|ESA Science & Technology||04-Jul-2005 14:25:10|
Knowledge before Cassini-Huygens
Virtually all the knowledge on Phoebe comes from a couple of images acquired by the Voyager 2 spacecraft in 1981. Unfortunately, the images are from a distance of around 2.2 million kilometres and reveal very little about the surface of the moon.
Phoebe is roughly spherical and has a diameter of 220 kilometres, which is equal to about one-fifteenth of the diameter of Earth's moon. Phoebe rotates on its axis every nine hours and it completes a full orbit around Saturn in about 18 months. Its irregular, elliptical orbit is inclined approximately 30 degrees toward Saturn's equator. Phoebe's orbit is also retrograde, which means it goes around Saturn in the opposite direction of most other moons - as well as of most other objects in the Solar System. Its average distance from the planet is 13 million kilometres, which is almost four times farther away from Saturn than its nearest neighbour, the moon Iapetus. Phoebe and Iapetus are the only major moons in the Saturn region that do not orbit close to the plane of Saturn's equator.
Unlike most major moons orbiting Saturn, Phoebe is very dark and reflects only 6 percent of the sunlight it receives. Its darkness and irregular, retrograde orbit suggest that Phoebe is most likely a captured object, which is defined as a celestial body that is trapped by the gravitational pull of a much bigger body, generally a planet.
Phoebe's darkness in particular suggests that the small moon comes from the outer Solar System, a region known as the Kuiper Belt, where many celestial bodies contain dark material.
Some scientists think Phoebe could be a captured Centaur. Centaurs are thought to be Kuiper Belt objects that migrated into the inner solar system and exist between the asteroid belt and the Kuiper Belt. If Phoebe is indeed a captured Centaur, images and scientific data taken by the Cassini-Huygens spacecraft will give scientists the first intensive opportunity to study a Kuiper Belt object.
*Note: in the following section all images courtesy NASA/JPL/Space Science Institute unless stated otherwise.
9 June 2004Cassini Instrument: Imaging Science Subsystem
10 June 2004Cassini Instrument: Imaging Science Subsystem
A large crater, roughly 50 kilometres across, is visible in the image on the left. The image on the right shows a body heavily pitted with craters of varying sizes, including very large ones, and displaying a substantial amount of variation in surface brightness. Features that appear to be cliffs may be the boundaries between large craters. Despite its exaggerated topography, Phoebe is more round than irregular in shape.
12 June 2004Cassini Instrument: Imaging Science Subsystem
On 11 June 2004 Cassini had its first close flyby of Phoebe. The moon has revealed itself to be a rugged, heavily cratered body, with overlapping craters of varying sizes. This morphology suggests an old surface. There are apparently many craters smaller than 1 km, indicating that projectiles probably smaller than 100 metres once pummeled 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.
13 June 2004Cassini Instrument: Imaging Science Subsystem
The image shows evidence for the emerging view that Phoebe may be an ice-rich body coated with a thin layer of dark material. Small bright craters in the image are probably fairly young features. This phenomenon has been observed on other icy satellites, such as Ganymede at Jupiter.
When impactors slammed into the surface of Phoebe, the collisions excavated fresh, bright material (probably ice) underlying the surface layer. Further evidence for this can be seen on some crater walls where the darker material appears to have slid downwards, exposing more light-colored material. Some areas of the image that are particularly bright - especially near lower right - are over-exposed.
The sharply-defined crater at above center exhibits two or more layers of alternating bright and dark material. Imaging scientists on the Cassini mission have hypothesized that the layering might occur during the crater formation, when ejecta thrown out from the crater buries the pre-existing surface that was itself covered by a relatively thin, dark deposit over an icy mantle. The lower thin dark layer on the crater wall appears to define the base of the ejecta blanket. The ejecta blanket itself appears to be mantled by a more recent dark surface lag.
This high-resolution image of Phoebe's pitted surface taken very near closest approach shows a 13-kilometre diameter crater with a debris-covered floor. Part of another crater of similar size is visible at left, as is part of a larger crater at top and many scattered smaller craters.
The radial streaks in the crater are due to down slope movements of loose fragments from impact ejecta. Also seen are boulders ranging from about 50 to 300 metres in diameter. The building-sized rocks may have been excavated by large impacts, perhaps from some other region of Phoebe rather than the craters seen here. There is no visible evidence for layering of ice and dark material or a hardened crust in this region, as on other parts of this moon.
Some of the relatively bright spots are from small impacts that excavated bright material from beneath the dark surface. Images like this provide information about impact processes on Phoebe.
14 June 2004Cassini Instrument: Imaging Science Subsystem
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.
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.
The image 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 kilometres high.
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 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 kilometeres long and 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.
23 June 2004Cassini Instrument: Visible and Infrared Mapping Spectrometer
The infrared images show a large range of bright and dark features. Water ice appears to be associated with the brighter regions, while the other two materials are more abundant in the darker regions. Carbon dioxide on the surface of Phoebe is distributed globally, although it appears to be more prevalent in the darker regions of the satellite. The existence of carbon dioxide strongly suggests that Phoebe did not originate in the asteroid belt, but rather in much colder regions of the Solar System such as the Kuiper Belt.Cassini Instrument: Ultraviolet Imaging Spectrograph
Cassini Instrument: Composite Infrared Spectrometer
In the middle panel this brightness is used to estimate the surface temperature distribution across Phoebe. Temperatures are given in Kelvin, and vary from 107 K in the late morning near the equator to less than 75 K in the northern hemisphere in the pre-dawn hours. The ragged edge of Phoebe in this region is an instrumental artifact.
Temperatures are affected strongly by topography, as can be seen by comparison with the visible-wavelength image (right). Some of the coldest temperatures are found in the shadowed region inside the large depression in the northern hemisphere (upper right).
Equatorial temperatures peak in the early afternoon near 112 K, plunging to 78 K before dawn, and are even colder at higher latitudes. The large day/night temperature contrasts imply that Phoebe's surface is covered in loose dust or ice particles that store little heat and thus cool off rapidly at night. Regions of Phoebe's surface that were not observed are shown in black.Cassini Instrument: Imaging Science Subsystem
Despite Phoebe's bumpy, irregular topography, the moon has a fairly round shape. The four views of the model are each separated by a 90 degree rotation - the upper left is centered at 0 degrees West longitude. The others show regions of the moon centered at 90, 180 and 270 degrees West longitude, as labeled. The coloring of the models corresponds to the height of Phoebe's surface, relative to the lowest point - a range of about 16 km.
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