Leonids Meteor Shower: Origin of Meteorites

Origin of Meteorites

The mineral composition of meteorites gives us a clue as to their origin. Most have survived since the earliest days of the Solar System. Somehow, they seem to have survived unchanged for billions of years.

What may sound confusing is that meteors and meteorites seem to have different origins. Usually, on any clear night, in a dark sky, you can see about ten meteors per hour. But now and then, that hourly number increases to fifty or more per hour and we have a meteor shower.

Meteor showers are known to occur when the Earth passes near the orbit of a comet and so are then associated with that comet. We know that as a comet gets closer to the Sun, it heats up and sheds some dust and debris. Any of the dust or debris that enters the Earth's atmosphere, burns up and we see this as a meteor. The bits of dust and debris are so small (mostly like grains of sand) that they are vapourized completely and never fall as meteorites.

The meteorites recovered from the ground appear to have a different origin than the meteors that burn up in the atmosphere. They are structurally stronger than comet dust. And because we have several types of meteorites, they appear to have come from different bodies in the SS. These bodies have melted enough to differentiate and have an iron core surrounded by silicates.

If a fairly large object, say one hundred km in diameter, the outer layers of rock would have insulated the molten iron core. The core would lose heat very slowly with the result that large crystals would grow. A sample of such material, once sliced, polished and etched, would show Widmanstatten lines.

Collisions would break up such an object and any fragments from the core area would look exactly like iron meteorites. Fragments from the object's mantle, which would have melted enough to melt chondrules would also be consistent with some stony meteorites. Fragments from the crust would not have completely melted would contain chondrules and would look very much like the meteorites we call chondrites.

It is likely that carbonaceous chondrites have their origins in smaller and cooler objects. These objects formed further from the Sun, far enough so that the object did not get hot enough to blow off volatiles. The objects had to be small enough that they did not get hot enough to differentiate.

These theories on the origins of meteorites point not to comets, but to larger bodies which the meteorites were once part of. They could not be fragments of the planets themselves, or they would have been swept up by the planets as they moved along in their orbits. They had to have come from bodies which had a different orbit other than the planets. That led astronomers to look towards asteroids.

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Origin of Meteorites

Constellations

The Solar System