solar system

Meteorites

Most of us probably have seen meteors or shooting stars. A meteor is the flash of light that we see in the night sky



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Meteorites

What is the difference between a meteor, a meteoroid, a meteorite, an asteroid and a comet?

Most of us probably have seen meteors or shooting stars. A meteor is the flash of light that we see in the night sky when a small chunk of interplanetary debris burns up as it passes through our atmosphere. "Meteor" refers to the flash of light caused by the debris, not the debris itself.

The debris is called a meteoroid. A meteoroid is a piece of interplanetary matter that is smaller than a kilometer and frequently only millimeters in size. Most meteoroids that enter the Earth's atmosphere are so small that they vaporize completely and never reach the planet's surface.

If any part of a meteoroid survives the fall through the atmosphere and lands on Earth, it is called a meteorite. Although the vast majority of meteorites are very small, their size can range from about a fraction of a gram (the size of a pebble) to 100 kilograms or more (the size of a huge, life-destroying boulder).

Asteroids are generally larger chunks of rock that come from the asteroid belt located between the orbits of Mars and Jupiter.

Comets are asteroid-like objects covered with ice, methane, ammonia, and other compounds that develop a fuzzy, cloud-like shell called a coma and sometimes a visible tail whenever they orbit close to the Sun.

Meteorite Meteorites are the main source of mass exchange between planets and carry with them characteristic clues about the nature and history of the planets or planetesimals where they originated, the impacts that dislodged them, and the time they spent in space.

Kyoungwon Kyle Min, postdoctoral fellow in geology, reported an innovation for determining the timing and temperatures of ancient impacts that liberate meteorites from extraterrestrial bodies such as Mars.

To measure both the age and thermal history of the piece of Martian rock, Min assayed the natural radioactive decay of uranium and thorium to the gas helium in these meteorites, and combined it with knowledge of how temperature affects helium loss over time. This (U-Th)/He dating method, used on single grains of minerals in the "Los Angeles" Martian meteorite gave a far more accurate picture than the conventional method of analyzing chunks of meteorite. The "helium age" of about three million years corresponds with the estimated cosmogenic space exposure age.

bollide1

bollide

Meteorites are essentially rocks from space that fall to Earth. The very small ones burn up in the atmosphere, leaving light trails we call meteors, or “shooting stars”, but the larger ones that make it to the Earth’s surface are called “meteorites”. Meteoriticists are particularly interested in what the meteorites are made of, which types they represent, and where they fit into the meteorite classification. They have developed an incredibly complex classification system for meteorites that is extremely difficult to remember, so most of us fall back on the simple division into meteorites made of iron, those made of stone and those made of mixtures of the two (stony irons). Geologically, this translates into the inner and outer parts of rocky planets such as the Earth, Venus or Mars.

Meteorites

Meteorite

A meteorite is the remnant of an asteroid or comet which survives passage through the Earth's atmosphere to land on the surface. Meteorites are classified based on their composition as iron or stony. Stony meteorites are further classified as chondrites, achondrites, or carbonaceous chondrites based on their carbon content and texture.

meteorite

While stony meteorites are more common than irons, irons are discovered with disproportionate frequency due to their obviously anomalous composition compared to common Earth rocks. As meteorites fall to Earth, they can appear as a shooting star or a brilliant display called a fireball. As it falls, the meteorite is called a meteor. Both shooting stars and fireballs are caused by melting and ionization of the outer layers of the meteorite, leaving a blackened layer called a fusion crust.

asteroid belt

The asteroid belt spans a narrow area between the orbits of Mars and Jupiter. It is comprised of billions of individual dust grains, rocks, boulders and planetoids up to 800 km long. It is believed to be the remants of a destroyed or "failed" planet

Meteorite Impact Sights On Mercury

Mercury's Caloris Basin

caloris

The circular Caloris Basin, the largest feature found on the surface of Mercury, is believed to be a huge impact crater. Numerous craters have since been formed within the basin. In this image taken by Mariner 10, only half of the basin is visible.

Impact craters dominate the surfaces of Mercury. Both bodies lack liquid water on their surfaces that would erode impact craters over time. They also lack an atmosphere which, on planets like the Earth and Venus, could disintegrate meteoroids before they impact the surface. However, old craters can be eroded by new impact events. Mercury and the Moon have very old surfaces. One of the youngest large craters on the Moon is Tycho, which was formed about 109 million years ago.

