you
watch the fireball, or is their some delay?
There are two reported types of sounds generated by very bright fireballs,
both of which are quite rare. These are sonic booms, and electrophonic
sounds.
If a very bright fireball, usually greater than magnitude -8, penetrates to
the stratosphere, below an altitude of about 50 km (30 miles), and explodes
as a bolide, there is a chance that sonic booms may be heard on the ground
below. This is more likely if the bolide occurs at an altitude angle of
about 45 degrees or so for the observer, and is less likely if the bolide
occurs overhead (although still possible) or near the horizon. Because
sound travels quite slowly, at only about 20 km per minute, it will
generally be 1.5 to 4 minutes after the visual explosion before any sonic
boom can be heard. Observers who witness such spectacular events are
encouraged to listen for a full 5 minutes after the fireball for potential
sonic booms.
Another form of sound frequently reported with bright fireballs is
"electrophonic" sound, which occurs coincidentally with the visible
fireball. The reported sounds range from hissing static, to sizzling, to
popping sounds. Often, the witness of such sounds is located near some
metal object when the fireball occurs. Additionally, those with a large
amount of hair seem to have a better chance of hearing these sounds.
Electrophonic sounds have never been validated scientifically, and their
origin is unknown. Currently, the most popular theory is the potential
emission of VLF radio waves by the fireball, although this has yet to be
verified.
7. How bright does a meteor have to be before there is a chance of it
reaching the ground as a meteorite?
Generally speaking, a fireball must be greater than about magnitude -8 to
-10 in order to potentially produce a meteorite fall. Two important
additional requirements are that (1) the parent meteoroid must be of
asteroidal origin, composed of sufficiently sturdy material for the trip
through the atmosphere, and (2) the meteoroid must enter the atmosphere as
a relatively slow meteor. Meteoroids of asteroid origin make up only a
small percentage (about 5%) of the overall meteoroid population, which is
primarily cometary in nature.
Photographic fireball studies have indicated that a fireball must usually
still be generating visible light below the 20 km (12 mile) altitude level
in order to have a good probability of producing a meteorite fall. Very
bright meteors of magnitude -15 or better have been studied which produced
no potential meteorites, especially those having a cometary origin.
8. Can a meteorite dropping fireball be observed all the way to impact with
the ground?
No. At some point, usually between 15 to 20 km (9-12 miles or 48,000-63,000
feet) altitude, the meteoroid remnants will decelerate to the point that
the ablation process stops, and visible light is no longer generated. This
occurs at a speed of about 2-4 km/sec (4500-9000 mph).
>From that point onward, the stones will rapidly decelerate further until
they are falling at their terminal velocity, which will generally be
somewhere between 0.1 and 0.2 km/sec (200 mph to 400 mph). Moving at these
rapid speeds, the meteorite(s) will be essentially invisible during this
final "dark flight" portion of their fall.
9. Are meteorites "glowing" hot when they reach the ground?
Probably not. The ablation process, which occurs over the majority of the
meteorite's path, is a very efficient heat removal method, and was
effectively copied for use during the early manned space flights for
re-entry into the atmosphere. During the final free-fall portion of their
flight, meteorites undergo very little frictional heating, and probably
reach the ground at only slightly above ambient temperature.
For the obvious reason, however, exact data on meteorite impact
temperatures is rather scarce and prone to hearsay. Therefore, we are only
able to give you an educated guess based upon our current knowledge of
these events.
10. How frequently do meteorite falls occur?
Our best estimates of the total incoming meteoroid flux indicate that about
10 to 50 meteorite dropping events occur over the earth each day. It should
be remembered, however, that 2/3 of these events will occur over ocean,
while another 1/4 or so will occur over very uninhabited land areas,
leaving only about 2 to 12 events each day with the potential for discovery
by people. Half of these again occur on the night side of the earth, with
even less chance of being noticed. Due to the combination of all of these
factors, only a handful of witnessed meteorite falls occur Each year.
As an order of magnitude estimation, each square kilometer of the earth's
surface should collect 1 meteorite fall about once every 50,000 years, on
the average. If this area is increased to 1 square mile, this time period
becomes about 20,000 years between falls.
11. How big are most meteorites, and do they fall as single objects or
clusters of objects?
Meteorite finds range in size from particles weighing only a few grams, up
to the largest known specimen: the Hoba meteorite, found in South Africa in
1920, and weighing about 60 tons (54,000 kg). As with the magnitude
distribution of meteors, the number of meteorites decreases exponentially
with increasing size. Thus, the majority of falls will produce only a few
scattered kilograms of material, with large meteorites being quite rare.
