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Phoebe flyby preview
This animation shows Cassini during its encounter with the tiny moon Phoebe on the route to Saturn. (42sec file)
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Cassini preview
The Cassini spacecraft's arrival at Saturn is previewed in this detailed news conference from NASA Headquarters on June 3. (50min 01sec file)
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Saturn arrival explained
Cassini's make-or-break engine firing to enter orbit around Saturn is explained with graphics and animation. Expert narration is provided by Cassini program manager Robert Mitchell. (3min 33sec file)
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Cassini mission science
The scientific objectives of the Cassini mission to study the planet Saturn, its rings and moons are explained by Charles Elachi, director of the Jet Propulsion Laboratory. (4min 54sec file)
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Huygens mission science
After entering orbit around Saturn, the Cassini spacecraft will launch the European Huygens probe to make a parachute landing on the surface of the moon Titan. The scientific objectives of Huygens are explained by probe project manager Jean-Pierre Lebreton. (3min 14sec file)
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Phoebe's surface gives scientists clues to its origin
CASSINI PHOTO RELEASE
Posted: June 14, 2004

Images collected during Cassini's close flyby of Saturn's moon, Phoebe, have yielded strong evidence that the tiny object may contain ice-rich material, overlain with a thin layer of darker material perhaps 300 to 500 meters (980 to 1,600 feet) thick.

The surface of Phoebe is also heavily potholed with large and small craters. Images reveal bright streaks in the ramparts of the largest craters, bright rays which emanate from smaller craters, and uninterrupted grooves across the face of the body.

"The imaging team is in hot debate at the moment on the interpretations of our findings," said Dr. Carolyn Porco, Cassini imaging team leader at the Space Science Institute in Boulder, Colo. "Based on our images, some of us are leaning towards the view that has been promoted recently, that Phoebe is probably ice-rich and may be an object originating in the outer solar system, more related to comets and Kuiper Belt objects than to asteroids."

In ascertaining Phoebe's origin, imaging scientists are noting important differences between the surface of Phoebe and that of rocky asteroids which have been seen at comparable resolution. "Asteroids seen up close, like Ida, Mathilde, and Eros, and the small martian satellites do not have the bright 'speckling' associated with the small craters that are seen on Phoebe," said Dr. Peter Thomas, an imaging team member from Cornell University, Ithaca, N.Y.

The landforms observed in the highest resolution images also contain clues to the internal structure of Phoebe. Dr. Alfred McEwen, an imaging team member from the University of Arizona, Tucson, said, "Phoebe is a world of dramatic landforms, with craters everywhere, landslides, and linear structures such as grooves, ridges, and chains of pits. These are clues to the internal properties of Phoebe, which we'll be looking at very closely in order to understand Phoebe's origin and evolution."

"I think these images are showing us an ancient remnant of the bodies that formed over four billion years ago in the outer reaches of the solar system," said Dr. Torrence Johnson, an imaging team member from NASA's Jet Propulsion Laboratory, Pasadena, Calif. "Battered and beat-up as it is, it is still giving us clues to its origin and history."

Phoebe may be an icy interloper from the distant outer solar system which found itself captured by giant Saturn in its earliest, formative years. Final conclusions on Phoebe's origins await a combination of the results on Phoebe's surface structures, mass and composition gathered from all 11 instruments, which collected data during the flyby on June 11, 2004.

"This has been an impressive whirlwind flyby and it's only a curtain raiser on the events about to begin," said Porco.


Credit: NASA/JPL/Space Science Institute
Download larger image version here

 
FIRST IMAGE: Images like this one, showing bright wispy streaks thought to be ice revealed by subsidence of crater walls, are leading to the view that Phoebe is an ice-rich body overlain with a thin layer of dark material. Obvious down slope motion of material occurring along the walls of the major craters in this image is the cause for the bright streaks, which are over-exposed here. Significant slumping has occurred along the crater wall at top left.

The slumping of material might have occurred by a small projectile punching into the steep slope of the wall of a pre-existing larger crater. Another possibility is that the material collapsed when triggered by another impact elsewhere on Phoebe. Note that the bright, exposed areas of ice are not very uniform along the wall. Small craters are exposing bright material on the hummocky floor of the larger crater.

Elsewhere on this image, there are local areas of outcropping along the larger crater wall where denser, more resistant material is located. Whether these outcrops are large blocks being exhumed by landslides or actual 'bedrock' is not currently understood.

The crater on the left, with most of the bright streamers, is about 45 kilometers (28 miles) in diameter, front to back as viewed. The larger depression in which the crater sits is on the order of 100 kilometers (62 miles) across. The slopes from the rim down to the hummocky floor are approximately 20 kilometers (12 miles) long; many of the bright streamers on the crater wall are on the order of 10 kilometers (6 miles) long. A future project for Cassini image scientists will be to work out the chronology of slumping events in this scene.

This image was obtained at a phase, or Sun-Phoebe-spacecraft, angle of 78 degrees, and from a distance of 11,918 kilometers (7,407 miles). The image scale is approximately 70 meters (230 feet) per pixel. No enhancement was performed on this image.


Credit: NASA/JPL/Space Science Institute
Download larger image version here

 
SECOND IMAGE: A mosaic of two images of Saturn's moon Phoebe taken shortly after Cassini's flyby on June 11, 2004, gives a close-up view of a region near its South Pole. The view, taken about 13,000 kilometers (8,000 miles) from Phoebe, is about 120 kilometers (74 miles) across and shows a region battered by craters. Brighter material, likely to be ice, is exposed by small craters and streams down the slopes of large craters. The skyline is a combination of Phoebe's shape and the formation of impact craters. Walls of some of the larger craters are more than 4 kilometers (2.5 miles) high. The image scale is 80 meters (264 feet) per pixel.


Credit: NASA/JPL/Space Science Institute
Download larger image version here

 
THIRD IMAGE: Shown here is a mosaic of seven of the sharpest, highest resolution images taken of Phoebe during Cassini's close flyby of the tiny moon. The image scales range from 27 to 13 meters (90 to 43 feet) per pixel. Smaller and smaller craters can be resolved as resolution increases from left to right. The number of blocks, or bumps on the surface also increases to the right. The Sun is coming from the right, so the bright-dark pattern is reversed between blocks and small craters. Grooves or chains of pits are seen on the left portion of the mosaic, which may mark fractures or faults induced by large impact events. Many of the small craters have bright rays, similar to recent craters on the Moon. There are also bright streaks on steep slopes, perhaps where loose material slid downhill during the seismic shaking of impact events. There are also places where especially dark materials are present, perhaps rich in carbon compounds.


Credit: NASA/JPL/Space Science Institute
Download larger image version here

 
FOURTH IMAGE: On June 11, 2004, during its closest approach to Phoebe, Cassini obtained this extremely high resolution view of a dark, desolate landscape. Regions of different reflectivity are clearly visible on what appears to be a gently rolling surface. Notable are several bright-rayed impact craters, lots of small craters with bright-colored floors and light-colored streaks across the landscape. Note also the several sharply defined craters -- probably fairly young features -- near the upper left corner.

This high-resolution image was obtained at a phase, or Sun-Phoebe-spacecraft, angle of 30.7 degrees, and from a distance of approximately 2,365 kilometers (1,470 miles). The image scale is approximately 14 meters (46 feet) per pixel. The image was high-pass filtered to bring out small scale features and then enhanced in contrast.

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