Mystery moon.
Pluto, Charon, and the sun line up in this artist's impression.

Charon's Shadow Illuminates Its True Nature

As NASA's New Horizons mission to Pluto readies for launch this month, new results published today are shining the spotlight on the ninth planet's largest moon Charon. By measuring the moon's eclipse of a distant star, two groups have obtained the most precise measurements yet of Charon's size and composition. The data may provide clues to how the moon formed in the early solar system.

Pluto is the only traditional planet in our solar system that has not been seen up close by spacecraft. What little astronomers know about its largest moon Charon makes it a bit of a lunar curiosity: The moon hovers very close to the planet and has roughly an eighth of Pluto's mass, making it the largest satellite with respect to its planet so far discovered (ScienceNOW, 1 November 2005).

A rare event that occurred on 11 July 2005 is finally helping astronomers understand Charon's curious characteristics. On this night, Charon passed in front of--or occulted--a relatively faint star, allowing several large telescopes in South America to witness the resulting stellar shadow. By comparing when the eclipse happened at different locations, two separate groups--one led by the Massachusetts Institute of Technology and another by the Paris Observatory--measured the radius of the moon to be about 605 kilometers, as reported in the 5 January issue of Nature.

Both teams also saw the light from the star turn sharply off and on--rather than fade more softly--as Charon passed in front of it. This suggests that the moon has little or no atmosphere. The lack of atmosphere is not surprising, says planetary scientist Roger Yelle of the University of Arizona, Tucson, because the moon's gravity is too weak to keep gases from escaping into space.

Combined with previous estimates of its mass, both groups found the density of Charon to be about 1.7 times the density of water, which implies that the moon is approximately 60% rock and 40% ice. Because Pluto is only slightly more rocky (70%), the moon and planet were likely two proto-planets that long ago collided with each other in such a way that they wound up orbiting each other, says planet modeller Robin Canup of the Southwest Research Institute in Boulder, Colorado. The other alternative, where the collision was more violent and the moon congealed out of the smashed-up bits, would probably have resulted in a less dense, icier Charon, she says.

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