Meteorites are known to fall as single, discreet objects; as showers of
fragments from a meteor which breaks up during the atmospheric portion of
its flight; and (rarely) as multiple individual falls. The initial mass and
composition of the meteoroid primarily determine its eventual fate, along
with its speed and angle of entry into the atmosphere.
12. How fast are meteorites traveling when they reach the ground?
Meteoroids enter the earth's atmosphere at very high speeds, ranging from
11 km/sec to 72 km/sec (25,000 mph to 160,000 mph). However, similar to
firing a bullet into water, the meteoroid will rapidly decelerate as it
penetrates into increasingly denser portions of the atmosphere. This is
especially true in the lower layers, since 90 % of the earth's atmospheric
mass lies below 12 km (7 miles / 39,000 ft) of height.
At the same time, the meteoroid will also rapidly lose mass due to
ablation. In this process, the outer layer of the meteoroid is continuously
vaporized and stripped away due to high speed collision with air molecules.
Particles from dust size to a few kilograms mass are usually completely
consumed in the atmosphere.
Due to atmospheric drag, most meteorites, ranging from a few kilograms up
to about 8 tons (7,000 kg), will lose all of their cosmic velocity while
still several miles up. At that point, called the retardation point, the
meteorite begins to accelerate again, under the influence of the Earth's
gravity, at the familiar 9.8 meters per second squared. The meteorite then
quickly reaches its terminal velocity of 200 to 400 miles per hour (90 to
180 meters per second). The terminal velocity occurs at the point where the
acceleration due to gravity is exactly offset by the deceleration due to
atmospheric drag.
Meteoroids of more than about 10 tons (9,000 kg) will retain a portion of
their original speed, or cosmic velocity, all the way to the surface. A
10-tonner entering the Earth's atmosphere perpendicular to the surface will
retain about 6% of its cosmic velocity on arrival at the surface. For
example, if the meteoroid started at 25 miles per second (40 km/s) it would
(if it survived its atmospheric passage intact) arrive at the surface still
moving at 1.5 miles per second (2.4 km/s), packing (after considerable mass
loss due to ablation) some 13 gigajoules of kinetic energy.
On the very large end of the scale, a meteoroid of 1000 tons (9 x 10^5 kg)
would retain about 70% of its cosmic velocity, and bodies of over 100,000
tons or so will cut through the atmosphere as if it were not even there.
Luckily, such events are extraordinarily rare.
All this speed in atmospheric flight puts great pressure on the body of a
meteoroid. Larger meteoroids, particularly the stone variety, tend to break
up between 7 and 17 miles (11 to 27 km) above the surface due to the forces
induced by atmospheric drag, and perhaps also due to thermal stress. A
meteoroid which disintegrates tends to immediately lose the balance of its
cosmic velocity because of the lessened momentum of the remaining
fragments. The fragments then fall on ballistic paths, arcing steeply
toward the earth. The fragments will strike the earth in a roughly
elliptical pattern (called a distribution, or dispersion ellipse) a few
miles long, with the major axis of the ellipse being oriented in the same
direction as the original track of the meteoroid. The larger fragments,
because of their greater momentum, tend to impact further down the ellipse
than the smaller ones. These types of falls account for the "showers of
stones" that have been occasionally recorded in history. Additionally, if
one meteorite is found in a particular area, the chances are favorable for
there being others as well. 13. How can I recognize a meteorite, and where
should I hunt for them?
The classic concept of a meteorite is a heavy, black rock. This stereotype
is true in some cases, but many, many more meteorites resemble nothing more
than mundane terrestrial rocks. These will attract attention only by being
different from all others around them.
To understand what a meteorite might look like on the ground, we must first
examine the numerical distribution of the three major types of meteorites.
Of the known meteorite classes (combining falls and finds):
* stones (Aerolites) comprise about 69 per cent;
* irons (siderites) comprise about 28 per cent;
* and stony-irons (siderolites) comprise the remaining 3 per cent.
First of all, if a meteorite is found fairly quickly after it falls, most
will exhibit an overall blackened surface, called a fusion crust. This
fusion crust is a souvenier of ablation heat from the meteorite's rapid
atmosphere transit. Depending on the composition of the meteorite, the
fusion crust may appear glassy, or dull. Irons develop a fusion crust
consisting of magnetite, and having the appearance of a fresh weld on
steel.