Meteorite Impact Sights On Venus

Crater Riley

This Magellan full resolution radar mosaic centered at 14 degrees north latitude, 72 degrees east longitude, shows an oblique view of the impact crater Riley, named for Margaretta Riley, a 19th Century botanist. This view was prepared from two left-looking Magellan radar images acquired with different incidence angles. Because the relief displacements of the two images are different, depths from the crater rim to the crater floor and heights of the crater rim and flanks above the surrounding plains can be measured. The crater is 25 kilometers (15.5 miles) in diameter. The floor of the crater is 580 meters (1,914 feet) below the plains surrounding the crater. The crater's rim rises 620 meters (2,046 feet) above the plains and 1,200 meters (3,960 feet) above the crater floor. The crater's central peak is 536 meters (1,769 feet) high. The crater's diameter is 40 times the depth resulting in a relatively shallow appearance. The topography is exaggerated by 22 times to emphasize the crater's features.

Crater Isabella

Crater Isabella

Crater Isabella, with a diameter of 175 kilometers (108 miles), seen in this Magellan radar image, is the second largest impact crater on Venus. The feature is named in honor of the 15th Century queen of Spain, Isabella of Castile. Located at 30 degrees south latitude, 204 degrees east longitude, the crater has two extensive flow-like structures extending to the south and to the southeast. The end of the southern flow partially surrounds a pre-existing 40 kilometer (25 mile) circular volcanic shield.

The southeastern flow shows a complex pattern of channels and flow lobes, and is overlain at its southeastern tip by deposits from a later 20 kilometer (12 mile) diameter impact crater, Cohn (for Carola Cohn, Australian artist, 1892-1964). The extensive flows, unique to Venusian impact craters, are a continuing subject of study for a number of planetary scientists. It is thought that the flows may consist of 'impact melt,' rock melted by the intense heat released in the impact explosion. An alternate hypothesis invokes 'debris flows,' which may consist of clouds of hot gases and both melted and solid rock fragments that race across the landscape during the impact event. That type of emplacement process is similar to that which occurs in violent eruptions on Earth, such as the 1991 Mount Pinatubo eruption in the Philippines.

Meteorite Impact Sights On Earth

Barringer's Meteor crater, Arizona, USA

arizona_crater.gif

The Barringer's Meteor Crater which is near Winslow, Arizona, is one the worlds best known craters. It is estimated to have been created at least 50,000 years ago. The crater is named after 'Daniel Monroe Barringer' because he was the first to theorize that the depression was created by a meteor. He based his conclusion on the observation of the many minerals surrounding the impact.

Nearly a mile wide, and 570 feet deep, it was originally thought to have been caused by an explosion of super heated steam resulting from volcanic activity that might have occurred far below the surface. But this was wrong.

The absence of any volcanic rocks in the area, coupled with the discovery of 'meteoritic iron' in the crater rim, and finely pulverised silica, convinced Barringer it was an impact crater.

Manicougan crater, Quebec, Canada

Manicougan crater

The Manicougan crater in Quebec, Canada, is believed to have been caused by an impact around 200 to 300 million years ago. At roughly 60 miles across, the Manicougan crater ranks as joint 5th largest crater in the world, along with the Popigai crater in Russia, which is about the same size.

It has been thought previously that the impact which caused this crater was responsible for the Triassic-Jurassic extinction event, but scientists have pointed out that, at approx 214 million years old, this impact occurred well before what has become known as 'the TJ event'.

Kara-Kul crater, Tajikistan, Central Asia

Kara Kul crater

Partially filled with the waters of the Kara-Kul Lake, this impact crater is estimated to be less than 10 million years old. It is located in the 'Pamir Mountain Range' of Tajikistan, close to the border with Afghanistan in central Asia. The crater is 28 miles in diameter, and the lake is 16 miles across.

NASA's Johnson Space Center in Houston, Texas, has made available a collection of photographs of the Karakul crater taken by astronauts. You can access these at the website of the Astronaut Photography of Earth - Display Record.

