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No. 91: Jan-Feb 1994

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Two-faced planets and moons

Several solar-system objects present asymmetrical visages to our telescopes. Mars is a classic case, being much more heavily cratered in its southern hemisphere than its northern. But the dichotomies are not restricted to cratering, as we shall now see.

Neptune . Recently, H.B. Hammel, using the University of Hawaii's 2.2-meter telescope, discovered that Neptune's northern hemisphere is now brighter than its southern -- something never observed before. During the past eight years, the southern hemisphere has been consistently brighter, although the hemispheres were of roughly equal brightness during the late 1970s. The cause of these brightness changes remains a mystery.

(Cowen, Ron; "Neptune's Northern Half Grows Brighter," Science News, 144:287, 1993.)

Iapetus . This satellite of Saturn is dark on one half and light on the other. Quantitatively speaking, the bright side reflects ten times more incident light than the other. An explanation is suggested by the fact that the dark side points in the satellite's direction of motion. A recent study of 12 Voyager images of Iapetus also imply an exogenous (externally imposed) origin of the dark surface, because they show a gradual rather than sharp transition between the dark and light regions. The thought of planetary scientists is that micrometeoroids bombard the leading hemisphere of Iapetus preferentially and in the process volatilize considerable surface material. The residual deposit:

"...may be an example of the dark, reddish, possibly organic-rich material which is found on other satellites in the outer solar system and on the D-type asteroids.

(Buratti, Bonnie J., and Mosher, Joel A.; "The Dark Side of Iapetus: New Evidence for an Exogenous Origin," Eos, 74:193, 1993.)

Comment. Here is still another hint that astronomical rather than terrestrial processes may perform that basic chemistry essential for the origin and prosperity of life. Apparently, such prebiotic infrastructure is widespread in the solar system and, most likely, the entire universe.

From Science Frontiers #91, JAN-FEB 1994. 1994-2000 William R. Corliss

Science Frontiers Sourcebook Project Reviewed in:


  • "A sourcebook of unexplained phenomena is therefore a valuable addition to a collection of scientific literature. William R. Corliss has provided this in the past with his source books of scientific anomalies in several subjects, and now he has provided it for astronomy. He has done an excellent job of collecting and editing a large amount of material, taken in part from scientific journals and in part from scientific reporting in the popular or semi-scientific press." -- "The Mysterious Universe: A Handbook of Astronomical Anomalies", reviwed by Thomas Gold, Cornell University, in Icarus, vol.41, 1980

  • "An interesting, systematic presentation of unusual weather [..] This book is recommended for a general audience" --"Corliss, William R., Tornados, Dark Days, Anomalous Precipitation, and Related Weather Phenomena, Sourcebook Project, 1983.", revieweed in Choice, September 1983
  • "..the science is necessarily somewhat speculative, but Corliss's symthesis is based on reputable sources." -- "Corliss, William R. (Compiler). Lightning, Auroras, Nocturnal Lights, and Related Luminous Phenomena" reviwed by Joseph M. Moran, Univ. of Wisconsin in Science Books and Films, Sep/Oct 1983

  • "Before opening the book, I set certain standards that a volume which treads into dangerous grounds grounds like this must meet. The author scrupulously met, or even exceeded those standards. Each phenomenon is exhaustively documented, with references to scientific journals [..] and extensive quotations" -- "Book Review: The moon and planets: a catalog of astronomical anomalies", The Sourcebook Project, 1985., Corliss, W. R., Journal of the Royal Astronomical Society of Canada, Vol. 81, no. 1 (1987), p. 24., 02/1987