Once a meteorite is on the surface, all the normal weathering effects that
erode earthly rocks affect meteorites, too. A fusion crust will weather,
and on a stone, lighten in color to a brownish hue. Chemical weathering, or
oxidation, will attack meteorites. Irons will quickly rust. Stones will
lose their fusion crusts entirely. Water will seep into the interior, and
chemically alter the minerals. Mechanical weathering, by frost, sun, and
wind will reduce the meteorite further. This is why most ancient meteorites
found are irons, most able to resist these processes.
Most suspected meteorites, by the percentages above, are stony, and the
finder's attention was drawn to them by their contrasting appearance with
their surroundings. The indisputable identification of a stony meteorite
requires chemical tests which are beyond the scope of this article.
Iron meteorites may frequently be recognized by their shape. The melting of
the exterior of the body will sometimes cause iron meteoroids to arrive at
the surface carved into fantastic shapes. Complete rings and segments of
arcs have been found. An iron will be pitted, as portions of the alloy with
a lower melting temperature will be scooped out by the heat and pressure.
There will sometimes be sharp points surrounding these pits, an ablation
effect. Positive identification of an iron requires a grinding and acid
etching process that is again, beyond the scope of this article.
Anyone with a serious interest in searching for meteorites should arrange a
visit to a large museum with a meteorite collection, in order to view not
the spectacular specimens on display, but the more "ordinary" specimens
kept in the institutions' collection. By examining many specimens, the
seeker will gain a good understanding of the varied appearance that
meteorites may present.
The most successful areas for hunting for meteorites are open, flat, arid
regions, usually having a light background color. Such regions have the
lowest rates of mechanical and chemical weathering, preserving the
meteorite for much longer periods of time. Some irons and stony-irons have
been found in desert regions more than 10,000 years after the fall which
produced them. Arid regions also offer great advantages in visual searches
due to the relative lack of vegetation or bodies of water, as well as a
light contrasting background color.
The best areas for meteorite searching (although rather impractical for
most persons) are the regions of the earth covered by continental glaciers,
such as Greenland and Antarctica. These ice packs offer the highest degree
of preservation of a meteorite after its fall, high background contrast,
and few competing terrestrial rocks. Many of the meteorites used in
research today were recovered during Antarctic expeditions.
For those without access to arid deserts or continental glaciers, perhaps
the best place to do meteorite hunting is in freshly plowed farmer's
fields, especially following a recent rain. Native-American arrowhead
hunters frequently employ this technique as well. Farmers have plowed up
many of the more famous meteorite finds in history. Iron meteorites are the
easiest to recognize and are most frequently found. Stony meteorites are
more difficult to recognize and to differentiate from terrestrial rocks,
such as (ice age) glacial erratics.
The majority of meteorites, including the stone varieties, contain
sufficient amounts of iron (Fe) and nickel (Ni) to cause them to be
paramagnetic. Meteorite hunters often employ metal detectors, or very
strong magnets attached to a walking stick, to aid them in their searches.
Meteorites have been known to literally "jump" out of loose soil in the
presence of a strong magnet.
14. Where can I get a potential meteorite authenticated?
Below is a brief list of academic institutions and museums which might be
contacted about authenticating a potential meteorite find.
Readers are highly advised to first contact the institution and obtain
information about their individual policies regarding such testing and
potential fees prior to shipping any actual material. Since the American
Meteor Society does not normally deal in meteorites, we cannot make
recommendations or give advice on the selection of a testing facility.
Readers must use their own discretion in this matter.
Academic Institutions:
Center for Meteorite Studies
Arizona State University
Temple, AZ 85281
Institute of Geophysics and Planetary Sciences
University of California
Los Angeles, CA 90024
Institute of Meteoritics
Department of Geology
University of New Mexico
Albuquerque, NM 87131
Lunar and Planetary Laboratory
Space Sciences Building
University of Arizona
Tucson, AZ 85721
Museums:
The American Museum of Natural History
Central Park West at 79th St
. New York, NY, 10024
The Field Museum of Natural History
S. Lake Shore Dr.
Chicago, IL 60605
National Museum of Natural History
Dept. of Mineral Sciences
Smithsonian Institution
Washington, DC 20560
15. What do fireballs and meteorites tell us about their origins?
Most of our current knowledge about the origin of meteoroids comes from
photographic fireball studies (meteors > magnitude -4) done over the last
50 years or so. This may sound like a long time, but good data has been
collected on only about 800 fireballs so far. Of these, only 4 have been
recovered on the ground as meteorites. A meteorite-causing fireball is very
rare and must be at least magnitude -8 to have sufficient mass to survive
the trip. Even with an accurate photographic or video trajectory, it is
still a matter of finding a needle in a haystack once the meteorite is on
the ground. In recorded scientific history, un-photographed (eyewitnessed)
falls have resulted in only about 900 meteorite finds.
