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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

Joseph A. Nuth III

Keywords:
comet, solar nebula, cometesimal, olivine, Comet Halley

Abstract:
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.

Joseph A. Nuth III has been head of the Astrochemistry Branch at NASA’s Goddard Space Flight Center since 1990. He often retreats to the laboratory to conduct experiments designed to further our understanding of the behavior of solids in astrophysical environments. He obtained B.S. degrees in astronomy and chemistry, an M.S. in geochemistry and a Ph.D. in chemistry from the University of Maryland at College Park. Following several years as an NAS/NRC Resident Research Associate at NASA Goddard and as an NAS/NRC Research Management Associate at NASA Headquarters, he joined the civil service at Goddard in 1986 as an astrophysicist. Address: Laboratory for Extraterrestrial Physics, Code 691, NASA Goddard Space Flight Center, Greenbelt, MD 20771. Internet: u1jan@lepvax.gsfc.nasa.gov