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Jun 2004
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XMM Press Release SNR 11-2004
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Phoebe hi-res
Size hi-res: 2280 kb Credits: NASA/JPL/Space Science Institute
Peering at Phoebe Shown here is a mosaic of seven of the sharpest, highest resolution images taken of Phoebe during the Cassini-Huygens close fly-by of the tiny moon. The image scales range from 27 to 13 metres per pixel. Smaller and smaller craters can be detected 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.
Phoebe hi-res
Size hi-res: 265 kb Credits: NASA/JPL/Space Science Institute
Dark desolation On 11 June 2004, during its closest approach to Phoebe, Cassini-Huygens 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-coloured floors and light-coloured 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 with an angle of 30.7 degrees between the Sun, Phoebe and spacecraft and from a distance of approximately 2365 kilometres. The image scale is approximately 14 metres per pixel. The image was high-pass filtered to bring out small-scale features and then enhanced in contrast.
Phoebe hi-res
Size hi-res: 311 kb Credits: NASA/JPL/Space Science Institute
A view to the south A mosaic of two images of Saturn's moon Phoebe taken shortly after Cassini's fly-by on 11 June 2004, gives a close-up view of a region near its South Pole. The view, taken about 13 000 kilometres from Phoebe, is about 120 kilometres across and shows a region battered by crater impacts. 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 roundish shape and the formation of impact craters. Walls of some of the larger craters are more than four kilometres high. The image scale is 80 metres per pixel.
Phoebe hi-res
Size hi-res: 136 kb Credits: NASA/JPL/Space Science Institute
A skyline view 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 icy-rich body overlain with a thin layer of dark material. Obvious downslope 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 been caused 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 kilometres in diameter, front to back as viewed. The larger depression in which the crater sits is on the order of 100 kilometres across. The slopes from the rim down to the ‘hummocky’ floor are approximately 20 kilometres long; many of the bright streamers on the crater wall are on the order of 10 kilometres 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 with an angle of 78 degrees between the Sun, Phoebe and the spacecraft, from a distance of 11 918 kilometres. The image scale is approximately 70 metres per pixel. No enhancement was performed on this image.
On Friday 11 June, at 22:56 CET, the Cassini-Huygens spacecraft flew by Saturn's outermost moon Phoebe, coming within approximately 2070 kilometres of the satellite's surface. All eleven on-board instruments scheduled to be active at that time worked flawlessly and acquired data. The first high-resolution images show a scarred surface, covered with craters of all sizes and large variation of brightness across the surface.
Phoebe is a peculiar moon amongst the 31 known satellites orbiting Saturn. Most of Saturn's moons are bright but Phoebe is very dark and reflects only 6% of the Sun's light. Another difference is that Phoebe revolves around the planet on a rather elongated orbit and in a direction opposite to that of the other large moons (a motion known as 'retrograde' orbit). All these hints suggested that Phoebe, rather than forming together with Saturn, was captured at a later stage. Scientists, however, do not know whether Phoebe was originally an asteroid or an object coming from the 'Kuiper Belt'.
The stunning images obtained by Cassini's high-resolution camera now seem to indicate that it contains ice-rich material and is covered by a thin layer of dark material, probably 300-500 metres thick. Scientists base this hypothesis on the observation of bright streaks in the rims of the largest craters, bright rays radiating from smaller craters, grooves running continuously across the surface of the moon and, most importantly, the presence of layers of dark material at the top of crater walls.
"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, USA. "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."
The high-resolution images of Phoebe show a world of dramatic landforms, with landslides and linear structures such as grooves, ridges and chains of pits. Craters are ubiquitous, with many smaller than one kilometre. "This means, besides the big ones, lots of projectiles smaller than 100 metres must have hit Phoebe," said Prof. Gerhard Neukum, Freie Universitaet Berlin, Germany, and a member of the imaging team. Whether these projectiles came from outside or within the Saturn system is debatable.
There is a suspicion that Phoebe, the largest of Saturn's outer moons, might be parent to the other, much smaller retrograde outer moons that orbit Saturn. They could have resulted from the impact ejecta that formed the many craters on Phoebe.
Besides these stunning images, the instruments on board Cassini collected a wealth of other data, which will allow scientists to study the surface structures, determine the mass and composition of Phoebe and create a global map of it. "If these additional data confirm that Phoebe is mostly ice, covered by layers of dust, this may well mean that we are looking at a 'leftover' from the formation of the Solar System about 4600 million years ago," said Dr Jean-Pierre Lebreton, ESA Huygens Project Scientist.
Phoebe might indeed be an icy wanderer from the distant outer reaches of the Solar System, which, like a comet, was dislodged from the Kuiper Belt and captured by Saturn when the planet was forming. Whilst studying the nature of Phoebe may give scientists clues on the origin of the building blocks of the Solar Syste
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