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Something about Comets

1996; John Benjamins Publishing Company; Linguagem: Inglês

10.1093/oso/9780195101058.003.0007

1385-7908

Gerrit L. Verschuur,

Astro and Planetary Science

Discovery of the iridium layer in the K/T boundary clay was the first clue that pointed to a cosmic impact as the trigger of the mass extinction that wiped out the dinosaurs and their associates. But what hit the earth? Was it an asteroid or a comet? To answer, we need to know something about their differences. Unfortunately, the distinction is very blurred. Comets are thought to be huge icy objects, probably with cores made of a mix of water ice and silicates (sandy material), pristine examples of the type of material out of which the solar system was formed. Some of them are hundreds of kilometers in size and they may have been built in the envelopes of gas and dust that surround cool, supergiant stars at the end of their lives. Part of the doubt about distinctions comes from trying to decide what a comet would be like after the ice evaporates. Would it then be like an asteroid? Around the end of the nineteenth century the British astronomer Sir Richard Gregory pictured comets as made up of a cloud of meteorites. He thought that when such an object was first pulled into the solar system from interstellar space it began to glow because of internal heat created as particles began to jostle one another. As the object drew closer to the sun a tail was supposed to be formed as the particles between the meteorites bumped into one another and began to escape. He did consider the potential risk to earth if it were to run into the head of a comet made up of lots of meteorites. The picture he painted was based on what an earlier astronomer, Sir Simon Newcomb, had written about this possibility. Newcomb admitted that, although there were more likely ways to die than as a result of comet collision, such a fate was real. Should such a collision, occur, Gregory conjured up a picture of what might happen. On the one hand, if the comet head was made up of dust, the earth’s inhabitants would experience nothing more than a stunning display of shooting stars. But if the comet head was made of cannonball sized objects the consequences would be dire.