Phoebe (moon)

Orbit and orbital interactions

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 is 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. Phoebe's orbit is significantly different from those of the other irregular moons, and it was the innermost known orbit among all retrograde satellites of Saturn, until the announcement of S/2023 S 50 and S/2023 S 38 in 2025. There are a number of satellites with similar orbits that are speculated to be fragments from past collision events involving Phoebe, such as S/2006 S 20, S/2006 S 9, S/2019 S 2, and S/2007 S 2.

Once captured in orbit around Saturn, Phoebe had a significant effect on the irregular moon population of Saturn. Due to occupying a relatively much smaller orbital space, irregular satellites are expected to collide amongst themselves with a frequency four orders of magnitude greater than do main-belt asteroids. Phoebe is expected to be involved in half of these collisions, mostly due to its large cross-section. As any objects that crashed into it would have been eliminated, Phoebe may have cleared its orbital surroundings of other moons in this way, in a sense similar to the action of a major planet. This may have reduced the original irregular population from a value 30% greater than today, and decreased Phoebe's orbital distance from a value 30% larger than its present value. Some two thirds of the irregular moons present today were found to count as potential future impactors. 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.

Phoebe ring

The Phoebe ring is tilted 27 degrees from Saturn's equatorial plane (and the other rings of Saturn). 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

Phoebe has a diameter of 212.8±1.0 km (132.2 mi), approximately one-sixteenth that of the Moon. It is Saturn's ninth-largest moon, but it is the eighth-most massive; Hyperion has a larger radius, but is less massive than Phoebe. Phoebe rotates every 9 hours and 16 minutes, and unlike its orbit, Phoebe rotates in the prograde direction. Its surface temperature generally ranges between 82–112 K (−191.2 – −161.2 °C), though it can drop below 75 K (−198.2 °C) at the poles at night. Phoebe is roughly spherical; its shape is consistent within error to an oblate spheroid. Its oblateness is consistent with an object in hydrostatic equilibrium with its rotation period.

Most of Saturn's inner moons have very bright surfaces, but Phoebe's albedo is much lower in comparison (around 8–9%), though relatively bright compared to the measured albedos of other irregular moons. The Phoebean surface is heavily scarred. The largest crater, Jason, is roughly 100 km in diameter. Phoebe's dark coloring initially led to scientists surmising that it was a captured asteroid, as it resembled the common class of dark carbonaceous asteroids. These are chemically very primitive and are thought to be composed of original solids that condensed out of the solar nebula with little modification since then.

However, images from Cassini indicate that Phoebe's craters show a considerable variation in brightness, which indicate the presence of large quantities of ice below a relatively thin blanket of dark surface deposits some 300 to 500 metres (980 to 1,640 ft) thick. In addition, quantities of carbon dioxide have been detected on the surface, a finding that has never been replicated for an asteroid. It is estimated that Phoebe is about 50% rock, as opposed to the 35% or so that typifies Saturn's inner moons. For these reasons, scientists are coming to think that Phoebe is in fact a captured centaur, one of a number of icy minor planets from the Kuiper belt that orbit the Sun between Jupiter and Neptune. 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

Formation and evolution

Phoebe is usually assumed to have its origins in the outer Solar System, then becoming captured in orbit around Saturn later. 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. Despite its small size, Phoebe is thought to have attained a differentiated interior, before solidifying and being battered into its current, slightly non-equilibrium shape. However, it is asserted by some studies that Phoebe may have instead have an origin with C-type asteroids.

Phoebe is significantly larger than the next largest retrograde moon of Saturn (Ymir), which has been found difficult to explain. Its capture by Saturn may have been a fluke, or it may have been captured in a different way to that of the other irregulars. Alternatively, due to its density and C-type nature, Phoebe may have been more difficult to disrupt in a collision than D or P-type objects, resulting in one surviving object as opposed to splitting into many fragments.

Observation and exploration

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 2,068 kilometres (1,285 mi) 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 and its flight path was deliberately designed to take it close by; otherwise, Cassini would likely not have returned images much better than Voyager's. Because of Phoebe's short rotation period of approximately 9 hours, 17 minutes, Cassini was able to map nearly the entire surface of Phoebe. The close fly-by enabled the mass of Phoebe to be determined with an uncertainty of only 1 in 500.