Introduction - Asteroid Belt
The asteroid belt is the region of the Solar System located roughly between
the orbits of the planets Mars and Jupiter. It is occupied by numerous
irregularly shaped bodies called asteroids or minor planets. The asteroid belt
region is also termed the main belt to distinguish it from other
concentrations of minor planets within the Solar System, such as the Kuiper
belt and scattered disc.
More than half the mass of the main belt is contained in the four largest
objects: Ceres, 4 Vesta, 2 Pallas, and 10 Hygiea. All of these have mean
diameters of more than 400 km, while Ceres, the main belt's only dwarf planet,
is about 950 km in diameter. The remaining bodies range down to the size of a
dust particle. The asteroid material is so thinly distributed that multiple
unmanned spacecraft have traversed it without incident. Nonetheless,
collisions between large asteroids do occur, and these can form an asteroid
family whose members have similar orbital characteristics and compositions.
Collisions also produce a fine dust that forms a major component of the
zodiacal light.
Formation
The asteroid belt formed from the primordial solar nebula as a group of
planetesimals, the smaller precursors of the planets. Between Mars and
Jupiter, however, gravitational perturbations from the giant planet imbued the
planetesimals with too much orbital energy for them to accrete into a planet.
Collisions became too violent, and instead of sticking together, the
planetesimals shattered. As a result, most of the main belt's mass has been
lost since the formation of the Solar System. Some fragments can eventually
find their way into the inner Solar System, leading to meteorite impacts with
the inner planets. Asteroid orbits continue to be appreciably perturbed
whenever their period of revolution about the Sun forms an orbital resonance
with Jupiter. At these orbital distances, a Kirkwood gap occurs as they are
swept into other orbits.
In 1802, shortly after discovering Pallas, Heinrich Olbers suggested to
William Herschel that Ceres and Pallas were fragments of a much larger planet
that once occupied the Mars-Jupiter region, this planet having suffered an
internal explosion or a cometary impact many million years before.
Exploded Planet theory
Over time
however, this hypothesis has fallen from favor. The large amount of energy
that would have been required to destroy a planet, combined with the belt's
low combined mass, which is only a small fraction of the mass of the Earth's
Moon, do not support the hypothesis. Further, the significant chemical
differences between the asteroids are difficult to explain if they come from
the same planet. Today, most scientists accept that, rather than fragmenting
from a progenitor planet, the asteroids never formed a planet at all.
In general in the Solar System, planetary formation is thought to have
occurred via a process comparable to the long-standing nebular hypothesis: a
cloud of interstellar dust and gas collapsed under the influence of gravity to
form a rotating disk of material that then further condensed to form the Sun
and planets. During the first few million years of the Solar System's history,
an accretion process of sticky collisions caused the clumping of small
particles, which gradually increased in size. Once the clumps reached
sufficient mass, they could draw in other bodies through gravitational
attraction and become planetesimals. This gravitational accretion led to the
formation of the rocky planets and the gas giants.
Planetesimals within the region which would become the asteroid belt were
too strongly perturbed by Jupiter's gravity to form a planet. Instead they
continued to orbit the Sun as before, while occasionally colliding. In regions
where the average velocity of the collisions was too high, the shattering of
planetesimals tended to dominate over accretion, preventing the formation of
planet-sized bodies. Orbital resonances occurred where the orbital period of
an object in the belt formed an integer fraction of the orbital period of
Jupiter, perturbing the object into a different orbit; the region lying
between the orbits of Mars and Jupiter contains many such orbital resonances.
As Jupiter migrated inward following its formation, these resonances would
have swept across the asteroid belt, dynamically exciting the region's
population and increasing their velocities relative to each other.
Evolution
The current asteroid belt is believed to contain only a small fraction of
the mass of the primordial belt. Computer simulations suggest that the
original asteroid belt may have contained mass equivalent to the Earth.
Primarily because of gravitational perturbations, most of the material was
ejected from the belt within about a million years of formation, leaving
behind less than 0.1% of the original mass. Since their formation, the size
distribution of the asteroid belt has remained relatively stable: there has
been no significant increase or decrease in the typical dimensions of the main
belt asteroids.
Collisions
The high population of the main belt makes for a very active
environment, where collisions between asteroids occur frequently
(on astronomical time scales). Collisions between main belt bodies
with a mean radius of 10 km are expected to occur about once every
10 million years. A collision may fragment an asteroid into
numerous smaller pieces (leading to the formation of a new
asteroid family). Conversely, collisions that occur at low
relative speeds may also join two asteroids together. After more
than 4 billion years of such processes, the members of the
asteroid belt now bear little resemblance to the original
population.
Meteorites
Some of the debris from collisions can form meteoroids that
enter the Earth's atmosphere.
More than 99.8 percent of the 30 000 meteorites found on Earth
to date are believed to have originated in the asteroid belt.
Every night
hundreds of meteors fly across the sky.
Image source:
http://science.howstuffworks.com/10-memorable-meteor-crashes.htm
Every day hundreds of meteors, commonly known as shooting
stars, can be seen flying across the night sky. Upon entering
Earth's atmosphere, friction heats up cosmic debris, causing
streaks of light that are visible to the human eye. Most burn up
before they ever reach the ground. But if one actually survives
the long fall and strikes Earth, it is called a meteorite.
