When scientists announced last year that they had discovered the most-remote object known in the solar system, they reported that the icy body probably had a moon. That’s because the object, named Sedna, seemed to spin unusually slowly, just once every 20 days. Only the tug of a small companion body could slow the rotation to such a leisurely rate, the astronomers reasoned.

FRINGE SCIENCE. Moonfree Sedna, an artist’s depiction. J. Lomberg

The Hubble Space Telescope, however, failed to find a moon (SN: 4/24/04, p. 262: Puzzle on the Edge: The moon that isn’t there). Now, new measurements show that Sedna rotates some 50 times as fast as earlier observations indicated.

Scott Gaudi of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and his colleagues studied Sedna’s spin using the MMT Telescope on Mount Hopkins in Arizona. With that telescope’s sensitivity, the researchers could discern tiny periodic variations in Sedna’s brightness, providing a better estimate of the body’s rotation than was possible before. The researchers describe their findings online (http://arxiv.org/abs/astro-ph/0503673).

In a separate study, scientists using the Gemini North Telescope atop Hawaii’s Mauna Kea found no evidence of ice or frozen methane on Sedna. That’s unlike conditions on two other remote solar system bodies, Pluto and its moon Charon.

Astronomers propose that Sedna initially had an icy surface but that bombardment by cosmic rays and the sun’s ultraviolet light produced a dark, hydrocarbon veneer. Because Pluto and Charon lie closer to the sun than Sedna does, they’re subject to more collisions with solar system debris. Those collisions either prevent the formation of a dark coating or deliver fresh supplies of bright ice on top of the coating.

Chad Trujillo of the California Institute of Technology in Pasadena and his colleagues report their findings in an upcoming Astrophysical Journal.

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