Phoebe (moon)

Template:Short description Template:More references needed {{#invoke:infobox|infoboxTemplate | class = vcard | titleclass = fn org | title = Phoebe | image = {{#invoke:InfoboxImage|InfoboxImage|image=Phoebe cassini full.jpg|upright={{#if:||1.1}}|alt=}} | caption = Cassini image of Phoebe. Jason crater occupies much of the upper image | headerstyle = {{#if:|background-color:|background-color:#E0CCFF}} | labelstyle = max-width:{{#if:||11em}}; | autoheaders = y

| header1 = Discovery

| header10 = {{#if:|Designations|Designations}}

| label11 = Template:Longitem | data11 = Saturn IX | label12 = Pronunciation | data12 = Template:IPAc-en Template:Respell<ref>Template:OED</ref> | label13 = Template:Longitem | data13 = {{#invoke:Lang|lang}} Phoíbē | label14 = Template:Longitem | data14 = | label15 = Template:Longitem | data15 = | label16 = Adjectives | data16 = Phoebean Template:IPAc-en Template:Respell<ref>Template:OED</ref> | label17 = Symbol | data17 =

| header20 = Orbital characteristics{{#ifeq:|yes| (barycentric)}}<ref name=MPC>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

| data21 = | data22 = {{#if: |Epoch {{{epoch}}}}} | data23 = {{#if: | Uncertainty parameter {{{uncertainty}}}}} | label24 = Observation arc | data24 = | label25 = Earliest precovery date | data25 = | label26 = {{#switch:{{{apsis}}} |apsis|gee|barion|center|centre|(apsis)=Apo{{{apsis}}} |Ap{{#if:|{{{apsis}}}|helion}}}} | data26 = | label27 = Peri{{#if:|{{{apsis}}}|helion}} | data27 = | label28 = Peri{{#if:|{{{apsis}}}|apsis}} | data28 = | label29 = {{#switch:{{{apsis}}} |helion|astron=Ap{{{apsis}}} |Apo{{#if:|{{{apsis}}}|apsis}}}} | data29 = | label30 = Periastron | data30 = | label31 = Apoastron | data31 = | label32 = Template:Longitem | data32 = 12 960 000 km | label33 = Template:Longitem | data33 = | label34 = Eccentricity | data34 = 0.1562415 | label35 = Template:Longitem | data35 = Template:Val | label36 = Template:Longitem | data36 = | label37 = Template:Longitem | data37 = | label38 = Template:Longitem | data38 = | label39 = Template:Longitem | data39 = | label40 = Inclination | data40 = 173.04° (to the ecliptic)
151.78° (to Saturn's equator) | label41 = Template:Longitem | data41 = | label42 = Template:Longitem | data42 = | label43 = Template:Longitem | data43 = | label44 = Template:Longitem | data44 = | label45 = Template:Longitem | data45 = | label46 = Template:Nowrap | data46 = | label47 = Satellite of | data47 = Saturn | label48 = Group | data48 = Norse group | label49 = {{#switch: |yes|true=Satellites |Known satellites}} | data49 = | label50 = Star | data50 = | label51 = Earth MOID | data51 = | label52 = Mercury MOID | data52 = | label53 = Venus MOID | data53 = | label54 = Mars MOID | data54 = | label55 = Jupiter MOID | data55 = | label56 = Saturn MOID | data56 = | label57 = Uranus MOID | data57 = | label58 = Neptune MOID | data58 = | label59 = T_Jupiter | data59 =

| header60 = Proper orbital elements

| label61 = Template:Longitem | data61 = {{#if: |{{{p_semimajor}}} AU}} | label62 = Template:Longitem | data62 = | label63 = Template:Longitem | data63 = | label64 = Template:Longitem | data64 = {{#if: |{{{p_mean_motion}}} deg Template:\yr}} | label65 = Template:Longitem | data65 = {{#if:|{{#expr:360/1 round 5}} yr
({{#expr:365.25*360/1 round 3}} d) }} | label66 = Template:Longitem | data66 = {{#if:|{{{perihelion_rate}}} arcsec Template:\yr }} | label67 = Template:Longitem | data67 = {{#if:|{{{node_rate}}} arcsec Template:\yr}}

| header70 = Template:Anchor{{#if:| Physical characteristics|Physical characteristics}}

