Phoebe (moon)

History

Discovery

Phoebe was discovered by William Henry Pickering on 18 March 1899 from photographic plates that had been taken starting on 16 August 1898 at the Boyden Observatory near Arequipa, Peru, by DeLisle Stewart. It was the first satellite to be discovered photographically.

Naming

Phoebe is named after Phoebe, a Titaness in Greek mythology associated with the Moon, who was the sister of Cronus (the Greek equivalent of the Roman god Saturn). It is also designated Saturn IX in some scientific literature. The IAU nomenclature standards have stated that features on Phoebe are to be named after characters in the Greek myth of Jason and the Argonauts. In 2005, the International Astronomical Union officially named 24 craters (see Named features).

Toby Owen of the University of Hawaii at Mānoa, chairman of the International Astronomical Union Outer Solar System Task Group said:

We picked the legend of the Argonauts for Phoebe as it has some resonance with the exploration of the Saturn system by Cassini–Huygens. We can't say that our participating scientists include heroes like Hercules and Atalanta, 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.

Orbit

Phoebe's orbit is retrograde; 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'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 inner moons of Saturn (except for Hyperion) do. Phoebe orbits within 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

Main article: Rings of Saturn#Phoebe ring

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.{{Cite web | access-date = 2009-12-22 | archive-date = 2012-02-03 | archive-url = https://web.archive.org/web/20120203132024/http://ciclops.org/view.php?id=6033 | url-status = dead

Material displaced from Phoebe's surface by microscopic meteor impacts may be responsible for the dark areas on the surface of Hyperion. 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 is roughly spherical and has a diameter of (213 km), approximately one-sixteenth that of the Moon. 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 9 hours and 16 minutes, and completes a full orbit around Saturn in about 18 months. Its surface temperature is on average 75 K.

Most of Saturn's inner moons have very bright surfaces, but Phoebe's albedo is much lower in comparison (), though relatively bright compared to other irregular moons with measured albedos. 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 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 planetoids 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.

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.

Named 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

Phoebe 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.

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 2068 km 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.

Notes

References

• Phoebe Profile at NASA's Solar System Exploration site
• Cassini–Huygens Multimedia: Images: Moons: Small Moons
• Cassini Pass Reveals Moon Secrets, BBC News, June 14, 2004
• The Planetary Society: Phoebe
• Asaravala, A.; Saturn's Odd Moon Out, Wired, (May 4, 2005)
• NASA: Natural Satellite Physical Parameters
• Cassini images of Phoebe
• Images of Phoebe at JPL's Planetary Photojournal
• Movie of Phoebe's rotation from the National Oceanic and Atmospheric Administration
• Phoebe basemap (December 2005) from Cassini images
• Phoebe atlas (March 2006) from Cassini images
• Phoebe nomenclature from the USGS planetary nomenclature page
• Phoebe on T. Denk's Outer Saturnian moons websites

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