Phoebe was the first target encountered upon the arrival of theCassini spacecraft in the Saturn system in 2004, and is thus unusually well-studied for an irregular moon of its size.Cassini's trajectory to Saturn and time of arrival were chosen to permit this flyby.[10] After the encounter and its insertion into orbit,Cassini did not go much beyond the orbit ofIapetus.
Phoebe is roughly spherical and has a differentiated interior. It was spherical and hot early in its history and was battered out of roundness by repeated impacts. There is some evidence that it may be a capturedcentaur that originated in theKuiper belt.[11] Phoebe is the second-largest retrograde satellite in theSolar System afterTriton.[12]
We picked the legend of the Argonauts for Phoebe as it has some resonance with the exploration of the Saturn system byCassini–Huygens. We can't say that our participating scientists include heroes likeHercules andAtalanta, but they do represent a wide, international spectrum of outstanding people who were willing to take the risk of joining this voyage to a distant realm in hopes of bringing back a grand prize.
Animation of Phoebe's orbit. Saturn· Phoebe· Titan
Phoebe's orbit isretrograde; that is, it orbits Saturn opposite to Saturn's rotation. 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'sregular moons except Iapetusorbit very nearly in the plane of Saturn's equator. The outerirregular satellites, including Phoebe, follow orbits that can be moderately to highlyeccentric, and none are expected torotate synchronously as all the inner moons of Saturn (except forHyperion) do. Phoebe orbits within a group of irregular satellites called theNorse 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 asS/2006 S 20,S/2006 S 9,S/2019 S 2, andS/2007 S 2.[20]
The Phoebe ring is one of therings 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[21] 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.[22] Because the ring's particles are presumed to have originated frommicrometeoroid impacts on Phoebe, they should share itsretrograde orbit,[23] 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 itstwo-tone coloration.[24][25][26][27] Although very large, the ring is virtually invisible—it was discovered usingNASA'sinfraredSpitzer Space Telescope.
Material displaced from Phoebe's surface by microscopic meteor impacts may be responsible for the dark areas on the surface ofHyperion.[a] Debris from the biggest impacts may be the origin of some of the other moons of theNorse group—almost all of which are less than 10 km in radius.
Phoebe is roughly spherical and has a diameter of213±1.4 km[4] (132 mi), approximately one-sixteenth that of theMoon. It is Saturn's ninth-largest moon, but it is the eighth-most massive. Hyperion, another one of Saturn's moons, has a larger radius, but is less massive than Phoebe. Phoebe rotates every nine hours and 16 minutes, and completes a full orbit around Saturn in about 18 months. Its surface temperature is on average 75 K (−198.2 °C).
Cassini's closeup of Phoebe from 13 June 2004; the crater Euphemus is at top center
Most of Saturn's inner moons have very bright surfaces, but Phoebe'salbedo is much lower in comparison (0.100±0.005), though relatively bright compared to other irregular moons with measured albedos.[8] The Phoebean surface is heavily scarred. The largest crater, Jason, is roughly 100 km in diameter.[28]
Phoebe's dark coloring initially led to scientists surmising that it was a capturedasteroid, as it resembled the common class of darkcarbonaceous asteroids. These are chemically very primitive and are thought to be composed of original solids that condensed out of thesolar nebula with little modification since then.[29]
However, images fromCassini 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 capturedcentaur, one of a number of icyplanetoids from theKuiper belt thatorbit theSun betweenJupiter andNeptune.[30][31] Phoebe is the first such object to be imaged as anything other than a dot.
Spectroscopic observations of Phoebe by theJames Webb Space Telescope and the VIMS instrument onCassini 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 ofKuiper 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 asSiarnaq andAlbiorix.[32][33]
Despite its small size, Phoebe is thought to have been a spherical body early in its history, with a differentiated interior, before solidifying and being battered into its current, slightly non-equilibrium shape.[34]
Apart from oneregio named after Phoebe's daughter,Leto, all named features are craters named after characters from the Greek legend ofJason and theArgonauts.[35]
Phoebe formed in theKuiper belt within three million years after theorigin 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.[34]
Phoebe (withNGC 4179 in the lower right corner) as imaged with a 24"telescopeImages of different hemispheres of Phoebe byVoyager 2
Unlike Saturn's other moons, Phoebe was not favorably placed for theVoyager 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.[36]
Cassini passed 2,068 kilometres (1,285 mi) from Phoebe on 11 June 2004,[37] returning many high-resolution images, which revealed a scarred surface. BecauseVoyager 2 had not been able to produce any high-quality images of Phoebe, obtaining them was a priority for theCassini mission[10] and its flight path was deliberately designed to take it close by; otherwise,Cassini would likely not have returned images much better thanVoyager's. Because of Phoebe's short rotation period of approximately 9 hours, 17 minutes,Cassini was able to map virtually 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.[38]
^The composition implied by spectra does not seem to support the earlier suggestion that Phoebe could be the source of the dark material deposited onIapetus.
^abKovas, Charlie (18 March 1899)."On This Day".What Happened on March 18, 1899. Unknown.Archived from the original on September 12, 2015. RetrievedSeptember 21, 2017.
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