At around 20,000 feet above sea-level it is one of the highest impact craters in the world, and only recently has impact-shock features been found in local 'breccias' and 'cataclastic rocks'. As the country opens up to co-operation with Western scientists, more data will become available.

Clearwater Lakes crater, Canada

clearwater crater

The Clearwater Lakes craters are the only examples on Earth of a pair of impact craters that were formed simultaneously by two separate meteorite impacts. This shows that in the past our planet has suffered from at least one major 'multiple bombardment' of cometary debris from space.

It has been estimated that these twin craters were formed some 290 million years ago and research scientist at the Geological Survey of Canada began studying craters 25 years ago on the Apollo program. Many more impact craters all over Canada have come to light since that time.

Clearwater Lake East (left) has a raised central area covered with impact melts, and these form the circle of islands. The lake to the right is also fomred by an impact crater, but this one has a central peak that is submerged.

Bosumtwi crater, Ghana, West Africa

Bosumtwi crater

The Bosumtwi crater in Ghana, West Africa, shown here partly obscured by clouds, has a rim diameter of 6.5 miles. It is estimated to be around 1.3 million years old, and is filled almost entirely by Lake Bosumtwi.
Beginning January 2000, researchers from the University of Syracuse started to explore the 'crater lake' to gather data that would help them assess the changes that occur after a major impact by a meteorite or asteroid.

Led by Professor Christopher Scholz, the expedition had one major objective, as explained by Prof. Scholtz:
"Our data should provide information about what happens when an impact hits hard, pre-Cambrian, crystalline rocks that are a billion years old"

Mistastin Lake crater, Newfoundland/Labrador, Canada

Mistastin Lake crater

The view of this crater lake to the left was taken by astronauts aboard the Space Shuttle. The crater has been heavily eroded over time, especially by the eastward-moving glaciers, which have reduced the rim exposing the crater floor. The breccias have almost disappeared as a result.

At the centre of the crater lake is 'Horseshoe Island', which is formed by the central uplift of the crater floor. This winter view shows the crater-lake, and the island at its centre, starkly contrasted against the snow.

Scientists estimate the crater to be around 38 million years old, and the rim diameter is a little under 17.5 miles. Just beyond the margins of the lake are vestiges of the impact melt sheet that contains evidence of meteoritic features in quartz, feldspar and diaplectic glasses.

Wolfe Creek crater, Australia

wolf creek

Discovered during an aerial survey in 1947, Wolfe Creek meteorite crater is just over half a mile wide - rim to rim. The crater is now thought to have been formed by a meteorite or asteroid impact around 300,000 years ago.
It has been known by Aboriginal peoples as 'Kandimalal' for countless centuries, and Aboriginal Dreamtime lore tells of:

" ... two rainbow snakes who formed the nearby Sturt and Wolfe Creeks as they crossed the desert. The crater is believed to be the place where one snake emerged from the ground."

Now a protected reserve, Wolfe Creek was named in 1889 after Robert Wolfe, chairman of the Kimberley Goldfields Roads Board, a prospector and storekeeper of Halls Creek.

Impact Features

Craters are the most widespread landforms in the solar system. Craters are found on all of the terrestrial planets - Mercury, Venus, Earth and Mars. The surfaces of asteroids and the rocky, ice covered moons of the outer gas planets are cratered as well. The craters left by impacting objects can reveal information about the age of a planet's surface and the nature and composition of the planet's surface at the time the crater was formed.

Tycho Crater, Moon

Tycho Crater

Impact craters dominate the surfaces of the Earth's Moon. Both bodies lack liquid water on their surfaces that would erode impact craters over time. They also lack an atmosphere which, on planets like the Earth and Venus, could disintegrate meteoroids before they impact the surface. However, old craters can be eroded by new impact events. Mercury and the Moon have very old surfaces. One of the youngest large craters on the Moon is Tycho, which was formed about 109 million years ago.

Meteorite Impact Sights On Mars

Argyre basins

The largest craters or impact basins on Mars may be buried beneath the northern smooth plains. In the southern hemisphere, the bombardment history typical of all the inner planets is recorded by a few large basins, such as Argyre. The ring of mountains surrounding the basin probably rises 5 kilometers (3 miles) above the basin floor. The Mars Global Surveyor mapping mission has provided accurate elevation data that have answered many questions about Mars.

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