http://www.sciencenews.org/articles/20060415/fob8.asp
Xena, unofficially called the 10th planet, is the second-most-shiny
known object in the solar system, new observations show. Scientists
are scrambling to explain where Xena got its sparkle. Some suggest
that it might have enough heat to belch methane, despite being in
the coldest region of the solar system.
The new notion of Xena arises from Hubble Space Telescope images
that were released this week. The images reveal that Xena, the most
distant known object in our solar system, isn't quite the big shot
that scientists had thought it was.
The chilly outpost's diameter—2,384 kilometers—makes it about 5
percent larger than Pluto, Mike Brown of the California Institute
of Technology in Pasadena and his colleagues announced April 11.
That's still large enough for Xena to retain its unofficial status
as a planet, Brown says, but considerably smaller than ground-based
observations had indicated.
Researchers have difficulty determining the size of remote denizens
of the solar system because a large object that reflects a small
amount of sunlight looks the same as a small object reflecting a
lot of light.
But for Xena, the sharp Hubble pictures erase that ambiguity.
The relatively small size shown in those images indicates that the
body reflects 86 percent of sunlight. Brown says he was "thoroughly
shocked" by that finding. Researchers had assumed that Xena's
surface was similar to that of Pluto, which reflects 60 percent of
sunlight. Saturn's moon Enceladus, recently shown to be shooting
out a geyser of water vapor is the only solar system object known
to have a higher reflectivity, notes Brown.
Scientists have proposed two scenarios to explain Xena's high
reflectivity. In one, a jet of methane leaks continuously from
Xena. The methane jet freezes as it emerges, continually blanketing
the surface with fresh snow.
What's the heat source that could drive such activity? "Beats me,"
says planetary scientist Rick Binzel of the Massachusetts Institute
of Technology. He notes that gravitational tugs from a neighboring
moon sometimes generate heat within a body, but Xena's moon is too
small to do that.
Another source of heat, sunlight, would penetrate only a few tens
of meters below Xena's surface and would probably have long ago
depleted the reserves of methane there.
In the other model, the planet has a methane-rich atmosphere
created during the portion of its 560-year-long orbit when it's
nearest the sun. As Xena speeds away, the atmosphere freezes on the
surface as a bright frost. However, Brown says, it's not clear that
such frost would be bright enough to account for the shininess of
Xena's surface.
Some articles to refer:
http://www.sciencenews.org/articles/20060107/note11.asp
http://www.sciencenews.org/articles/20050806/fob2.asp