A brief,
non-technical synopsis of their paper was published in Astronomy,
Sky and Telescope, and CCD News. The synopsis from CCD
News follows:
A brief,
non-technical synopsis of their paper was published in Astronomy,
Sky and Telescope, and CCD News. The synopsis from CCD
News follows:
Ronald Tabak and WarrenYoung from Youngstown State University believe they have solved a mystery that has baffled astronomers for more than 300 years.
Shortly after Giovanni Cassini discovered Saturn's moon Iapetus in 1671, he noted that its brightness changes dramatically as it orbits the planet. Iapetus, like most satellites, rotates on its axis in the same period of time as it revolves. This means one hemisphere leads and the other trails in its orbit. The leading face is ten times darker than the trailing one. The leading side is as dark as coal and the trailing as white as snow.
Arthur C. Clark used this feature in his science fiction novel 2001. Iapetus was the location of the star gate that took the astronaut Dave Bowman to the land of the aliens. The star gate was surrounded by a region coated with a brilliant white material to mark its location.
Most attempted explanations of the nature of this two-faced planet have been more prosaic. Recent attempts to explain this dichotomy have concentrated on Saturn's next outer moon, Phoebe. In this scenario micrometeorites striking Phoebe eject material from its dark surface which are later swept up by Iapetus. However, the orbital planes of the two satellites are quire different so most of this debris would miss Iapetus.
Also, the material that hit the satellite would be distributed in a very different pattern than that shown in the Voyager photographs. Voyager 2 images show the dark material extends for over 220 degrees in longitude and only 110 degrees in latitude.
Tabak and Young proposed that a comet struck Iapetus in the past 100 million years. The resulting violent impact sent a cloud of debris into orbit about Saturn. Both cometary nuclei and the surfaces of icy satellites like Iapetus contain a mixture of bright and dark material. The bright material is a mixture of ice and would have been vaporized by the explosion and then dispersed. As Iapetus orbits Saturn, the dark material would have collected on its leading hemisphere.
If the impact did not occur on the apex of Iapetus, it would have caused the satellite to oscillate with a period of 5 years. This oscillation would have spread some of the infalling dark material on to the trailing side as observed in the Voyager photographs.