- 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.
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- 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.
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- 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.
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- 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.
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- Hydrocarbon sludge
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- 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.
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- 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.
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- "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.
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- Less crowded environment
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- 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.
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- 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.
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- 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.
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- The study by Trujillo and colleagues will be published
in an upcoming issue of the Astrophysical Journal.
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- http://www.newscientist.com/article.ns?id=dn7272
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