Early images returned from the first detailed reconnaissance of Saturn's small outer moon, Phoebe, show breathtaking details in the moon's pockmarked surface that already have imaging scientists puzzling over the body's history. The images are only a preview of what to expect from the high resolution images to be examined later today which will show details about 10 times smaller.
Phoebe has revealed itself to be a rugged, heavily cratered body, with overlapping craters of varying sizes. This morphology suggests a very ancient surface. There are apparently many craters smaller than 1 km, indicating that projectiles probably smaller than 100 meters once pummelled Phoebe. Whether these objects were cometary or asteroidal in origin, or were the debris that resulted from impacts on other bodies within the Saturn system, is hotly debated. There is also variation in surface brightness across the body. In parts of both images, the brightness is so great the image is overexposed
In the first image (at left) in which Phoebe looks somewhat like a sideways skull, the large crater near the bottom displays a complex and rugged interior. The lower right hand part of Phoebe appears to be covered by bright wispy material.
The second, higher resolution image further reveals the moon's battered surface, including a crater near the right hand edge with bright rays that extend outward from its center. This suggests that dark material coats the outside of what may be a predominantly icy body, and the underlying ice has been exposed and ejected by a recent impact. Features reminiscent of those seen on the Martian moon Phobos -- such as linear grooves are faintly visible in the upper part of this image. There are suggestions of linear ridges or grooves and of chains of craters, perhaps radial to a large crater just hidden on the unilluminated region in the upper left.
Left to right, the two views were obtained at phase, or Sun-Phoebe-spacecraft, angles of approximately 86 degrees, and from distances ranging from 143,068 kilometers (88,918 miles) to 77,441 kilometers (48, 130 miles); for reference, Cassini's closest approach to Phoebe was approximately 2,000 kilometers (1,240 miles). The image scale ranges from 0.86 to 0.46 kilometers (0.53 to 0.29 miles) per pixel. No enhancement of any kind has been performed on the images.
In 2012, Cassini scientists published their findings, derived from theoretical modeling using data from a variety of Cassini instruments, that Phoebe almost surely is a large planetesimal which formed in the early Kuiper Belt before being captured into orbit around Saturn. The new analysis showed Phoebe likely was heated early in its history and consequently is nearly spherical and has denser rock-rich material concentrated near its center. Its average density, in fact, is about the same as Pluto, another object in the Kuiper Belt. Phoebe was likely captured by Saturn when it got close to the giant planet in the past.
[Caption updated April 26, 2012.]
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA's Office of Space Science, Washington, D.C. The imaging team consists of scientists from the US, England, France, and Germany. The imaging operations center and team lead (Dr. C. Porco) are based at the Space Science Institute in Boulder, Colo.