The distant planetoid Sedna appears to be covered in a tar-like sludge that gives it a distinctly red hue, a new study reveals. The findings suggests the dark crust was baked-on by the Sun and has been untouched by other objects for millions of years.

Sedna appears to be nearly the size of Pluto and was discovered in November 2003. It is the most distant object ever seen within the solar system and travels on an elongated path that stretches from 74 to 900 times the distance between the Sun and the Earth.

Astronomers have struggled to explain such an extreme orbit, but many believe a star passing by the Sun about 4 billion years ago yanked the planetoid off its original, circular course.

Now, observations by the same team that discovered Sedna suggest the object has since led an uneventful life. Infrared spectra taken with the Gemini Observatory in Hawaii show the surface of the planetoid contains little methane ice, found in significant amounts on Pluto, and little water ice, seen on Pluto’s moon, Charon.

Hydrocarbon sludge

Chad Trujillo, the team's lead researcher at the Gemini Observatory, says collisions with other objects may have helped expose the icy interiors of Pluto and Charon and believes a lack of collisions might explain Sedna's ice-free surface.

He says Sedna, which is probably made up of an equal mixture of ice and rock, may be covered with a metre or so of hydrocarbon sludge. This sludge is produced when the Sun's ultraviolet radiation and charged particles alter the chemical bonds between atoms in the ice.

"You just get this big tangle of carbon and hydrogen bonds, which turns the surface dark like asphalt or tar," he told New Scientist. A similar "space weathering" process occurs on a 200-kilometre-wide object called Pholus, which lies near Saturn and is also very red.

Less crowded environment

Scott Gaudi, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, US, says the new work supports previous theories showing Sedna evolved in a more distant, less crowded environment than Pluto and Charon’s. "Maybe it was lifted to its higher orbit early on and lived out there for a long time," he says.

But Gaudi recently discovered that, in at least one way, Sedna appears more conventional than previously thought. When Sedna was discovered, astronomers used a 1.3-metre telescope to observe the planetoid's period of rotation, concluding it rotated once every 20 days - an abnormally slow rate which they attributed to the gravitational tugs of a moon. But in March 2004, the mystery deepened when the Hubble Space Telescope failed to detect any moon.

Now, Gaudi and colleagues have taken more than 140 images of Sedna with a 6.5-metre telescope and found that actually Sedna rotates once every 10 hours. "Most things in the solar system rotate with periods of 10 hours or less, so this is what you’d expect," says Gaudi.

The study by Trujillo and colleagues will be published in an upcoming issue of the Astrophysical Journal.

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