Earth - A Moving Target(Evidences for Meteoroid Hit) In past

 

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.

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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

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Terrestrial Impact Craters

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 ]

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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

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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

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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.