Meteorites are a fascinating topic of study. However, there is a great amount of science fiction misinformation swirling around. Every other week or so for the next several months I will be writing articles on as many meteorite related topics as I can think of for our Meteorites 101 course for the beginning enthusiast. So let’s get right into it.
First thing to discuss is a few terms. I still see and hear these misused in print and movies so it must still need to be talked about. Meteorites are the space rocks that actually make it to the ground to be found. Meteors are the shooting stars that can be seen in the night sky where it is dark away from our bright urban areas. Very bright fireballs even the ones from which meteorites may land and be recovered are still meteors. The word meteor is used correctly to describe only the visual event in the sky. The chunk of space rock is called a meteoroid while it is still in space. There is a fuzzy line for where a meteoroid becomes big enough to be called an asteroid. Individuals have tried to fix exact sizes to these bodies, but the rocks themselves make this a difficult problem. In the last few years cosmic bodies quite small have entered our atmosphere and done things once expected only of larger bodies. In general however, asteroids are larger objects and meteoroids are smaller bodies. Micrometeoroids are even tinier, and are hitting the Earth all the time. In fact thousands of tons of weight is added to the Earth every year from the countless micrometeorites that filter down through the atmosphere after hitting us from space.
Pieces of the Carancas meteorite were found in and around a crater after it fell arrived September 15, 2007. The rock was pulverized into only small fragments. It was a very small cosmic body. Normal thinking would have dictated this object be slowed by the atmosphere and break up into hundreds or thousands of small individual pieces that would land as meteorites. This is not what happened at all. This small body entered the atmosphere and maintained its high velocity and hit the ground intact exploding out a crater. Carancas is a chondrite stone type meteorite and the question of how it survived intact all the way to the ground with much of its cosmic speed has not been well answered yet.
On February 15th 2013 the world saw and heard on television the tremendous explosion of the Chelyabinsk object. It was immediately called an asteroid explosion. But, it was not a huge object, in astronomical terms. So there is a blur of terms. If it causes a lot of damage when it explodes the media uses asteroid. If it detonates high in the atmosphere and causes no damage beyond the rain of stones and some holes in roofs then it may be called a meteoroid or large meteoroid but maybe not and asteroid.
Where do these rocks from space spend their time before landing on Earth. Here is where another common bit of science fiction has led to misunderstanding. Asteroids are found all over the solar system. There is of course the large Asteroid Belt which is between the orbits of Mars and Jupiter. There are millions of asteroid in the Asteroid Belt but, these are not the direct source of meteorites. Other groups of asteroids travel in zones and paths closer to Earth. Appropriately called Near Earth Asteroids, most meteorites come from these bodies. The members of these NEO groups are probably displaced members of the Asteroid Belt. They were kicked out of the Asteroid Belt long ago. Collisions between asteroids, gravitational encounters with Jupiter, and other causes have sent these asteroids into obits that now bring them near the Earth.
Scientists believe that we’re sampling the Asteroid Belt when examining meteorites since they originated there. Reasoned guesses have been made for the place of origin for some meteorite types. For example several achondrite type meteorites the Diogenites being one are thought to originate with Vesta. Vesta is one of the very large asteroids with a mean diameter of 525 kilometers. Of course Vesta never comes near Earth so the conclusion is that fragments from great impacts into Vesta have melted and shattered off pieces. These fragments now smaller asteroids and meteoroids wander the space near Earth until they are finally on a collision course. Tatahouine is a Diogenite believed to be from Vesta. Tatahouine broke up into thousands of minute fragments when it entered the atmosphere over Tunisia on June 27, 1931.
Most meteorites are fragments of asteroids but not all. In fact one of the other sources is just around the corner in astronomical terms. The Moon has provided pieces of itself as meteorites. As a note of clarification the Apollo astronauts that landed on the Moon returned a large amount of moon rocks. Those rocks were not meteorites with the exception of two microscopic bits found in soil samples. The Lunar meteorites that are studies are pieces of the Moon that have been blasted free of the Moon’s gravity by tremendous impacts that threw massive amount of rock into space. Mush of the rock fell back but some pieces achieved escape velocity in impacts and left the Moon to orbit the Sun near Earth. Because we have the lunar sample brought back by the Apollo astronauts and Russian robot sample return missions scientists are able to recognize the signature of the Moon in meteorite rocks also.
The Moon is bombarded by meteoroids all the time. It has no atmosphere to protect it as the Earth does so every impact no matter how large or small the body is a hypervelocity impact. And with no weather every crater remains unchanged for a very long time. Eventually the surface is pulverized by further impacts and the resulting powder does cover up small craters. As the accompanying picture shows the Moon is covered with millions of craters from meteoroids and asteroids.
There is one other source for meteorites. Mars like the Moon has been struck by huge asteroids. Fragments have also been launched into space from Mars. It is far more difficult for fragments to reach Earth after being ejected from Mars. And from what we know it has taken them much longer to get here. But, they have made it to Earth. Unlike the Moon we have not returned any samples from Mars to compare with the Martian meteorites. Yet most rocks are porous enough that they have gases trapped inside them. This is the case with Martian meteorites. They are mostly igneous rocks of types similar to the common basalts of Earth’s volcanos. However, the gases trapped inside them matches the composition of the Martian atmosphere. We know this from probes we have sent to our outer planetary neighbor. It is more than just the simple gases it is the ratios of isotopes of many gases, some of them quite exotic gases. These ratios vary by the placement of where they are created in the Solar System. Gas ratios out at Mars are very different from those closer to the Sun at the Earth.
One location for possible meteorites has been highly sought after but no meteorite has been found yet. That place is the Earth. We have also been hit by some tremendous impactors in the past. Craters over a hundred miles in diameter still remain faintly recorded on our world. It is likely that chunks of Earth rock were blasted free of here and are out there wandering in space. The problem is to recognize them as meteorites. Since they are Earth rocks they would blend rather well into the background. Even though they would be highly shocked and probably very melted they would be hard to identify unless seen to fall. So far no witnessed meteorite fall has been of rocks that seem to have originated here.
It is also possible that meteorites could be found which came from Mercury and maybe even Venus, but none have been declared to so far. Angrite type meteorites have the closest guess to what scientist think meteorites from Mercury would be. Nothing has been proposed for any meteorites from Venus.
Meteorites bring to Earth a record of their experiences in space. Some have suffered a lot of abuse while others arrive almost pristine. This rock record will be discussed in great detail in future installments of these articles.