A September 2007 study by a joint US-Czech team has suggested that
a large-body collision undergone by the asteroid 298 Baptistina
sent a number of fragments into the inner solar system. The
impacts of these fragments are believed to have created both the
Tycho crater on the Moon and the Chicxulub crater in Mexico, the
remnant of the massive impact which triggered the extinction of
the dinosaurs 65 million years ago.
Largest asteroids
Although their location in the asteroid belt excludes them from
planet status, the four largest objects, Ceres, Vesta, Pallas, and
Hygiea, hover on the edge of hydrostatic equilibrium, the boundary
that separates objects from planethood. They share many
characteristics common to planets, but also show qualities more
akin to rock-like asteroids.
Ceres is the only object in the belt large enough for its
gravity to force it into a roughly round shape, and so, according
to the IAU's 2006 resolution on the definition of a planet, it is
now considered a dwarf planet. The other three may also eventually
be reclassified as well. Ceres has a much higher absolute
magnitude than the other asteroids, of around 3.32, and may
possess a surface layer of ice.Like the planets, Ceres is
differentiated: it has a crust, a mantle and a core. Vesta, too,
has a differentiated interior, though it formed inside the Solar
System's "snow line", and so is devoid of water; its composition
is mainly of basaltic rock such as olivine. Pallas is unusual in
that, like Uranus, it rotates on its side, with one pole facing
the Sun and the other facing away. Its composition is similar to
that of Ceres: high in carbon and silicon. Hygiea is a
carbonaceous asteroid and, unlike the other largest asteroids,
lies relatively close to the ecliptic plane.
Source:
http://en.wikipedia.org/wiki/Asteroid_belt
Don't miss our article:
Asteroid Belt - it includes info about
Exploded Planet Thery and Bode's Law
Internal Links
External Links
Craters - the evidence of intensive "bombardment" of the planets
and moons of the solar system
Virtually all planetary surfaces are cratered from the
impact of interplanetary bodies. It is now clear from planetary
bodies that have retained portions of their earliest surfaces that
impact was a dominant geologic process throughout the early solar
system. For example, the oldest lunar surfaces
are literally saturated with impact craters, produced by an
intense bombardment which lasted from 4.6 to approximately 3.9
billion years ago, at least a 100 times higher than the present
impact flux.
http://www.daviddarling.info/encyclopedia/P/potentially_hazardous_asteroid.html
Impact craters are geologic structures formed when a large
meteoroid, asteroid or comet smashes into a planet or a satellite.
All the inner bodies in our solar system
have been heavily bombarded by meteoroids throughout their
history. The surfaces the Moon, Mars and Mercury, where other geologic processes stopped
millions of years ago record this bombardment clearly. Below are
few examples
Mercury
Below is a map of Mercury constructed by re-projecting and
overlaying three Mariner 10 mosaics created by Mark S. Robinson of
the Center for Planetary Sciences, Northwestern University on top
of a previous map combining Mariner data from the USGS with
Earth-based radar data.
Fragment of larger map of Mercury:
http://laps.noaa.gov/albers/sos/mercury/mercury/mercury_rgb_cyl_www.jpg
Venus
Map of Venus constructed by overlaying three left-ward looking Magellan mosaics created by
Emily Lakdawalla of The Planetary Society on top of a previous map from Calvin Hamilton
Fragment of larger map of Mercury:
http://laps.noaa.gov/albers/sos/venus/venus4/venus4_rgb_cyl_www.jpg
Mars
The image below shows the striking Maunder crater lying at
approximately 50° South and 2° East, in the Noachis Terra region
on Mars. With a diameter of 90 kilometers and a depth of barely
900 meters, the crater is not one of the largest impact craters on
Mars at present, but it used to be much deeper. It has since been
filled partially with large amounts of material.
Maunder crater, named after the British astronomer Edward W.
Maunder,
is located halfway between Argyre Planitia and Hellas
Planitia
on the southern highlands of Mars.
Credits: ESA/DLR/FU Berlin (G. Neukum)
Maunder Crater on Mars (photo credit: NASA)
Click for Hi-res image
Hale Crater on Mars
http://esamultimedia.esa.int/images/marsexpress/134-021104-0533-6-co-02-HaleCrater_hires.jpe
The Moon
Below is portion of the map of the Moon created by Jens
Meyer. He processed a map derived from Clementine spacecraft data
that is available on the USGS PDS web site:
http://pdsmaps.wr.usgs.gov/PDS/public/explorer/html/mmfront.htm
Moon surface.
Fragment of larger image (4096x8192):
http://laps.noaa.gov/albers/sos/moon/moon_8k_color_brim16.jpg
Related Links
Impact craters are geologic structures formed when a
large meteoroid, asteroid or comet smashes into a planet
or a satellite. Virtually all planetary surfaces are
cratered from the impact of interplanetary bodies. The
most intense bombardment lasted from 4.6 to approximately
3.9 billion years ago, and it was at least a 100 times
higher than the present impact flux.
The Earth, as part of the solar system, experienced
the same bombardment as the other planetary bodies.
http://www.bertsgeschiedenissite.nl/geschiedenis%20aarde/komeetinslag.html
On Earth, the atmosphere slows small meteorites so much
that they merely pit the ground. Only meteorites that are
house-sized or bigger blast out a crater many times larger
than the meteorite itself. From the size and structure of
a crater, we get some indication of the size and speed of
the body that formed it. A giant impact yields an enormous
crater with central peaks or multiple rings.
