PLEASE NOTE:
*
CCNet DIGEST, 23 June 1998
--------------------------
"To me, the comet-shower theory is
exciting because it is not just
a theory about the Sun and the stars. It is
also a theory about the
history of life on Earth. If the theory is
right, it changes the
way we think about life and its evolution.
[...] Creatures which
were too successful in adapting themselves to
a stable environment
were doomed to perish when the environment
suddenly changed.
Creatures which were unspecialized and
opportunistic in their
habits had a better chance when Doomsday
struck. We humans are
perhaps the most unspecialized and the most
opportunistic of all
existing species. We thrieve on ice ages and
environmental
catastrophes (sic). Comet showers must have
been one of the major
forces that drove our evolution and made us
what we are."
(Friedman J Dyson, 1988)
(1) COMMENT ON GRONDINE'S REPORT
David Morrison <dmorrison@arc.nasa.gov>
wrote:
(2) WHAT SPEAKERS FAILED TO MENTION AT THE LUNCHEON: THE TAURID
METEOR
STREAM & DISINTEGRATED GIANT COMETS CAN
ALSO AFFECT YOUR HEALTH
James Marlin <rmarlin@network-one.com>
(3) SPACEGUARD TM
Duncan Steel <dis@a011.aone.net.au>
(4) FAQ ON FIREBALLS AND METEORITE FALLS
Jim Bedient (wh6ef@pixi.com)
=============================
(1) COMMENT ON GRONDINE'S REPORT
From David Morrison <dmorrison@arc.nasa.gov>
wrote:
Comment on Grondine's report
Ed Grondine sees fit to criticize Carl Pilcher and NASA generally
for
the estimates given of impact rates, for both Tunguska-class
impacts
and larger civilization-threatening impacts. In fact, Pilcher's
estimates at the Congressional forum on impact rates and their
uncertainties are exactly those of the scientific consensus, as
reflected in peer-reviewed scientific papers such as Chapman and
Morrison (Nature 1994), Morrison, Chapman and Slovic (Hazards
book
1994), Rabinowitz, Bowell, Shoemaker and Muinonen (Hazards book
1994),
Shoemaker, Weissman and Shoemaker (Hazards book 1994) and Toon et
al.
(Rev of Geophysics 1997). These are also the values quoted in the
two
official NASA reports (Morrison et al. 1992; Shoemaker et al.
1995).
If Grondine or others think these estimates are too low, they
should
write up their arguments and submit them to peer-reviewed
scientific
journals. Until that happens, they (and the rest of your
readers)
should recognize that the figures quoted by Pilcher do correctly
represent the scientific consensus as it is today (including Gene
Shoemaker's extensive contributions to these studies). That
doesn't
mean they are necessarily correct, but they are the best we have
to go
on.
On another subject, I would add that the favorable
characterization
Grondine gives of the Russian planetary defense proposals as
presented
at the Space Protection of the Earth conferences in Snezhinsk in
1994
and 1996 (not 1992 as stated by Grondine) do not agree with what
I
heard at these meetings. I attended both the SPE-94 and SPE-96
meetings, which were interesting and constructive, but I am
afraid that
Grondine, who did not attend them, may have been misinformed on
some of
these areas.
David Morrison
+++++++++++++++++++++++++++++++++++++++++++
David Morrison, NASA Ames Research Center
Tel 650 604 5094; Fax 650 604 1165
david.morrison@arc.nasa.gov
or dmorrison@mail.arc.nasa.gov
website: http://space.arc.nasa.gov
website: http://astrobiology.arc.nasa.gov
website: http://impact.arc.nasa.gov
=====================
(2) WHAT SPEAKERS FAILED TO MENTION AT THE LUNCHEON: THE TAURID
METEOR
STREAM & DISINTEGRATED GIANT COMETS CAN
ALSO AFFECT YOUR HEALTH
From James Marlin <rmarlin@network-one.com>
Just a reminder:
We are entering the Taurid daytime shower.
It is interesting to note that during this shower, the
Farmington, KS,
meteorite fell, the Tunguska Event occured, and five monks saw an
impact
on the Moon.
According to IRAS, there may be small asteroids in the core of
this
shower, though we will not pass through this core anytime soon.
What
does the List think of a possible link between Tunguska and this
shower?
==================
(3) SPACEGUARD TM
From Duncan Steel <dis@a011.aone.net.au>
Dear Benny,
Regarding: "By the way, is Spaceguard any kind of a NASA
trademark?"
During the deliberations of the NASA NEO Detection Workshop
(actually,
whilst the report was being drafted), I mentioned the project
'Spaceguard' as visualized in Sir Arthur C. Clarke's book
'Rendezvous
with Rama' (1973), and that was adopted as the real (one hopes)
project
name. I have had the business/company name 'Spaceguard' (now
'Spaceguard Australia') legally registered in Australia since
1988. I
assume that in other countries the people involved have protected
their
fledgling organisations by similarly registering the names
(Spaceguard
UK, Spaceguard Germany etc.).
Duncan
================
(4) FAQ ON FIREBALLS AND METEORITE FALLS
From Ron Baalke <BAALKE@kelvin.jpl.nasa.gov>
Forwarded from Jim Bedient (wh6ef@pixi.com)
Subject: FAQ - Fireballs and Meteorite Dropping Fireballs
-----------------------------------------------------------------------------
The American Meteor Society, Ltd.
Frequently Asked Questions (FAQ)
About Fireballs and Meteorite Dropping Fireballs
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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Fireball Networks".
Astronomical Institute of the
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© 1997 American Meteor Society, Ltd.
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