New Hubble images show similar colors for Pluto's moons
Fri Mar 10, 2006 at 16:24 UTC
Finding supports theory that single collision created ninth planet's three satellites.
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 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 green/red filter.
"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," says Weaver.
Image Credit: NASA, ESA, H. Weaver (Johns Hopkins University/Applied Physics Lab), A. Stern (Southwest Research Institute), and the HST Pluto Companion Search Team
This pair of NASA Hubble Space Telescope images shows the motion of Pluto's satellites between February 15th and March 2nd, 2006. Both images were taken through a red filter (F606W) using the High Resolution Channel (HRC) of the Advanced Camera for Surveys (ACS). During this 15-day period, Pluto's newly-discovered satellite S/2005 P 2 (P2 for short) moved counterclockwise from the noon position to the 5 o'clock position, while the other newly-discovered satellite S/2005 P 1 (P1 for short) moved counterclockwise from the 1 o'clock position to the 7 o'clock position.
During that same period, Pluto's much larger and closer moon, Charon, started near the 2 o'clock position, made more than two complete counterclockwise revolutions around Pluto, and ended up near the 10 o'clock position. The motions of P1 and P2 are in excellent agreement with the predicted motions based on earlier Hubble observations, thereby confirming that P1 and P2 are orbiting Pluto in the same plane as Charon's orbit, as expected if all three moons were created during a single, giant impact event.
Image Credit: NASA, ESA, A. Stern (Southwest Research Institute), H. Weaver (Johns Hopkins University/Applied Physics Lab), and the HST Pluto Companion Search Team
These NASA Hubble Space Telescope images of Pluto were taken on March 2, 2006, using the High Resolution Channel (HRC) of the Advanced Camera for Surveys (ACS). The image on the left was taken through a blue filter (F435W), and the one on the right was taken through a red filter (F606W). By comparing these two images in detail, astronomers discovered that the surfaces of Pluto's two newly-discovered satellites (S/2005 P 1 and S/2005 P 2, or P1 and P2 for short) have essentially the same color as Charon's surface.
All three satellites have surfaces that reflect sunlight with equal efficiency at all wavelengths, which means they have the same color as Earth's moon (in the absence of Earth's atmospheric effects which can alter the apparent color of our moon). In contrast, Pluto's surface has a reddish hue. The remarkable similarity in the colors of the satellites supports the idea that they were all created from material stripped from the surface layers of Pluto during the giant impact that created the entire system more than 4 billion years ago. (Note that the color schemes used to display the images are not meant to represent the colors of the objects. Rather, a blue intensity scale is used for the image taken through the F435W filter and a red intensity scale is used for the F606W image simply to highlight that the images were obtained through two different filters.)
Image Credit: NASA, ESA, A. Stern (SwRI) and Z. Levay (STScI)
The new HST/ACS observations made on March 2nd reveal that all three of Pluto's satellites are neutrally colored, unlike reddish Pluto itself. Pluto's reddish color is believed to be due to reddening agents created by the effects of sunlight acting on its nitrogen and methane surface ices. Charon's surface is known to consist primarily of water ice; the similar color of P1 and P2 may indicate they too have water ice surfaces. The color similarity of Pluto's two small satellites to one another and to Charon is consistent with their all having been born as a result of a single giant impact, as previously indicated by their orbits and Charon's large mass.
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," 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."
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, 2006, 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.
The Johns Hopkins University Applied Physics Laboratory, Laurel, Md., manages the mission for NASA's Science Mission Directorate and operates the New Horizons spacecraft.
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