A smaller one leaves a simple, bowl-shaped crater.
Sometimes the impacting body (meteoroid or comet)
disintegrates or explodes before reaching the surface.
Explosion caused by such an airburst near the Earth's
surface will create slightly different crater (horseshoe).
Erosion and plate tectonics have destroyed most of Earth's
craters. Fortunately, many ancient impacts left behind distinctive
changes in the rocks lying deep beneath the crater. Like the scars
from old wounds, these imprints have remained long after the
surface structures were eroded away. These signs of collision, and
the few actual craters that still exist, give us our closest look
at the impact process and shed light on how collisions have shaped
our own and other worlds.
http://www.mnh.si.edu/earth/text/5_3_4_0.html
So far
only about 160 terrestrial impact craters have been recognized,
the majority in geologically stable aras of North America,
Europe and Australia where most exploration has taken place.
Spacecraft orbital imagery has helped to identify structures in
more remote locations for further investigation.
Meteor Crater (also know as Barringer Crater) in Arizona was
the first-recognized terrestrial impact crater. It was identified
in the 1920s by workers who discovered fragments of the meteorite
impactor within the crater itself. Several other relatively small
craters were also found to contain impactor fragments; for many
years, these remnants were the only accepted evidence for impact
origin. However, scientists have come to realize that pieces of
the impactor often do not survive the collision intact.
Barringer Crater in Arizona
 |
Crater: Meteor
Crater [ Canyon Diablo Crater, Barringer Crater ]
Location: Arizona, United States
Size: 1,200 m (4,000 ft) in diameter, some 170 m deep
(570 ft), rim rises 45 m (150 ft) above the plains
Age: 50,000 years old
Meteorites: 45-m (150-ft) wide nickel-iron meteorite
traveling more than 40,000 kph (25,000 mph). The
crater is the freshest, best-preserved impact
structure on Earth. |
In massive events caused by a large impactor, tremendous
pressures and temperatures are generated that can vaporize the
meteorite altogether or can completely melt and mix it with melted
target rocks. Over several thousand years, any detectable
meteoritic component might erode away. In some cases,
nonterrestrial relative abundance of siderophile elements can be
detected in the impact melt rocks within large craters - a
chemical signature of the meteorite impactor.
Impact craters are divided into two groups based on morphology:
simple craters and complex craters. Simple craters are relatively
small with depth-to-diameter ratios of about 1:5 to 1:7 and a
smooth bowl shape. In larger craters, however, gravity causes the
initially steep crater walls to collapse downward and inward,
forming a complex structure with a central peak or peak ring and a
shallower depth compared to diameter (1:10 to 1:20). The diameter
at which craters become complex depends on the surface gravity of
the planet: The greater the gravity, the smaller the diameter that
will produce a complex structure. On Earth, this transition
diameter is 2 to 4 kilometers (1.2 to 2.5 miles) depending on
target rock properties; on the Moon, at one-sixth Earth's gravity,
the transition diameter is 15 to 20 kilometers (9 to 12 miles).
The central peak or peak ring of the complex crater is formed
as the initial (transient) deep crater floor rebounds from the
compressional shock of impact. Slumping of the rim further
modifies and enlarges the final crater. Complex structures in
crystalline rock targets will also contain coherent sheets of
impact melt atop the shocked and fragmented rocks of the crater
floor. On the geologically inactive lunar surface, this complex
crater form will be preserved until subsequent impact events alter
it. On Earth, weathering and erosion of the target rocks quickly
alter the surface expression of the structure; despite the
crater's initial morphology, crater rims and ejecta blankets are
quickly eroded and concentric ring structures can be produced or
enhanced as weaker rocks of the crater floor are removed. More
resistant rocks of the melt sheet may be left as plateaus
overlooking the surrounding structure.
Large terrestrial impacts are of greater importance for the
geologic history of our planet than the number and size of
preserved structures might suggest. For example, recent studies of
the Cretaceous/Tertiary boundary, which marks the abrupt demise of
a large number of biological species including dinosaurs, revealed
unusual enrichments of siderophile elements and shock metamorphic
features that are markers of meteorite impact events. Most
researchers now believe that a large asteroid or comet hit the
Earth at the end of the Cretaceous Period 66 million years ago. An environmental crisis triggered by the gigantic collision
contributed to the extinctions. Based on apparent correspondences
between periodic variations in the marine extinction record and
the impact record, some scientists suggest that large meteorite
impacts might be the metronome that sets the cadence of biological
evolution on Earth - an unproven but intriguing hypotheses.
http://web.archive.org/web/20011129113848/http://www.star.le.ac.uk/edu/solar/tercrate.html#intro
Erosion and plate tectonics have destroyed most of
Earth's craters. Fortunately, many ancient impacts left
behind distinctive changes in the rocks lying deep beneath
the crater. Like the scars from old wounds, these imprints
have remained long after the surface structures were
eroded away. These signs of collision, and the few actual
craters that still exist, give us our closest look at the
impact process and shed light on how collisions have
shaped our own and other worlds.
http://www.mnh.si.edu/earth/text/5_3_4_0.html
There is extensive evidence that Earth has already been hit by asteroids
many times throughout history. To-date, over 160 impact craters have been identified
on Earth.
