No. 91: Jan-Feb 1994
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.