Pluto Is so tiny and distant that telescopes on Earth cannot distinguish its disk; thus no surface features may be discerned on it. But based on light variations as Pluto spins on its axis in 6.4 days and as it revolves around the Sun, Marc W. Buie created a computer model of the surface of Pluto that shows bright polar ice caps, a darker equatorial belt, and two equatorial spots: one bright and the other very large and dark. Here Buie shows the rotation of Pluto bringing the light and dark spots into view. Pluto lies on its side as it revolves around the Sun. Its south pole is to the left. The hemisphere of Pluto with no spots is the one that always faces toward Charon. The present series of eclipses between Pluto and Charon should determine whether Buie's model is correct. Courtesy of Marc W. Buie. UH/SDSC
Using his computer model of Pluto. Marc W. Buie illustrates the March 19, 1987, transit of Charon across the face of Pluto. Pluto's south pole is to the left, notice how large Charon is compared to Pluto and that Charon is darker In color. The amount by which the transit of Charon and its shadow reduce the light from Pluto tells astronomers how light or dark the eclipsed surface of Pluto is and allows a rough map to be drawn. The transit event shown here required approximately four hours.
Courtesy of Marc W. Buie. UH/SDSC
the spots is quite dark and has a width of 500 miles (800 kilometers), more than a third the diameter of Pluto. The bright polar caps and equatorial bright spot are interpreted as methane ice. The less reflec-
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