| label71 = Dimensions | data71 = (Template:Val) × (Template:Val) × (Template:Val) km<ref name=Thomas2010>Template:Cite journal</ref> | label72 = Template:Longitem | data72 = | label73 = Template:Longitem | data73 = Template:Val<ref name=Thomas2010/><ref name="Jacobson2022"/> | label74 = Template:Longitem | data74 = | label75 = Template:Longitem | data75 = | label76 = Flattening | data76 = | label77 = Circumference | data77 = | label78 = Template:Longitem | data78 = | label79 = Volume | data79 = | label80 = Mass | data80 = Template:Val<ref name="Jacobson2022">Template:Cite journal</ref> | label81 = Template:Longitem | data81 = Template:Val<ref name="Jacobson2022"/> | label82 = Template:Longitem | data82 = 0.038–0.050 m/s²<ref name=Thomas2010/> | label83 = Template:Longitem | data83 = | label84 = Template:Longitem | data84 = ≈ Template:V2 km/s | label85 = Template:Longitem | data85 = | label86 = Template:Longitem | data86 = Template:Val (9 h 16 min 25 s ± 3 s)<ref>Template:Cite journal</ref> | label87 = Template:Longitem | data87 = | label88 = Template:Longitem | data88 = 152.14° (to orbit)<ref>Template:Cite journal</ref> | label89 = Template:Longitem | data89 = | label90 = Template:Longitem | data90 = | label91 = Template:Longitem | data91 = | label92 = Template:Longitem | data92 = | label93 = {{#if: |Template:Longitem |Albedo}} | data93 = Template:Value<ref name="NEOWISE2015"/> | label94 = Temperature | data94 = ≈ 73(?) K

| data100 = {{#if:|

Surface temp.	min	mean	max
{{{temp_name1}}}
{{{temp_name2}}}
{{{temp_name3}}}
{{{temp_name4}}}

}}

| header110 = Atmosphere

| below = {{#if:||Template:Reflist }}

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Phoebe (Template:IPAc-en Template:Respell) is the most massive irregular satellite of Saturn with a mean diameter of Template:Convert. It was discovered by William Henry Pickering on 18 March 1899<ref name="ArtizLyfe/Charlie Kovas">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> from photographic plates that had been taken by DeLisle Stewart starting on 16 August 1898 at the Boyden Station of the Carmen Alto Observatory near Arequipa, Peru. It was the first natural satellite to be discovered photographically.

Phoebe was the first target encountered upon the arrival of the Cassini 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.<ref name="Martinez">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> After the encounter and its insertion into orbit, Cassini did not go much beyond the orbit of Iapetus.

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 captured centaur that originated in the Kuiper belt.<ref name="Jewitt2007">Template:Cite journal</ref> Phoebe is the second-largest retrograde satellite in the Solar System after Triton.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

HistoryEdit

DiscoveryEdit

File:William Henry Pickering.jpg

William Henry Pickering, discoverer of Phoebe

Phoebe was discovered by William Henry Pickering on 18 March 1899<ref name="ArtizLyfe/Charlie Kovas"/> from photographic plates that had been taken starting on 16 August 1898 at the Boyden Observatory near Arequipa, Peru, by DeLisle Stewart.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref name="Pickering1899">Template:Cite journal</ref><ref>Template:Cite journal (same as above)</ref><ref>Template:Cite journal</ref> It was the first satellite to be discovered photographically.

NamingEdit

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).<ref name="Pickering1899"/> 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 IAU officially named 24 craters (see Named features).<ref>Features on Saturn's moon Phoebe given names Template:Webarchive, Spaceflight Now, February 24, 2005</ref>

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

Template:Quote

OrbitEdit

File:Animation of Phoebe orbit around Saturn.gif

Animation of Phoebe's orbit.
Template:Legend2 Template:·Template:Legend2 Template:·Template:Legend2

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.<ref name="Ashton2025a">Template:Cite arXiv</ref>

