Meteorites
While the objection may be made that this page does not contain my own photographs of celestial objects (but wait until you get to the end!), I thought I would include a section on the solar system's smallest objects just to complete the story.
What are they?
Most people who take an interest in the night sky have probably seen the sudden, rapidly-moving, streaks of light known as meteors - not to be confused with the much slower-moving Iridium Flares which I have documented on another page! Meteors are small particles of rock (some no bigger than a grain of sand) burning up in the Earth's atmosphere, producing a brief flash of light as they are destroyed. Larger rocks can make it through the atmosphere without being entirely consumed and then land on the Earth's surface: these are known as meteorites. Their size can range from small pebbles to the massive objects which created the Barringer Crater in Arizona or caused the extinction of the dinosaurs in the so-called K-T Event. Fortunately, those at the higher end of the range are exceedingly rare!
Where do they come from?
The material that impacts the atmosphere to cause meteors has two main sources - cometary debris and the solar system more generally. Comets throw off large amounts of dust and ice as they warm up when near to the Sun, and this dust (which is what causes the comet's visible tail) tends to accumulate along the comet's orbital path. If the Earth should intercept this dust trail some of the particles will impact the atmosphere, causing meteors. Because the dust trail is relatively thin, the Earth will pass through it quite quickly and so all the meteor strikes will happen over a short period of time - this is the reason for the regular "meteor showers" such as the Perseids and the Geminids. It is even possible to predict how intense a particular shower will be, as we know the orbits of the dust trails and thus which of them the Earth will pass through on any particular occasion. However, cometary debris tends to be made up of small particles only, and so will not result in meteorites. These are produced by the second source of material - the rest of the solar system.
Why is there meteorite material in the solar system?
The solar system bodies were formed by the gradual accretion of material from a rotating disc of dust, gas and ice attracted to the Sun in the very early stages of its life. This process initially produced rocky bodies immersed in a "soup" of ices and gases, but most of the lighter components were then blown outwards when the Sun suddenly became much brighter as it passed through one of its developmental phases. The net result was a number of small rocky bodies near to the Sun and larger mainly gaseous bodies further out, where the lower temperatures meant they could hold on to the gases removed from the inner solar system.
Not all the material present in the initial rotating disc finally gathered into larger bodies though, either because of the disruptive effect of gravitational interaction with nearby bodies (particularly the giant planet Jupiter) or because proto-planets which formed close to each other collided and destroyed each other again - this is how the Moon is thought to have been formed, for example. The result of all these processes was that a fair amount of debris ended up orbiting between the major planets. Much of this was then swept up by the planets or deflected into stable orbits, giving the situation we have today - a set of planets orbiting in basically "empty space" plus well-defined bands of very small rocky bodies, of which the best known is the asteroid belt, together with a large amount of dust and very small grains distributed throughout the system. This dust can in fact still be seen - the effect known as the Zodiacal Light (a faint band of light stretching up from the horizon after dusk or before dawn) is caused by sunlight scattering off these particles.
Although the orbits of the asteroids and other similar small bodies are basically stable, many of them have high eccentricities (i.e. they are far from circular) and so asteroidal bodies can come quite close to the major planets on occasion. Gravitational effects can then cause their orbits to change considerably, which can set very small bodies (called "meteoroids") on a collision course with the planets. This is the process that produces almost all the meteorites that land on the Earth, as cometary debris is not usually large enough to reach the surface. The so-called "sporadic" meteors, i.e. those not associated with any particular meteor shower, come mainly from the dust and debris but may also be sent Earth-wards by gravitational influences.
Are there any other sources of meteorites?
The final way meteorites can be produced is similar to the above, but with material coming from a very different source. I said earlier that much of the inter-planetary debris was swept up by the planets at an intermediate stage of the formation of the solar system. In this case, "sweep up" of course means "crash into and absorb". Because the planets are so much larger than the "bits left over" this process does not disturb them greatly, but fragments of collision debris can be thrown off the planet and themselves start to wander the solar system. If these fragments then approach the Earth they can become meteorites. Such meteorites can only come from small rocky bodies, as larger bodies would have too great a gravitational field to allow them to escape and gaseous bodies would not create collision debris in the same way. This really reduces the field to Mercury, Venus, the Moon and Mars (together with mutual collisions between the asteroids, of course) but because Venus is thought to have acquired its dense atmosphere quite early on and also has a considerable gravitational field (only a little less than the Earth) it would be very difficult for collision debris to leave its surface with sufficient speed to escape its influence. The same considerations also apply to the Earth of course, and this means that any fragments blasted off its surface by asteroid bombardments are quite likely to arrive back on the Earth before too long.
Is it really possible to get meteorites from the planets?
In fact, a relatively small number of meteorites from both the Moon and Mars have indeed been found on Earth, with some suggestion that an even smaller number might possibly be from Mercury. There's even the possibility that some fragments are "returns" from the Earth itself. How do we know this? Mainly from physical, chemical and structural analyses. Their reflectance spectra (i.e. how they reflect different wavelengths of light) match that of their presumed parent body very well; the relative abundance of the isotopes of particular elements also match the parent body; the composition of the gases trapped in tiny bubbbles in some meteorites is a good match to their atmospheric abundance as measured by spacecraft, and their structure is indicative of rocks which have been both melted and differentiated (i.e. separated into different crystal types and phases): these last processes can only occur within large bodies. In addition, meteorites have also been found which almost certainly came from the large asteroid Vesta. The finding by the Dawn spacecraft of a gigantic impact crater at the south pole of Vesta gives a tantalisingly possible confirmation of this theory.
Further reading
The above is just a brief description of concepts common to most meteorites, but of course there is much more to be said about specific types. There would be little point in me repeating information here though, so I've added the following links so the interested reader can learn more about the "planetary" types already mentioned:-
 Mercury
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Evidence that meteorites could have reached the Earth from Mercury |
Moon
Mars
| | Information about lunar and martian meteorites, including their means of production; the reasons they are believed to come from the Moon and Mars; their geological classification; their chemical composition, and what they can tell us about the bodies from which they come. The martian meteorite page is particularly extensive and includes some excellent photographs of whole meteorites and thin sections |
Vesta
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Description of the three different types - Howardite, Eucrite and Diogenite |
To see some pictures of actual meteorites, and get further information about each of them, click here
