Phoebe (moon)

Phoebe was discovered by William Henry Pickering on 18 March 1899 Toby Owen of the University of Hawaii at Mānoa, chairman of the International Astronomical Union Outer Solar System Task Group said: ==Orbit==

For more than 100 years, Phoebe was Saturn's outermost known moon, until the discovery of several smaller moons in 2000. Phoebe is almost 4 times more distant from Saturn than its nearest major neighbor (Iapetus), and is substantially larger than any of the other moons orbiting planets at comparable distances. All of Saturn's regular moons except Iapetus orbit very nearly in the plane of Saturn's equator. The outer irregular satellites, including Phoebe, follow orbits that can be moderately to highly eccentric, and none are expected to rotate synchronously as all the regular moons of Saturn (except for Hyperion) do. Phoebe completes a full orbit around Saturn in about 18 months, and its orbit is retrograde; that is, it orbits Saturn in the opposite direction to Saturn's orbit. This categorizes it in a group of irregular satellites called the Norse group. There are a number of satellites with similar orbits that are speculated to be fragments from collision events Phoebe has experienced in the past, such as S/2006 S 20, S/2006 S 9, S/2019 S 2, and S/2007 S 2. Phoebe ring , which dwarfs the main rings The Phoebe ring is one of the rings of Saturn. This ring is tilted 27 degrees from Saturn's equatorial plane (and the other rings). It extends from at least 128 to 207 times the radius of Saturn; Phoebe orbits the planet at an average distance of 215 Saturn radii. The ring is about 40 times as thick as the diameter of the planet. Because the ring's particles are presumed to have originated from micrometeoroid impacts on Phoebe, they should share its retrograde orbit, which is opposite to the orbital motion of the next major moon inward, Iapetus. Inwardly migrating ring material would thus strike Iapetus's leading hemisphere, contributing to its two-tone coloration. Although very large, the ring is virtually invisible—it was discovered using NASA's infrared Spitzer Space Telescope. Material displaced from Phoebe's surface by microscopic meteor impacts may be responsible for the dark areas on the surface of Hyperion, another one of Saturn's moons. Debris from the biggest impacts may be the origin of some of the other moons of the Norse group—almost all of which are less than 10 km in radius. ==Physical characteristics==

and the Moon Most of Saturn's inner moons have very bright surfaces, but Phoebe's albedo is much lower in comparison (around 8–9%), Phoebe is the first such object to be imaged as anything other than a dot. Spectroscopic observations of Phoebe by the James Webb Space Telescope and the VIMS instrument on Cassini have confirmed the presence of water ice and carbon dioxide on its surface, with ambiguous evidence for organic compounds. The overall shape of the spectrum resembles that of Kuiper belt objects, providing a compositional confirmation that Phoebe is a captured body. Phoebe also appears to have distinctly more water ice than other similarly observed Saturnian irregular satellites, such as Siarnaq and Albiorix. Named surface features Apart from one regio named after Phoebe's daughter, Leto, all named features are craters named after characters from the Greek legend of Jason and the Argonauts. Maps Phoebe 2005 Mercator PIA07795.jpg|Map of Phoebe's middle latitudes. The higher latitudes have been clipped from the main map, but can be seen in the polar projections. Phoebe 2005 south polar projection PIA07797.jpg|Map of Phoebe's south polar region Phoebe north polar region PIA 07796.png|Map of Phoebe's north polar region Phoebe contour map PIA15507.jpg|3D map showing Phoebe's once spherical shape ==Formation and evolution==

Phoebe is usually assumed to have its origins in the outer Solar System. It may have formed in the Kuiper belt within three million years after the origin of the Solar System. This was early enough that sufficient radioactive material was available to melt it into a sphere and stay warm enough to have liquid water for tens of millions of years. Phoebe is expected to be involved in half of these collisions, mostly due to its large cross-section. Before the 2004 Cassini flyby, Phoebe was predicted to have a heavily cratered surface, with most of the large craters created from impacts with irregular moons. ==Observation and exploration==

in the lower right corner) as imaged with a 24" telescope Unlike Saturn's other moons, Phoebe was not favorably placed for the Voyager probes. Voyager 2 observed Phoebe for a few hours in September 1981. In the images, taken from a distance of 2.2 million kilometres at low phase angle, the size of Phoebe was approximately 11 pixels and showed bright spots on the otherwise dark surface. Cassini passed from Phoebe on 11 June 2004, returning many high-resolution images, which revealed a scarred surface. Because Voyager 2 had not been able to produce any high-quality images of Phoebe, obtaining them was a priority for the Cassini mission ==See also==