The ninth planet's two newest satellites shed light on the far outer corner of the solar system.

plutomoons.jpg Hubble Space Telescope images of Pluto taken on March 2, 2006.  Credit: NASA, ESA, A. Stern (Southwest Research Institute), H. Weaver (Johns Hopkins University/Applied Physics Lab), and the HST Pluto Companion Search Team

Through a series of discoveries that will help provide a roadmap for the New Horizons mission that launched in January, scientists are beginning to understand the make up and geologic history of Pluto‘s three moons.

In May 2005, researchers at Johns Hopkins University spotted two small moons—5,000 times fainter than Pluto and its largest moon, Charon—for the first time using the Hubble Space Telescope. Now, the same team of scientists have turned the Hubble back onto P1 and P2, as the moons are provisionally known, and found that the satellites are identical in color to one another, as well as Charon. This evidence supports the idea that all three moons were formed in a single cataclysmic event.

“Pluto’s so small: It’s only 70% the size of the Earth’s moon, and it’s got this little mini solar system around it,” said Harold Weaver, a scientist at the Johns Hopkins’ Applied Physics Laboratory and principal investigator of the study published in an International Astronomical Union Circular.

On March 2nd, as part of an effort to verify their initial discovery of Pluto’s smallest moons, Weaver and his team changed the viewing filter on the Hubble from red to blue in order to learn more about the surfaces of the two new moons.

“By knowing how it looks in these different colors,” Weaver said, “that gives you some information on what kind of surface it is; what the composition of the surface is, at least in some rough way.”

They found an exact similarity between Pluto’s three moons in terms of the way they reflect light—in other words, in terms of their color. Their neutral color is different from Pluto’s scarred “reddish gunk,” as Weaver put it, and very similar to that of Earth’s moon. The similarity between Charon, P1 and P2 suggested to Weaver that the moons originated as part of the same collision, which occurred nearly five billion years ago.

“The surfaces of Charon and the two new moons seem to be similar to each other,” Weaver said. “And that also suggests that they were formed during the same birthing event, which was this giant collision that we think took place probably around the time the whole solar system formed, about 4.6 billion years ago.”

The collision probably removed Pluto’s outer mantle and launched it into orbit, in much the same way that a long-ago collision ripped Earth’s outer mantle into orbit, where millennia of spinning shaped it into our moon.

The enhanced knowledge of Pluto’s lunar system will help guide NASA‘s New Horizons mission, headed by Weaver, which set off at the beginning of this year. Over the next nine years, as the mission advances toward Pluto, Weaver and others will have a chance to refine their understanding of the Kuiper Belt’s terrain. This mission, the first of its kind, is expected to enter Pluto’s system in July 2015.

“In the meantime, we’ll continue observing with whatever facilities are available here on Earth—and in orbit around Earth—and try and gain as much information as possible that can help us try to do a better job with the New Horizons mission as we fly by Pluto,” Weaver said.

Originally published March 14, 2006


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