No. 44: Mar-Apr 1986
All of a sudden it seem that astronomers are finding dark -- even "charcoal-black" -- materials in unlikely places in the solar system. Three "sludgy" sites have been high-lighted in the recent literature.
(1) Comets. "Black" comets certainly defy our expectations. Are all those pictures of white, flaming apparitions wrong? Not really. The comets approaching the sun are made visible by sunlight reflected from the gases and dust in the coma and tail, plus some direct emission. However, the heart of the comet, its nucleus, has long been considered a "dirty snowball"; that is, a mixture of dirt and ices. Now it appears that comets are more like "icy dirtballs"! And some of that dirt is pitch black. New measurments of the bare nuclei of comets, using a visual-infrared technique, find that the nuclei reflect as little as 2% of the incident sunlight. They are indeed charcoal black. Comet nuclei, according to W. Hartmann and his colleagues, are colored by a brownishblack primordial organic sludge, and have the appearance of "a very dark Hershey bar." The use of the adjective "organic" may be premature, but in light of the next item, maybe not.
(2) Carbonaceous chondrites. This well-known class of meteorites sometimes appears tarry and is characterized by carbon contents of up to 2% and more. The Japanese have just reported the analysis of Yamoto 791198, a carbonaceous chondrite picked up in the Antarctic. This meteorite is loaded with amino acids, 20 kinds of them. These extraterrestrial amino acids are not considered biogenic because they are half left-handed and half right-handed; whereas all amino acids synthesized by terrestrial animals, plants, etc. are left-handed. [This is obviously presumptious because we have no notion how extraterrestrial life works, assuming it exists at all.]
(3) The rings of Uranus. Voyager photos reveal that these rings are composed of unknown dark material quite unlike that in the high-albedo (bright) rings of Saturn.
References. (Kerr, Richard A.; "A Comet's Heart May Be Big But Black," Science, 229:372, 1985. Also: Emsley, John; "Amino Acids from Outer Space," New Scientist, p. 30, December 19/26, 1985. Also: Anonymous; "Fine Particles Viewed in Uranus' Rings Leave Scientists 'Happily Bewildered'." Baltimore Sun, January 28, 1986.)
Comment. How and where is star sludge manufactured? Again we have to venture that the venerable "primordial soup" in which life mysteriously assembled itself is located in outer space rather than in warm, sunlit earthly ponds. It also seems strange that all these extraterrestrial amino acids just seem to "fall together," with little urging, in seemingly hostile environments. More on this in the item that follows.
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