Phoebe, Approaching and Departing

As it entered the Saturn system, NASA's Cassini spacecraft performed its first targeted flyby of one of the planet's moons. On June 11, 2004, Cassini passed Phoebe, the largest of Saturn's outer or "irregular" moons, at an altitude of just 1,285 miles (2,068 kilometers). This was the sole close flyby of one of the outer moons of Saturn in the entire Cassini mission.

This montage of two views is published by the Cassini team to mark the 10th anniversary of the Phoebe flyby.

The image on the left side shows Cassini's view on approach to Phoebe, while the right side shows the spacecraft's departing perspective. Most of the left-side view was previously released as PIA06073; an area on its upper right side is newly filled in here. Most of the view on the right side has not previously been released, although the crater at upper left is seen in PIA06074.

Phoebe's shape is approximately spherical (see PIA06070 and PIA15507 for more details), with a diameter of 136 miles (219 kilometers) on its longest axis and 127 miles (204 kilometers) on its shortest axis, which is also the rotation axis. This is approximately 16 times smaller than Earth's moon.

For several reasons, Phoebe is thought to be a captured object that does not share a joint origin with Saturn and the inner, "regular" satellites. It orbits in a retrograde direction, opposite to the direction of Saturn's other major moons. Its overall density was determined by Cassini scientists to be quite large for a moon of Saturn. The prevailing view is that Phoebe might have formed in the Kuiper Belt, far beyond the orbit of Saturn. It might thus be a small cousin of the largest Kuiper Belt object, Pluto.

The image mosaic on the left, recorded about 45 minutes before closest approach to Phoebe, is composed of six frames from Cassini's Narrow-Angle Camera (NAC), plus one Wide-Angle Camera (WAC) image to fill the gap on the upper-right limb. The image has a spatial resolution of 260 feet (80 meters) per pixel. The sun-Phoebe-spacecraft, or phase, angle is 80 degrees.

The image at right, taken about half an hour after closest approach, is composed of eight NAC frames. The spatial resolution is 210 feet (65 meters) per pixel, and the phase angle is 83 degrees.

The images have been slightly rescaled from their original formats and sharpened. Because Phoebe is a very dark object, contrast enhancement was also necessary. At such high phase angles, the brightest parts of the surface, except where bright ice is exposed, reflect only about four percent of the incoming sunlight. The mosaics are composed of monochromatic, or single-color, images. Since Phoebe is a very dark object with no obvious coloration, a natural color view would probably look somewhat similar.

Image credit: NASA/JPL-Caltech/Space Science Institute

Iapetus

Iapetus is a moon of extreme contrasts. The light and dark features give the moon a distinctive "yin and yang" appearance. Scientists believe that a runaway migration of ice on the surface, triggered by a preferential initial darkening and consequential warming of the leading hemisphere of the moon by infalling debris from the outer moon Phoebe, may be responsible for the unusual and striking appearance.

For more on Iapetus (914 miles, or 1,471 kilometers across), see this press release about theories regarding this moon's unique color dichotomy.

This view looks toward the Saturn-facing hemisphere of Iapetus. North on Iapetus is up and rotated 30 degrees to the right. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on August 30, 2013.

The view was obtained at a distance of approximately 1.5 million miles (2.5 million kilometers) from Iapetus. The raw image scale is 9 miles (15 kilometers) per pixel. This image has been magnified by a factor of 1.5.

Photo credit: NASA/JPL-Caltech/Space Science Institute

The Face of Phoebe

Phoebe's true nature is revealed in startling clarity in this mosaic of two images taken during Cassini's flyby on June 11, 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 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.

An accurate determination of Phoebe's density -- a forthcoming result from the flyby -- will help Cassini mission scientists understand how much of the little moon is comprised of ices.

This spectacular view was obtained at a phase, or Sun-Phoebe-spacecraft, angle of 84 degrees, and from a distance of approximately 32,500 kilometers (20,200 miles). The image scale is approximately 190 meters (624 feet) per pixel. No enhancement was performed on this image.

Photo credit: NASA/JPL-Caltech/Space Science Institute

Phoebe

This panel of images shows the nearly spherical shape of Saturn's moon Phoebe, as derived from imaging obtained from NASA's Cassini spacecraft. Each image represents a 90-degree turn.

Photo credit: NASA/JPL-Caltech/SSI/Cornell

Note: For more information, see Cassini Finds Saturn Moon has Planet-Like Qualities.