Pluto's Moons are Indeed Related to Each Other

Pluto's three moons have formed as a result of a single cataclysmic event - the encounter with an asteroid more than 4 billion years ago. This has been confirmed by yet another observation. Astronomers drew the conclusion that Pluto's moons are probably related when they realized that all of them orbit in the same plane. Using new Hubble Space Telescope observations, a research team led by Dr. Hal Weaver of the Johns Hopkins University Applied Physics Laboratory and Dr. Alan Stern of the Southwest Research Institute has found that Pluto's three moons are essentially the same color - which indicates they are made from the same material.

"Everything now makes even more sense," says Stern. "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."

Astronomers compared the satellites images through blue and green/red filters. The fact that their luminosity has remained unchanged when viewed through each of these filters shows that their surface reflects sunlight with equal efficiency at all wavelengths, which means that the color of the satellites is something like the Moon's (in the absence of Earth's atmospheric effects which can alter the apparent color of our moon). On the other hand, Pluto seems smaller (less luminous) when viewed through the blue filter meaning that its color is reddish.

"The high quality of the new data leaves little doubt that the hemispheres of P1 and P2 that we observed have essentially identical, neutral colours," says Weaver. The team hopes to make additional Hubble color observations, with several more filters, to see if the similarity among the satellites persists to longer (redder) wavelengths. Another reason for observing the satellites in the domain of near-infrared wavelengths is that this can provide more specific information about their composition, about various ices and minerals.

Stern and Weaver are also working on the New Horizons mission to Pluto and the Kuiper Belt, which has been launched on Jan. 19, 2006 and which is expected to reach Pluto in July 2015.

Photo Credit: NASA, ESA, A. Stern (Southwest Research Institute), H. Weaver (Johns Hopkins University/Applied Physics Lab), and the HST Pluto Companion Search Team.