Meteorites

Introduction

When we look into the night sky, the impression we first get may be of an unchanging scene. Certainly, the Moon and planets change position against the backdrop of stars from night to night, and over a period of hours the satellites of Jupiter can be seen to shift around their orbits - but everything appears to happen slowly. Apart, that is, from the brief streak of light from a meteor. Capable of appearing in any part of the sky without warning, these objects can catch the observer completely off-guard. Many people have never seen one, but in fact meteors are visible every cloudless night - if only one has the patience to watch out for them. But these are more than just a means to a free firework display; for two centuries, meteors (or more correctly, the objects which caused them) provided science with its only source of material from beyond the Earth. Meteors are caused by pieces of debris floating through the solar system, entering the Earth's atmosphere at very high speed and exciting the atoms in the air. The piece of material itself is usually destroyed by the immense heat caused by friction with the air. Although the vast majority of such objects are destroyed long before they can reach the surface of the Earth, some of the larger pieces do survive the journey, and fall to the ground. These pieces of rock from beyond the reaches of our planet are known as meteorites.

Meteoroids, Meteors and Meteorites

The nature of meteors remained a mystery for a considerable time, and there was much debate about whether "shooting stars" were phenomena of the Earth, or space. However, the fact that meteorites really were visitors from beyond our atmosphere was proved by Ernest Chladni in 1794, and Jean-Baptiste Biot in 1803 when a meteorite was observed to fall near a French village.

There are three distinct stages to the phenomenon, and in the vast majority of cases, an object will only survive to the second. These stages are:

Meteoroid. When a piece of debris , later to enter the Earth's atmosphere, is travelling through space towards our planet, the object is called a meteoroid.

Meteor. The brief flash of light we see in the night sky and caused not by the material "burning" with friction from the atmosphere, but rather from the atoms which have been excited in the air from the object's high speed.

Meteorite. The name given to the relatively small number of objects which are not completely destroyed in the upper atmosphere, but survive to the ground.

Meteors and Meteor showers

The pieces of material which cause meteors enter the upper atmosphere of Earth at very high velocity - typically around 260,000 km/h. The flash of light which we see occurs when the object is approximately 100 km above the surface of the Earth. Perhaps the most surprising feature of these objects is their size. To create a flash which is visible to the naked eye, the particle must be at least the size of a grain of sand. A particle the size of a grape would be a spectacular object, and may even cast shadows on the ground. Such bright meteors are given the name of fireballs. The image here (left, courtesy of the Dutch Meteor Society) shows such a fireball.

Due to the Earth's motion, the best time to see meteors is after midnight, since before this time, only those objects which are travelling faster than the Earth can catch up and fall through the atmosphere. After midnight, the night time sky faces the direction of the Earth's motion, and in effect "scoops up" more meteors.

Meteor showers and their origins

At certain times of the year, many more meteors than usual can be seen at night. The reason for this is linked to the origins of the vast majority of meteors: comets. As a comet orbits near the sun, it loses material, the majority of which is ejected in the tail. The dust and rock particles ejected from the comet are spread behind it in a trail which follows the orbit, as shown on the diagram (left, courtesy of Cambridge University press). At certain times of the year, the Earth crosses these dirt-laden paths, and the particles of material which they contain are swept up by our planet's atmosphere to appear in the sky as a shower of meteors. The dates of these showers are accurately known, and astronomers can prepare for them in advance. Often, the comet responsible for particular showers is known: for instance, both the Eta Aquarid shower (which is at its height around May 4th) and the Orionid shower (October 20th) are both caused by the trail of debris left by Halley's comet. The name of each shower comes from the position of its radiant. Radiants are an effect of perspective - the effect which makes railway tracks seem to meet far in the distance. If the meteors from a particular shower are plotted on a map, and a line drawn through the tail and continuing on behind the meteor, all the lines appear to converge at a particular point on the sky. This point is called the radiant, and is shown in the figure below.

 

The shower is named after the constellation in which the radiant lies - such as Gemini for the Geminid shower (December 13th). In the case of the Eta Aquarids, the radiant is found to lie close to the star Eta Aquarii.

The movie presented here (courtesy of NASA) shows a fireball of the 2003 Perseid meteor shower.

Meteorites

The majority of meteors seen in our skies, including probably all those in the showers, are from these cometary trails, and most - even the very bright ones, are caused by particles very small in size (from dust grains to pieces a few centimetres across). These particles are, generally, too small to survive the journey to the surface of the Earth, since they are burned away too rapidly. However, much larger chunks of material do land, and it is believed that these meteorites are from a different source. Rather than a by-product of the passage of a comet, meteorites probably originate from the asteroids. Asteroids may collide with each other, and with other pieces of space debris. This may break pieces off the asteroid, and in some very severe cases, the asteroid may shatter completely. The result is a cloud of "rubble" - large pieces of rock and metals from the original asteroid which may wander into the path of the Earth and fall through the atmosphere. Far more rare than a "normal" comet related sighting, the resulting meteor is remarkably bright - again, capable of casting strong shadows. Although a great deal of the object will be burned away on its passage, some may still survive, and the resulting meteorite provides scientists with a mine of information about the composition of asteroids, and also that of the early solar system when these objects formed. Although travelling at great speed, recordings of injuries from meteorite falls are rare.

