Using new Hubble
Space Telescope observations, a research team led by
Hal Weaver of the Johns Hopkins
Laboratory and Alan Stern of the Southwest Research
Institute has found that Pluto's three moons are
essentially the same color, boosting the theory that the
Pluto system formed in a single, giant collision.
Publishing their findings in an International
Astronomical Union Circular (No. 8686), the team
determined that Pluto's two "new" satellites, discovered in
May 2005 and provisionally called S/2005 P 1 and S/2005 P
2, have identical colors to one another and are essentially
the same neutral color as Charon, Pluto's large moon
discovered in 1978.
All three satellites have surfaces that reflect
sunlight with equal efficiency at all wavelengths, which
means they have the same color as the sun or Earth's moon.
In contrast, Pluto has more of a reddish hue.
The new observations were obtained March 2 with the
high-resolution channel of the Hubble's Advanced Camera for
Surveys. The team determined the bodies' colors by
comparing the brightness of Pluto and each moon in images
taken through a blue filter with those taken through a
"The high quality of the new data leaves little doubt
that the hemispheres of P1 and P2 that we observed have
essentially identical, neutral colors," Weaver said.
The new results further strengthen the hypothesis that
Pluto and its satellites formed after a collision between
two Pluto-sized objects nearly 4.6 billion years ago.
"Everything now makes even more sense," Stern said. "If all
three satellites presumably formed from the same material
lofted into orbit around Pluto from a giant impact, you
might well expect the surfaces of all three satellites to
have similar colors."
The researchers hope to make additional Hubble color
observations, in several more filters, to see if the
similarity among the satellites persists to longer (redder)
wavelengths. They have proposed to obtain compositional
information on the new satellites by observing them at
near-infrared wavelengths, where various ice and mineral
absorptions are located. The researchers also hope to
better refine the orbits of P1 and P2 and measure the
moons' shapes and rotational periods.
The Hubble observations were made in support of NASA's
New Horizons mission to Pluto and the Kuiper Belt. New
Horizons launched on Jan. 19 and will fly through the Pluto
system in July 2015, providing the first close-up look at
the ninth planet and its moons. Stern leads the mission and
science team as principal investigator; Weaver serves as
the mission's project scientist. APL manages the mission
for NASA's Science Mission Directorate and operates the New
For more information on the mission, go to pluto.jhuapl.edu.