[ Earth Impact Database:
http://www.unb.ca/passc/ImpactDatabase/ ]
Below are few more examples of terrestrial craters.
 |
Crater: Wolf Creek
Location: Australia
Size/Shape: Simple crater nearly a kilometer
(.56 mi) across. Rim 25 m (80 ft) high.
Age: Less than 300,000 years old
Meteorites: Iron meteorite fragments still
found in vicinity. |
 |
Crater: Manicouagan
Location: Quebec, Canada
Size/Shape: 100-km-wide (60-mi), multi-ringed
structure. The ring-shaped lake encloses a central
uplift.
Age: 214 million years old
Meteorites: No meteorite fragments found. |
Kara-Kul Crater
38°57'N, 73°24'E;
rim diameter: 45 kilometers (28 miles);
age: <10 million years
The spectacular Kara-Kul structure is partly filled by the 25-kilometer (16-mile) diameter
Kara-Kul Lake. It is located at 6,000 meters (20,000 feet)
above sea level in the Pamir Mountain Range near the Afghan
border. Only recently have impact shock features been found in
local breccias and cataclastic rocks. (Courtesy NASA/LPI)
Kara-Kul crater. Image couresy of Google Earth
Horseshoe Crater in New Mexico
Horseshoe Crater is a Crater in the state of New Mexico
(county of Colfax), located at latitude - longitude
coordinates (also known as lat-long or GPS coordinates) of
N 36.68336 and
W -104.03303.
Horseshoe Crater is a crater in Colfax County, New
Mexico.
Latitude: 36 41 00N, Longitude: 104 01 57W
Image couresy of Google Earth
Wetumpka Impact Crater
Located only a dozen miles north of Montgomery, in the City of
Wetumpka, is an ancient meteor crater over twice as large as the
famous Barringer Crater near Flagstaff Arizona.
The asteroid impact at Wetumpka produced Alabama's greatest
natural disaster in the last 81.5 million years. Based on formulae
developed to study the effects of nuclear weapon detonations, the
area of total devastation (atmospheric shock wave with peak
overpressure exceeding 14 kPa) would be within a radius of 40 km.
Similarly, the maximum clearday infrared flashburn radius would
have been 47 km. These areas are indicated. Continuous ejecta
(rock particles from the impact) would have fallen over an area
within 7.5 km, and random rock falls would have covered a much
larger area. A Richter-magnitude 8.5 to 9 earthquake would have
occurred at crater center.
GEOLOGY AND TOPOGRAPHY OF WETUMPKA IMPACT CRATER
Impact Angle and Vector
An oblique impact on a compass bearing around 212
degrees and at an incident angle of 10 degrees or less in indicated by the shape
of the crater, a horseshoe. Horseshoe-shaped, or semicircular craters can be
reproduced experimentally at low incident angles.
Photos and caption provided by Dr. King
Wetumpka's impact probably occurred in 0 to 150 m of water and
0 to 50 km offshore from a barrier-island coastline. The impactor,
a stony and (or) iron asteroid, is estimated to have been about 350
m in diameter. When the asteroid vaporized approximately 10-2 sec
after surface impact, energy equivalent to between 102 and 103 MT
of TNT (or between 4.2 x 1024 and 4.2 x 1025 erg) was released and
thus opened a 3.3 to 4.2 km-diameter transient crater within
approximately 11.5 sec. On a vegetated plain, both atmospheric
peak over-pressure wave and infrared flash-burn combustion due to
impact would have devastated a region estimated to have comprised
2.15 to 9.95 x 105 hectares (or 830 to 3840 mi2). In shallow
seawater, this impact would have generated a tsunami-like wave
estimated to have been as much as 239 m high at a distance of five
asteroid radii (875 m) from the target center. Terrestrial
surface-wave magnitude of seismic shock-wave energy is estimated
to have been 8.4 to 9. Wetumpka's impact-crater melange, at least
25 m thick, was likely formed by mass movement from the transient
crater rim commencing after the first 18 to 21 sec. Wetumpka's
horseshoe-shaped crater rim and associated interior
northeast-trending aeromagnetic anomaly are evidence of an oblique
impact by a cosmic object arriving upon a low-angle,
northeast-to-southwest trajectory. The bearing of this trajectory
is normal to the one proposed for the Chicxulub impactor, thus any
connection between the Wetumpka and Chicxulub impacts, as has been
suggested recently, is doubtful. Including Wetumpka, there are
nine Late Cretaceous terrestrial impact craters known, ranging in
size from 6 to 170 km diameter, and all are situated in the
northern hemisphere. Two of these impact craters are temporally
correlated with global marine mass extinctions. Steen River impact
crater (25 km diameter; Canada) may be related to the terminal Cenomanian (91 Ma) mass extinction of 14 to 19 percent of all
marine genera. Chicxulub (170 km diameter; Mexico) is firmly
established as a significant causal factor in the well-documented
terminal Maastrichtian (65 Ma) mass extinction that included 39 to
47 percent of all marine genera and many terrestrial genera as
well.
Source:
http://www.auburnastro.org/wetu.htm
It has also been suggested that there are regular periods of
bombardment. By looking at the rocks from different depths in the
Earth's crust, scientists find layers of debris which indicate
massive planet-wide events. The depth at which these debris layers
are found is an indication of the date of the event, and some
scientists believe that these impacts occur at regular intervals.