Such an object fell to earth on August 14th, 1992 in Mbale, Uganda. The meteorite was made of stone and broke up on its fall, scattering debris over an area of around 3 x 7 km. At least 48 separate impacts were found, and it is thought that the original object must have had a mass of around 1000 kg, and so far, 150 kg of fragments have been found. The image here (left, courtesy of the Dutch Meteor Society) shows the largest fragment recovered.

Although most meteorites almost certainly came from the asteroids, there are some exceptions. Some are thought to have come from comets, and others have a composition which is very close to the rocks which are found on the Moon, which suggests that some of the impacts from objects which caused the craters on the Moon's surface, threw pieces of the lunar surface off, and some of these pieces ended up falling into the Earth's atmosphere. At least seven other meteorites in collections are of a type which match closely the materials found on the planet Mars, also probably as a result of impacts on the surface of that planet. A martian meteorite is shown in the image on the left (courtesy of LANL).

The composition of meteorites

Due to their asteroidal origins, meteorites provide us with information about the composition of their distant parents. The objects which fall to Earth are very diverse in their composition and appearance, reflecting the different materials in the original bodies.

Meteorites are generally divided into three classes according to their composition:

Irons: This is the most commonly encountered meteorite. The majority of the material (90%) is iron, with a smaller amount of nickel mixed in. The meteorites which fell at Barringer, Arizona, and Wolf Creek, Australia, were both irons. You can click here to see an image of a slice through an iron meteorite.

Stony meteorites are the most common type to fall; however, because they are very similar in composition to native Earth-rocks, they are difficult to spot, and so this class is not the most common type to find. Some stony meteorites contain small glassy spheres called chondrules, and objects with these spheres are known as chondrites, a sub-class of the stony meteorites. (Stony meteorites without the spheres are called achondrites). This image shows a fragment of the meteorite which fell in Peekskil, New York, USA. This is the same meteorite which was captured on video and was presented above. (Photo courtesy of AstroMall).

Stony Irons are the final major class of meteorite, and contain small pieces of stone embedded in a body of iron. The image here shows a slice of a meteorite of this class. The meteorite fell at Esquel Chabut, Argentina, and is pictured courtesy of AstroMall.

Impacts with the Earth

Today we are fortunate that the vast majority of debris from space which enters the Earth's atmosphere is small - so small that it rarely survives the fall to the ground. A very small fraction of the pieces do make it all the way, and are recovered as meteorites, usually causing no more damage than a hole in a house roof (as in the image on the right, courtesy of the Dutch Meteor Society), or a small pit in the ground.

However, things were not always this way; throughout the history of the Earth our planet has been hit by very much larger objects which have had a profound effect on its evolution, and in many cases have left visible scars on the surface. For example, some scientists believe that the disappearance of the Dinosaurs about 65 million years ago was caused by the impact of a large asteroid in an area now covered by the Indian Ocean - although this is by no means the only theory for their demise. Theories for the creation of the moon also include one in which the early Earth was hit by a huge body - possibly planet-like in size, which caused a large piece to fracture off our planet and form the satellite which we see today (again, this is only one of many theories put forward to explain the existence of the Moon).

Although photographs of the Earth show a surface remarkably free from the craters which scar the surface of our satellite, this appearance is deceptive. Whilst the Earth's atmosphere does shield it from the smaller bodies, it is inadequate to prevent large pieces of material from hitting the surface; the reason for an apparent lack of craters on our planet is erosion. Whilst the moon is a relatively quiet and inactive body, the Earth has always been far more restless. Craters from long ago can be "wiped off the face of the Earth" by the action of the oceans, weather, volcanoes, earthquakes and other large scale events. However, there are several impact sites still identifiable. These are shown on the chart below.

 

World crater sites - courtesy of the Canadian Geological Survey

Why are the major meteorite falls of today so small in comparison with these apocalyptic events? Early on in the history of the solar system, the pieces of debris which caused major impacts were far more common. The surface of the moon, pock-marked with huge craters, is evidence to the fact that the planets went through a period of very intense bombardment millions of years ago. Through time, the space around the planets has been "swept clear" of the majority of these objects, and so large events are more rare. However, the scars from some of these events are still visible, as shown in the images below, also from the Canadian Geological Survey.

 

Wolfe Creek, Western Australia.

Less than 0.3 million years old, this crater has a diameter of 0.9 km. You can also see an image of a fragment from the Wolf Creek meteorite.

Manicouagan, Quebec, Canada.

Around 214 million years old, this crater (pictured here from the Space Shuttle) has a diameter of about 100 km.

Barringer, Arizona, USA.

An artists impression of the impact of a large meteorite on the Earth. (Courtesy of Meteor Crater Enterprises)