Studies of meteoroid parent bodies, comets and asteroids, have been more
successful, using space probes and infrared telescope studies to greatly
increase our knowledge of these objects. What we have found is that, rather
than distinct differences between these two smaller solar system members,
there exists an entire spectrum of parent bodies, ranging from low-density
comets to large differentiated asteroids. The similarities between
asteroids and comets is made more apparent by the recent discovery of a
coma (a distinctly cometary phenomena) around the asteroid Chiron, at its
perihelion.
At the present time, meteoroid parent bodies can be roughly divided into
the following classes:
COMETS:
By far the most prevalent parent body of meteoroids, cometary meteoroids
form about 95% of the total meteor population, and include nearly ALL of
the shower meteor population. These parent bodies are composed of frozen
methane (CH4), ammonia (NH3), water (H2O), and common gases (such as carbon
dioxide, CO2), carbon dust and other trace materials. As a comet passes
near the sun in its orbit, the outer surface exposed to sunlight is
vaporized and ejected in spectacular jets and streams, freeing large
amounts of loosely aggregated clumps of dust and other non-volatile
materials.
These freshly generated cometary meteoroids, often called "dustballs" will
roughly continue to follow the orbit of the parent comet, and will form a
meteoroid stream.
Based upon photographic fireball studies, cometary meteoroids have
extremely low densities, about 0.8 grams/cc for class IIIA fireballs, and
0.3 grams/cc for class IIIB fireballs. This composition is very fragile and
vaporizes so readily when entering the atmosphere, that it is called
"friable" material. These meteoroids have virtually no chance of making it
to the ground unless an extremely large piece of the comet enters the
atmosphere, in which case it would very likely explode at some point in its
flight, due to mechanical and thermal stresses. If a piece did survive the
journey, it would be extremely black, float in water, and would rapidly
melt and sublime away (with the exception of some dust particles) in a
short amount of time.
NON-DIFFERENTIATED ASTEROIDS:
These parent bodies are the smaller asteroids, constructed of denser and
less volatile materials than the comets. Small meteoroids of this type are
produced through collisions. This class of parent bodies generate about 5%
of the total meteor population, generally as part of the non-shower, or
"sporadic" meteors. These meteoroids can make it through the atmosphere,
and as meteorites, they make up about 84% of all falls.
Stony meteorites from this source are called Chondrites, due to the rounded
nodules of material found within their structure, which are called
chondrules. Chondrite meteorites have two major groupings:
The first group, the Class II fireballs, are the carbon-rich Chondrites, or
Carbonaceous Chondrites, which help bridge the gap between comets and
asteroids. They make up about 4% of all observed falls, and have densities
of around 2.0 grams/cc. They are characterized by the presence of 2% or
more carbon, partly present as complex hydrocarbons, and of considerable
hydrogen (hydroxyl groups, OH-1, and water, H2O).
The second group, the Class I fireballs, are what is called the Ordinary
Chondrites, making up about 80% of all observed falls. They have an average
density of 3.7 grams/cc, and generally fall into two general types:
Olivine-Bronzite Chondrites (about equal amounts of bronzite and olivine)
and Olivine-Hypersthene Chondrites (less pyroxene than olivine).
DIFFERENTIATED ASTEROIDS:
These asteroids are physically the largest parent body for meteoroids, but
generate only a small fraction of the overall meteor population: less than
1%, and have no fireball classification. Due to their hardier composition,
however, they make up about 16% of the observed falls. A differentiated
asteroid is one with sufficient size to cause internal temperatures high
enough to melt and stratify the asteroid. The higher density materials
(mainly iron) gather in the core, the lighter basalt/silicate materials
gather in the outer layers, with thinner layers of various concentrations
of other materials stratified in between. Small meteoroids of these types
have been produced by what must have been spectacular collisions, breaking
up even the iron core of the asteroid.
The three major groups for these meteors are:
1. Achondrites (Basalt/Silicate non-chondritic stones); with a 3-4
grams/cc density, and comprising about 8% of observed falls. These
formed in the outer and crustal layers of the asteroid.
2. Siderolites (Stony-Irons); with a 5-7 grams/cc density, and comprising
about 2% of observed falls. These formed a thin layer between the core
and outer layers of the parent bodies. They generally consist of
round, translucent green crystals of olivine imbedded in a matrix of
iron.
