In 2007 a team of astronomers led by Mike Brown (California Institute of Technology,
Caltech) discovered a dwarf planet belonging to the Kuiper Belt. It was provisionally
called 2007 OR10 and was the fifth largest dwarf planet (about half the size of
Pluto), and Brown hypothesised that it was similar to the icy dwarf planet Haumea,
and was therefore a very white body. The object was accordingly nicknamed "Snow
White", while an official name was decided.
Later observations of 2007 OR10 revealed however that its surface was not white at
all, but rather a distinctly reddish colour, so much so that it is actually one of
the reddest objects in the Solar System.
To understand the origin of this colour,
that could have excluded the possibility of water ice on the surface, Brown's team,
unable to use the recently decommissioned Near Infrared Camera (NIRC) at the Keck
Observatory (Hawaii), made a new instrument, the Folded-port Infrared Echellette
(FIRE), for use on the Magellan Baade Telescope (Chile).
The results obtained with this instrument, to be published in the Astrophysical
Journal Letters, have confirmed the red colour of the surface, but also show that the
surface is actually covered in water ice. This is surprising, given that water ice is
not red!
The mystery is likely to be explained by the presence of methane, a remnant of an
atmosphere that has been lost to space over the last 2-3 billion years, a small part
of which was precipitated onto the surface. A thin deposit of methane then may cover
the water ice, just like on Quaoar, a dwarf planet that appears to be very similar to
Snow White. |