They have suggested that this may be caused by a companion star,
called Nemesis, orbiting the Sun every 26 million years. This star
periodically disturbs the Oort cloud, and sends many comets
hurtling in to the centre of the solar system, so that impacts
between planets and large meteorites become far more common.
But perhaps the most intriguing possibility is the one put
forward by some scientists who look into the origins of life: some
believe that the seeds of the life found on Earth today could have
been carried here on some unknown meteorite, millions of years
ago. [
Source ]
What is the chance of an asteroid hitting Earth and how do
astronomers calculate it?
Note: On March 3, an asteroid about the size
of one that blasted Siberia a century ago just buzzed the
Earth. The asteroid named 2009 DD45 was about 48,800 miles
from Earth when it zipped past early Monday,
NASA's Jet Propulsion Laboratory reported. That is
just twice as high as the orbits of some
telecommunications satellites and about a fifth of the
distance to the Moon. "This was pretty darn close,"
astronomer Timothy Spahr of the
Harvard-Smithsonian Center for Astrophysics said
Wednesday. But not as close as the tiny meteoroid
2004 FU162, which came within 4,000 miles in 2004.
The space
rock measured between 69 feet and 154 feet in
diameter. The
Planetary Society said that made it about the same
size as the asteroid that exploded over Siberia in 1908
and leveled more than 800 square miles of forest.
Perry A. Gerakines, an assistant professor in the department of
physics at the University of Alabama at Birmingham, explains.
We have extensive evidence that Earth has already been hit by
asteroids many times throughout history - the most famous (or
infamous) example is probably the asteroid or comet that created
the Chicxulub crater in the Gulf of Mexico and may have
contributed to the extinction of the dinosaurs at the end of the
Cretaceous Period 65 million years ago.
A more recent but less
devastating example, called the
Tunguska Event, occurred in 1908,
when a meteor or comet exploded over the wilderness of Siberia,
damaging farmland and leveling trees for miles around. Because
most of the earth is covered by oceans, there may also be many
small impacts that go unnoticed.
There are thousands of small bodies that we call asteroids or
meteoroids in orbit around the sun. Many of these objects are
called near-Earth asteroids (or NEAs) because they have orbits
that repeatedly bring them close to, or intersect with, Earth's
orbit.
Although the odds of any one particular asteroid ever impacting
Earth are quite low, it is still likely that one day our planet
will be hit by another asteroid. At the current rate of impacts,
we would expect about one large asteroid to impact Earth every 100
million years or so. For that reason several programs, such as the
Lincoln Near-Earth Asteroid Research (LINEAR) project at the
Massachusetts Institute of Technology, have been undertaken around
the world to discover and monitor potentially Earth-threatening
asteroids.
When a new asteroid is discovered, astronomers analyze it to
determine whether its orbit around the sun could bring it close to
the Earth. They take successive images of the asteroid over the
course of days after its discovery in order to predict its
probable orbital path for the near future. The predicted orbit is
then compared to the orbit and position of Earth to check for any
times when they might pass close to each other.
Although scientists can calculate a most-likely orbit from
these early observations, each single observation of the
asteroid's position contains some uncertainty. Most asteroids are
small objects, a few meters to a few tens of meters across, and
even the resolving power of a large telescope cannot determine
their positions exactly. The uncertainties in an asteroid's
position lead to uncertainties in how well we can determine its
speed and direction of travel. As a result, a large number of
possible orbits for an asteroid can be predicted within these
windows of uncertainty.
Careful computer simulations are used to calculate the future
orbital path of the asteroid, with randomly chosen initial
positions and velocities that fall within the margin of error of
the telescopic observations to date. A large number of these
simulations are generated for each asteroid. The probability that
any particular one will actually hit Earth is given by the
fraction of the extrapolated paths that leads to an impact. For
example, if one million different possible orbits are calculated,
and one of those leads to an impact, then we say that the odds of
the asteroid hitting our world are one million to one.
The uncertainties in an asteroid's orbit are greatest in the
hours just after its discovery, and thus the calculated
probability of an impact also tends to be the highest at these
times. As we monitor an asteroid over the course of the weeks or
months that follow, its orbit becomes more and more certain, and
we become more knowledgeable about its position at a given date in
the future. We can then rule out many possible paths it may take.
In most cases, monitoring the asteroid over a few weeks quickly
leads to an impact probability of very nearly zero.
Answer originally published October 18, 2004.
Source
http://www.sciam.com/article.cfm?id=what-is-the-chance-of-an
Links and resources:
Golf of Mexico - A 'Smoking Gun' for Dinosaur Extinction
There is evidence that the Chicxulub Crater buried underneath the Yucatn
Peninsula is an ancient impact crater. According to
a University of Arizona Chicxulub crater web page: "... a large 65
million years old impact crater was located on Mexico's Yucatan Peninsula. It is called Chicxulub
- a Maya word that roughly translates as "tail of the devil." The crater,
now buried beneath a kilometer-thick sequence of sediments, ... appears to
have a diameter of 145 to 180 km, which makes it one of the largest
confirmed impact structures on Earth. Only Sudbury in Canada and the Vredefort structure in South Africa could potentially be larger ... [ http://www.valdostamuseum.org/hamsmith/EarthCollisions.html
]
Scientists believe the crater was formed by an asteroid or comet which
slammed into the Earth more than 65 million years ago. This impact crater
has been linked to a major biological catastrophe where more than 50
percent of the Earth's species, including the dinosaurs, became extinct.