3. Siderites (Irons); with a 7.9 grams/cc density, and comprising about
6% of observed falls. These are the remains of the core of a
differentiated asteroid, and show signs of extremely slow cooling
(1-10 deg C per million years), and extremely high shock stresses,
presumably from collisions. These meteorites weather so well once on
the ground, they make up 54% of all meteorite finds despite their
small percentage of the fall population.
DIFFERENTIATED PLANETOIDS:
The very rarest of meteorites are those which are thought to have
originated from large differentiated bodies, such as moons and planets.
These Achondritic stones (basalt/silicate) are believed to have been
ejected from a moon or planet's surface, due to the impact of another very
large meteoroid. One sub-class of Achondrites show a very similar
composition to that of the earth's moon, and are believed to be Lunar
meteorites. Another class, the SNC (shergottite-nakhlite-chassignite)
meteorites, are believed to have been ejected from the crust of the planet
Mars.
16. Author's note on fireball / meteorite statistics.
Readers of this FAQ will notice that those particles which make up the
majority of the meteoroid population are those which are the least likely
to make it to the ground as a meteorite. Conversely, those particles which
make up a minority of the meteoroid population are the most likely to reach
the ground as a meteorite. This disparity becomes even more skewed when
weathering conditions on the ground are considered. Thus, the meteors which
are most often seen are not found on the surface, and the ones which are
most often found are uncommon in the sky.
It took scientists many years to realize this disparity, and published
texts frequently seem to conflict with one another with regard to the
percentile breakdown of meteorite types. This is especially true if the
author has combined old meteorite finds with fresh, observed falls. In an
attempt to help alleviate this confusion, we present a current breakdown of
the different meteoroid/meteorite types, in their various stages:
OVERALL METEOR POPULATION:
As a general rule, the smaller (fainter) is the meteoroid population under
consideration, the more likely is a cometary origin. As a very rough
estimation, the visible meteor population is composed of about 19 cometary
meteors for every 1 asteroidal meteor. This yields the following breakdown:
* Cometary meteoroids: ~95%
* Chondritic meteoroids: ~5%
* Non-chondritic meteoroids: <1%
FIREBALL POPULATION:
When only the population of meteors of > -4 magnitude are considered, the
more sturdy asteroidal meteoroids begin to make up an increasingly higher
percentage when compared to fainter magnitudes. There are four basic
fireball classes which are divided as follows:
* Cometary meteoroids: 38%
o Type IIIb fireballs, low density comets: 9%
o Type IIIa fireballs, high density comets: 29%
* Chondritic meteoroids: 62%
o Type II fireballs, Carbonaceous Chondrites: 33%
o Type I fireballs, Ordinary chondrites: 29%
* Non-chondritic meteoroids: <1%
o No fireball class
OBSERVED METEORITE FALLS / FRESH FINDS:
When only very fresh meteorite falls are considered, it becomes instantly
apparent how important the density and sturdiness of the meteoroid material
is to its likelihood of reaching the ground. The cometary meteoroid
population disappears, and the carbonaceous chondrite population is greatly
reduced. Thus, the ordinary chondrites and non-chondritic meteorites become
the primary constituents of this population:
* Cometary meteoroids: 0%
* Chondritic meteoroids: 84%
o Carbonaceous chondrites: 4%
o Ordinary chondrites: 80%
* Non-chondritic meteoroids: 16%
o Achondrites: 8%
o Siderolites: 2%
o Siderites: 6%
METEORITE FINDS:
Once they are on the ground, meteorites instantly begin to undergo
mechanical and chemical weathering. Again, those meteorites which are more
sturdy and dense tend to withstand these processes much better. In this
case, the iron meteorites (siderites) fare the best, despite their very
small proportion of the overall meteoroid population:
* Cometary meteoroids: 0%
* Chondritic meteoroids: 37%
o Carbonaceous chondrites: <1%
o Ordinary chondrites: 37%
* Non-chondritic meteoroids: 63%
o Achondrites: 3%
o Siderolites: 6%
o Siderites: 54%
This is an active field of study, and readers are reminded that all of the
above numbers are estimates, and subject to revision as our knowledge level
increases. We have attempted to select the most representative values for
each.
FAQ compiled by:
James Richardson
AMS Operations Manager / Radiometeor Project Coordinator
James Bedient
AMS Electronic Information Coordinator
FAQ References:
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© 1997 American Meteor Society, Ltd.
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