All About Saturn’s Moon, Phoebe

May 27, 2005

ESA — Data from the NASA/ESA/ASI Cassini-Huygens mission are providing convincing evidence that Saturn’s moon Phoebe was formed elsewhere in the Solar System, and was only later caught by the planet’s gravitational pull.

One way to unlock Phoebe’s secrets is using Cassini’s Visible and Infrared Mapping Spectrometer (VIMS), developed by a team of US (JPL), French and Italian (ASI) scientists and engineers. The science team is made by a large international group of US, Italian, French and German scientists led by the University of Arizona.

This instrument identifies the chemical compositions of the surfaces, atmospheres and rings of Saturn and its moons by measuring colours of visible light and infrared energy emitted or reflected (spectra).

The origin of Phoebe, which is the outermost large satellite of Saturn, is of particular interest because its orbit is in the opposite direction (retrograde) and inclined at a different angle to Saturn’s regular satellites (with “Ëœprograde’, low-inclination circular orbits). 

Phoebe’s generally dark surface shows evidence of water ice, but otherwise the surface most closely resembles that of asteroids and small outer Solar System bodies such as Chiron and Pholus that are thought to have originated in the Kuiper belt.

Recent results from VIMS suggest that Phoebe was gravitationally “Ëœcaptured’ by Saturn, having formed from ice and rocks “Ëœaccreting’, or joining together, outside the region of the “Ëœsolar nebula’ gas and dust in which Saturn formed.

The other moons probably accreted within the nebula in which Saturn itself formed. VIMS made its observations during the close fly-by of Phoebe by Cassini-Huygens on 11 June 2004.

The composition of Phoebe should reflect the composition of the region of the solar nebula where it formed. If it originated in the region of the main asteroid belt, it should consist largely of “Ëœmafic’ minerals, which are silicate rocks and magmas with relatively high amounts of heavier elements.

However, the presence of highly volatile substances (i.e. lots of water and carbon dioxide ice or other carbon-based compounds) does not support strongly this hypothesis. Alternatively then, it could have formed where the Kuiper belt objects originated in the “Ëœvolatile-rich’ outer solar nebula.

Spectra of Phoebe display a wealth of information, indicating a surface containing distinct locations iron-bearing minerals, bound water, trapped carbon dioxide, silicates, organics, nitriles and cyanide compounds. Phoebe is one of the most compositionally diverse objects yet observed in our Solar System. The only body imaged to date that is more diverse is Earth!

Mapping results from VIMS show that water ice is distributed over most of Phoebe’s observed surface, but generally shows stronger spectral signatures toward the southern polar region.

However, this water ice may only be a surface coating because some crater interiors show less ice deep in the crater and more near the surface.

By contrast with the moons around Jupiter, cratering tends to expose fresh ice in the subsurface. This raises the possibility that Phoebe is coated by material of cometary or outer Solar System origin, or that it is formed there.

Without information about its deep internal composition, we cannot conclusively say that Phoebe originated in the outer Solar System, but compositional data for the full Saturn system may help to narrow it down.

The same broad traces of iron on Phoebe are seen in Saturn’s rings, particularly in the Cassini division and the C-ring, and may imply that some materials are common to both Phoebe’s surface and the rings.

Angioletta Coradini, of the Instituto di Fisica dello Spazio Interplanetario, CNR, Rome, said: “Phoebe’s organic and cyanide compositions are unlike any surface yet observed in the inner Solar System, but organics and cyanides have still not yet been definitively detected by the VIMS in any of Saturn rings to date. This may mean that the materials on Phoebe and the rings have different origins.”

The detection of compounds with a similar absorption characteristics in their spectra on both Phoebe and Iapetus may indicate that material from Phoebe has struck Iapetus’s leading hemisphere. They may have collided or perhaps cometary material has coated both Phoebe and Iapetus.

Regardless of its origin, Phoebe’s diverse mix of materials is unique among Solar System surfaces observed to date, and the chances are very high that it probably does display primitive materials from the outer Solar System.

Image Captions

Image 1: Cassini-Huygens sees probable evidence on Phoebe of an ice-rich body overlain with a thin layer of dark material. The sharply-defined crater at above centre exhibits two or more layers of alternating bright and dark material.

Imaging scientists on the Cassini-Huygens mission have hypothesised 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 image was obtained on 11 June 2004 from a distance of 13 377 kilometres. The image scale is approximately 80 metres per pixel. No enhancement was performed on this image.

Credits: NASA/JPL/Space Science Institute

Image 2: Phoebe’s true nature is revealed in startling clarity in this mosaic of two images taken during the Cassini-Huygens fly-by on 11 June 2004.

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 around Jupiter.

When impacting bodies hit the surface of Phoebe, the collisions excavated fresh, bright material – probably ice – from under 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-coloured material. Some areas of the image that are particularly bright – especially near lower right – are over-exposed.

This view was obtained from a distance of approximately 32 500 kilometres. The image scale is approximately 190 metres per pixel. No enhancement was performed on this image.

Credits: NASA/JPL/Space Science Institute

Image 3: 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 “Ëœhummocky’ 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 “Ëœhummocky’ 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.

Credits: NASA/JPL/Space Science Institute


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