The 180- to
300-kilometer-diameter (110- to 180-mile) crater is buried by 300 to 1,000
meters (1,000 to 3,000 feet) of limestone. The exact size of the crater is
currently being debated by scientists. [
http://neo.jpl.nasa.gov/images/yucatan.html ]
The Gulf of Mexico is the ninth largest body of water Body of water. The shape
of its
basin is roughly oval. It was probably formed approximately 300 million
years ago.
Image courtesy of Google Earth.
The researchers modelled the asteroid impact believed to have led to the
demise of the dinosaurs
- this graphic shows tsunami wave heights 4 hours after the impact of the
10-kilomtre-wide asteroid (Image: Steve Ward)
Surface geology, ring locations from gravity data, and wells near the
Chicxulub impact basin. The crater center is indicated by
×. Hatchured lines represent the Ticul fault system. Dashed lines indicate trend
of ringlike zone of water-filled sinkholes.
Source:
http://www.agu.org/sci_soc/sharpton.html
It is hard to imagine that one of the largest impact craters on
Earth, 180-kilometers (112-mile) wide and 900-meters (3,000-feet)
deep, could all but disappear from sight, but it did.
Chicxulub, located on Mexico's Yucatan peninsula, eluded
detection for decades because it was hidden (and at the same time
preserved) beneath a kilometer of younger rocks and sediments.
Size isn't the only thing that makes Chicxulub special. Most
scientists now agree it's the "smoking gun" -- evidence that a
huge asteroid or comet indeed crashed into Earth's surface 65
million years ago causing the extinction of more than 70 percent
of the living species on the planet, including the dinosaurs. This
idea was first proposed by the father and son team of Luis and
Walter Alvarez in 1980.
Though the buried giant can't be seen, the impact crater has
left subtle clues of its existence on the surface. The view from space lets scientists see some of Chicxulub's
surface features that are not nearly so obvious from the ground.
Chicxulib Crater Yucatan Peninsula, Mexico.
In more recent times, the impact crater has affected the circulation of
groundwater on the Yucatan Peninsula. This groundwater, has in some areas,
dissolved the limestone in the Yucatan peninsula. Below ground, this has
produced caves. At the surface, this has produced cenotes which are
groundwater springs. The cenotes form a ring, like a blue pearl
necklace, that is nearly coincident with the rim of the Chicxulub structure
and is the only visible feature on the surface to indicate a huge crater lurks
below. ...
Satellite images showing a necklace of sink holes, called cenotes,
across the Yucatan's northern tip are what first caught the
attention of NASA researchers Drs. Kevin Pope, Adriana Ocampo and
Charles Duller in 1990. They were among the first to propose
Chicxulub as the impact site linked to the mass extinctions that
occurred at the end of the Cretaceous and beginning of the
Tertiary geological ages, called the K/T boundary.
"We were ignorant of the existence of a crater," says Pope, now
an independent geologist, "We were working on a project on surface
water and Mayan archaeology when we saw this perfect semi-circular
structure in images from the Landsat Thematic Mapper. "We were
fascinated and got the magnetic and gravity data from the area
collected earlier by the Mexican petroleum company, who had been
looking for oil. Their data showed a large, remarkably circular
structure that they had identified as an impact crater." Pope and
his colleagues reasoned that the cenotes resulted from fractures
in the buried crater's rim and that the area within the cenote
ring corresponded with the crater's floor.
Further studies by other researchers of the magnetic and
gravity data plus analysis of rocks and ocean sediments published
in 1991 helped convince the scientific world that Chicxulub was
the site of the impact that sent life on Earth in a new direction,
from the age of dinosaurs to the age of mammals.
Now researchers are getting their first look at detailed,
three-dimensional topographical data from the Shuttle Radar
Topography Mission. "This new image gives us both corroboration of
what we expected and also shows up things we haven't seen before,"
says Kinsland. "We'll be working to get as much out of the data as
possible. Anything we learn at the surface tells us more about the
buried crater."
Much about Chicxulub remains mysterious. "We don't know exactly
how the impact caused the mass extinctions," says Pope. "We
believe it did, but we don't know what the "kill mechanism" was.
One theory is that the impact threw up so much dust into the
atmosphere than it obscured the Sun and stopped plants from
growing. Another is that the sulfur released by the impact lead to
global sulfuric acid clouds that also blocked the Sun and fell as
acid rain. Global wild fires triggered by the atmospheric reentry
of red-hot debris from the impact are another possibility.
Also unresolved is whether Chicxulub is the result of a
collision with a comet or an asteroid. "There are arguments on
both sides," says JPL's Dr. Don Yeomans, who manages NASA's
Near-Earth Object Program Office. "There are far more asteroids in
Earth's orbital vicinity than comets," says Yeomans, "but most are
much smaller than the 10-kilometer size (6.2-mile) of the one that
hit Chicxulub. When you get to that size range, then comets are
about as prevalent. Most people say it was an asteroid, but it is
still not altogether clear. Whatever it was, once you're hit with
something that large, it's horrendous."
http://www.jpl.nasa.gov/news/features.cfm?feature=8
... The asteroid or comet that produced the Chicxulub crater was roughly
10 km in diameter. When an object that size hits Earth's surface ...[ as
shown in these paintings from a
National Museum of Natural History web page ]... it causes a tremendous
shock wave while transferring energy and momentum to the ground.
