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
Every once in a while a dirty snowball—in the form of a comet—swoops through the night sky from the outer reaches of our solar system. Though a comet is beautiful to behold, the sudden appearance of these celestial interlopers has a long history of terrifying our ancestors and, more recently, of worrying the modern public about doomsday-impact threats. To the astronomer, however, comets are a puzzle that must be solved. They contain crystalline dust grains that could only have formed at very high temperatures, yet they also contain ices that simply could not have survived the heat needed to make the crystals. How did the hot and cold parts come together to form the flying amalgam we call a comet? It turns out that the answer to the puzzle may require astronomers to revise their models of how our solar system formed.
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