How Were the Comets Made?
Explaining the composition of these 4.5 billion-year-old relics may require scientists to revise their models of the primitive solar nebula
In traditional Hindu mythology, the god Shiva is both creator and destroyer, bringing life and death to the world. It is an odd pairing of talents for a deity, and yet there may be a real-world counterpart in the heavens, an avatar if you will, in the form of comets. Some recent studies suggest that a rain of comets in the very early history of our planet, perhaps 4 billion years ago, may have seeded the young Earth with complex organic molecules from space—key ingredients necessary for life to arise. On the other hand, giant comet impacts may be responsible for some of the major extinction events in the history of life on Earth, including the demise of the dinosaurs 65 million years ago. Comets, it seems, may have been both creators and destroyers in our own history.
Yet the comets have a history of their own, and the more we find out about them the more enigmatic they seem. Although it has been said that "a comet is as close to being nothing as something can be"—in reference to the diffuse tail a comet emits near the sun—that bit of "something" holds important clues for the planetary scientist. As best as we can tell, comets are the most primitive bodies in the solar system. Some of the material inside a comet is preserved in nearly the same state it was in when the solar system was just taking shape, before the Sun and the planets were fully formed. Each comet is effectively a "grab bag" sample of the building blocks present in the nascent solar system—the solar nebula—at the time the comet was formed, about 4.5 billion years ago. Although the nebular material may have undergone considerable processing before it was incorporated into the comet, very little has been altered since. A comet is literally a little piece of the past.
Although we can't snatch a comet from the sky and examine it in the laboratory, there are ways to get the next best thing. We can measure the spectral properties of a comet as it swings by the Earth, and we can examine some of the particulate remains of comets in the form of interplanetary dust particles collected by special, high-flying aircraft in the stratosphere. On the basis of these studies we can then make analogues in the laboratory, and so understand something about how a comet must be made. In this article I will review what such studies have told us about the comets, and what the comets tell us about the processes that must have taken place in the early history of our solar system.