Images by
Mary
Parrish
How the crater perhaps got its trench is shown below. The 10 km asteroid
begins its transit through the atmosphere and rapidly becomes a 90-100 km
fireball at 5000 degrees Celsius (Sandia National Laboratory supercomputing
simulation
http://www.sandia.gov/media/comethit.htm ). This fireball is what melts a
groove in the crust just before impact. This explains why the trench is
crossed by the rings and the rings are crossed by the two jets that form near
the end of the impact sequence simulation. I imagined that this incandescent
shockwave trenched the crust a split second before the main impact and
illustrated it with Bryce 3D software. The bolide is shown at the low angle
(20 to 30 degrees indicated by core transects of the Chicxulub structure
showing directionally shocked quartz grains (doubled laminar planes).
Image source:
http://nicholnl.wcp.muohio.edu/DingosBreakfastClub/Extinction/Chicxulub2.html
The impact
was similar to a large explosion, although the energy of the Chicxulub impact
dwarfs anything modern civilization has experienced. The energy of the
impact was comparable to 100 million megatons of TNT, 6 million times more
energetic than the 1980 Mount St. Helens volcanic eruption. The impact ejected
rock from several kilometers beneath the surface of the Earth and carved out a
bowl-shaped crater nearly 100 km in diameter. In addition, the shock of the
impact produced magnitude-10 earthquakes, which were greater than the
magnitude of any we have ever measured in modern times. ...[ According to
a Miami University of Ohio web page: "... In contrast to the 2 to 3 cm
thick clay layer found worldwide, the K-T boundary in the Gulf of Mexico
region and in Haiti is composed of much thicker very coarse clastic deposits.
Sand beds indicative of high energy deposition at the K-T boundary at Brazos
River, Texas, have been interpreted to be the result of a major disturbance of
the depositional environment, such as a tsunami approximately 50 to 100
meters high. ...". ]... The initial bowl-shaped crater was very unstable,
and its walls quickly collapsed along a series of faults that enlarged the
final diameter to between 145 and 180 km. At the same time, the rock that had
been compressed beneath the crater by the impact rebounded, producing a
peak-ring structure in the crater's center. These dramatic changes, which
rapidly transported huge volumes of rock over distances of tens of kilometers,
occurred within only a few minutes. ...
... Because the impact site was in a shallow sea, water rushed in to fill the circular depression. Kilometer high waterfalls
tumbled over the rim of the crater and roared furiously across the floor of
the crater. Because seawater filled and covered the crater, sediment on the bottom of
the sea soon buried the impact scar. The crater is no longer visible today,
even when standing directly over it.
According to
a Miami University of Ohio web page: "... the image of the Chicxulub
impact crater ...[shows]... a groove leading into the crater which shows the
direction from which the asteroid or comet (rocky core type) hit. The image covers the Yucatán Peninsula.
Note the ejection plume pointing north and west out of the crater at the
southwestern United States. ...
The image above shows the LPI gravitational anomaly image combined with
another map to locate it on the Yucatan Peninsula. The various markings on it
indicate testing sites done by a group of scholars. The superimposed black
lines are from a map matched to the exact coastline of Yucatán.
Image source:
http://nicholnl.wcp.muohio.edu/DingosBreakfastClub/Extinction/Chicxulub.html
http://antwrp.gsfc.nasa.gov/apod/ap971012.html
The crater has some extraordinary features. A trench leads into
it, crossed by concentric rings which are in turn crossed by an
ejection plume. This gives an order of events, among other things,
and hints at a much larger catastrophe than has been supposed. The
ejection plume is a big clue to the dynamics that were generated
by the 10+ km asteroid...
Gravity Gradient.
http://www.unb.ca/passc/ImpactDatabase/images/chicxulub.htm
... The explosion that produced the Chicxulub crater excavated a huge
amount of material, which was then ejected upwards. Most of the debris was
deposited as a blanket of material that covered North America and possible
South America. Near the impact crater the debris is tens to hundreds of
meters thick, while as far away as Colorado (over 2000 km distance), the
debris is still a centimeter thick ... Additional material was lofted in
an expanding, vapor-rich plume that included gas from the vaporized asteroid
or comet. This plume rose far above the Earth's atmosphere, enveloping it,
and eventually depositing a thin layer of debris around the entire world.
...".
Multiple impact theory
In recent years, several other craters of around the same age as Chicxulub
have been discovered, all between latitudes 20°N and 70°N. Examples include the
Silverpit crater
Additional interesting links about the Chicxulub Crater:
Speculations
In 1912 Alfred Wegener published a theory to explain why the
Earth looked like a huge jigsaw. He believed the continents were
once joined forming a supercontinent he called Pangaea. Over 180
million years ago this supercontinent began to "break up" due to
continental drift. During the 20th Century, scientists developed the
theory of Plate Tectonics. The theory suggested that the crust of
the Earth is split up into seven large plates (see map below) and a
few smaller ones, all of which are able to slowly move around on the
Earth's surface. They float on the semi-molten mantle rocks, and are
moved around by convection currents within the very hot rocks. The
are two types of tectonic plates - continental plates and oceanic
plates.
