|
|
Cassini-Huygens looks at Phoebe's distant past   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.  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.  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.  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. Release date: 19 January 2005 |