A Map of the world's tectonic plates.
Gulf of Mexico is part of larger tectonic plate and it is not very likely
it was formed by continental drift. Perhaps the entire Golf of Mexico is
result of cosmic impact?
The theory of continental drift does not conclusively explain
the shape of the Gulf of Mexico.
The Following is an exert from the Smithsonian Magazine: “In
1978 a young geophysicist named Glen Penfield, who was working
with PEMEX, found himself assigned to fly over the Gulf of Mexico. Using a magnetometer, he was to measure the magnetic field of
rocks on the Gulf floor--specifically off the coast near Chicxulub
Pueblo. Like the findings of earlier PEMEX geologists, Penfield's
were intended to map out the rock composition beneath the surface
and determine the likelihood of finding oil. But what Penfield's
magnetometer let him see was very odd. More than a mile below the
surface of the Yucat n Peninsula, and for 70 miles out into the
Gulf of Mexico, was a saucer-shaped underground structure with a
magnetic field different from that of any known volcanic terrain.
It also had a most un-volcano-like symmetry. Put together, the old
land data and the new underwater data indicated the existence of a
huge ring, about 120 miles in diameter, half on land, half under
the Gulf of Mexico. It was ten times the size of any volcano, with
an upward bulge at its center similar to those seen on
known--though much smaller--impact craters. “
The article continues on to say that in 1990 it was in fact
proven that this crater was created by the impact of an asteroid.
So no the impact theory you are referring to is not an urban
legend it is in fact real and true. Most of the studies done on
the impact revolve around the extinction of the dinosaurs, as
there is some correlation to the time of both events. The crater
that was made by the impact of the asteroid(s) does have a
distinct magnetic field to it. It was first discovered by an oil
drilling company whom kept the findings as proprietary
information.
[
http://answers.google.com/answers/threadview/id/103238.html ]
The "horseshoe" shape of the Golf of Mexico is similar to Wetumpka Impact Crater
in Alabama, Horseshoe Crater in New Mexico, Kara-Kul crater
in Russia, Tunguska, and even
Maunder Crater on Mars. [ Craters on Earth ]
The horseshoe shape is most likely the result of the impacting body
(meteoroid or comet) hitting the ground surface at low angle and/or
disintegrating/exploding above the ground.
Sandia National Labs ran several versions of the one kilometer
asteroid hit (see story below) entering the atmosphere at a 45
degree angle, steeper than the Chicxulub asteroid. The image at
right compares four frames in series at roughly comparable times.
Note the rapid expansion of one kilometer to about 13 km or about
3 miles of shockwave and fireball. The jet of steam and water from
an ocean hit is 12 miles high. The ocean is three miles deep in
the simulation. Acoustic waves shock disintegrate the asteroid;
the initial tsunami wave is almost half a mile high and propagates
at about 380 miles per hour. At a ratio of approximately1:10, a 10
km asteroid would have a 100 km shockwave, which agrees with the
90+ km wide trench found at the impact site .
Click to enlarge. Image source:
http://nicholnl.wcp.muohio.edu/DingosBreakfastClub/Extinction/Chicxulub2.html
Study of the Atlantic Ocean off Florida, reveals a gravitational anomaly
arc which centers on the impact crater, as do several arcs in the Gulf of
México. In fact, features such as the Alacrán Reef and Florida appear to be
parts of the extended structure of the complex crater.
http://nicholnl.wcp.muohio.edu/DingosBreakfastClub/Extinction/Chicxulub.html
Golf of Mexico - the evidence of big asteroid impact?
Close up of the sea bottom in the northern part of Golf
of Mexico.
The rocks appear to be melted (and cooled by water) as if exposed
to bombardment by super hot debris of exploded asteroid.
Note: pillow lava looks quite different.
Atlantis in Golf of Mexico area?
Gene D. Matlock seems to have discovered many
language and culture similarities between many
pre-Columbian civilizations and people from Afghanistan to
far east Asia. Also giving arguments that support the idea
of extensive commercial relations between pre-Columbian
America with the rest of the world, long before Christ,
has Matlock tries to find rather possible or not. He might
be quite convinced that ancient cities from Mexican gulf
to Tenochtitlan are related to some Atlántepec, Tolan, A-tolan
and other TLAN like cities:
Quote:_"Should we continue our fun guessing games about
Atlantis for another few millenniums? Or should we confidently begin
our search for the submerged half of Atlantis from Atlántepec's
(Mexico's) southeast coast? Will the ruins that we'll surely find be
those of the real Atlantis?"
He believes we should be conducting the search for
Atlantis in the Yucatan region of Mexico. Matlock cites place names
as one of the compelling proofs:
Atlán,
Autlán, Mazatlán, Cihuatlán, Cacatlán, Tecaltitlán, Tihuatlán,
Atitlán, Zapotlán, Minititlán, Ocotlán, Miahuatlán, Tecaltitlán,
Tepatitlán, Tihuatlán, Texiutlán, and the like. Notice that the
Nahuatl Tlán root of these place names is exactly like the Tlan in
"Atlantis."
Read more:
http://www.world-mysteries.com/mpl_10.htm#Mexico
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