Phoebe ringEdit

File:Saturn largest ring Spitzer telescope 20091006.jpg

Artist's impression of the Phoebe ring, which dwarfs the main rings

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} 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<ref name="Verbiscer">Template:Cite journal</ref> 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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Because the ring's particles are presumed to have originated from micrometeoroid impacts on Phoebe, they should share its retrograde orbit,<ref name=sciencenews_large_ring>Template:Cite journal</ref> 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.<ref>Largest ring in solar system found around Saturn Template:Webarchive, New Scientist</ref><ref name = "Mason">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name = "Denk">Template:Cite journal</ref><ref name = "Spencer">Template:Cite journal</ref> 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.Template:Efn 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 characteristicsEdit

File:PocketSizedPhoebe.jpg

Saturn IX Phoebe compared to 1 Ceres and the Moon<ref name=Thomas2010>Template:Cite journal</ref>

File:Cratersofphoebe.jpg

Named craters on Phoebe

Phoebe is roughly spherical and has a diameter of Template:Val<ref name=Thomas2010/> (Template:Convert), 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 nine hours and 16 minutes, and completes a full orbit around Saturn in about 18 months. Its surface temperature is on average Template:Convert.

File:Phoebe 13-06-2004.jpg

CassiniTemplate:'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's albedo is much lower in comparison (Template:Value), though relatively bright compared to other irregular moons with measured albedos.<ref name="NEOWISE2015">Template:Cite journal</ref> The Phoebean surface is heavily scarred. The largest crater, Jason, is roughly 100 km in diameter.<ref name="Rambaux2020"/>

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.<ref name="Castillo-Rogez2019"/>

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 Template:Convert 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.<ref>Template:Cite journal</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>

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.<ref name=planetlike>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Named featuresEdit

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.<ref>Phoebean craters Template:Webarchive, USGS</ref>

Regios
Name	Pronunciation	Greek	Coordinates	Diameter (km)	Approval Date	Named After	Ref
Leto Regio	Template:IPAc-en	Λητώ	Template:Coord	95	2000	Daughter of Phoebe in Greek mythology	Template:WGPSN

Named Phoebean craters
Name	Pronunciation	Greek	Coordinates	Diameter (km)	Approval Date	Named After	Ref
Acastus	Template:IPAc-en	Ἄκαστος	Template:Coord	34	2006	Son of Thessalian king Pelias, took part in the Calydonian boar hunt	Template:WGPSN
Admetus	Template:IPAc-en	Ἄδμητος	Template:Coord	58	2006	Founder and king of Pherae in Thessaly	Template:WGPSN
Amphion	Template:IPAc-en	Ἀμφῑ́ων	Template:Coord	18	2006	Son of Hyperasius and Hypso	Template:WGPSN
Butes	Template:IPAc-en	Βούτης	Template:Coord	29	2006	Son of Teleon, bee-master	Template:WGPSN
Calais	Template:IPAc-en	Κάλαϊς	Template:Coord	31	2006	Son of Boreas, the north wind	Template:WGPSN
Canthus	Template:IPAc-en	Κάνθος	Template:Coord	44	2006	Son of Kanethos or Cerion, the only member of the expedition to die in combat	Template:WGPSN
Clytius	Template:IPAc-en	Κλυτίος	Template:Coord	52	2006	Son of Eurytus, skilled archer who was killed by Apollo for challenging the god to a shooting match	Template:WGPSN
Erginus	Template:IPAc-en	Ἐργῖνος	Template:Coord	38	2006	Son of Neptune, helmsman of the Argo after the death of Tiphys	Template:WGPSN
Euphemus	Template:IPAc-en	Εὔφημος	Template:Coord	23	2006	Son of Neptune and Europa	Template:WGPSN
Eurydamas	Template:IPAc-en	Εὐρυδάμᾱς	Template:Coord	19	2006	Son of Ctimenus	Template:WGPSN
Eurytion	Template:IPAc-en	Εὐρυτίων	Template:Coord	14	2006	Son of Kenethos or Cerion	Template:WGPSN
Eurytus	Template:IPAc-en	Εὔρυτος	Template:Coord	89	2006	Son of Mercury and Antianira	Template:WGPSN
Hylas	Template:IPAc-en	Ὕλας	Template:Coord	30	2006	Son of Theiodamas/Theodamas, king of the Dryopes	Template:WGPSN
Idmon	Template:IPAc-en	Ἴδμων	Template:Coord	61	2006	Son of Apollo and the nymph Cyrene, or of Abas, a prophet	Template:WGPSN
Iphitus	Template:IPAc-en	Ἴφιτος	Template:Coord	22	2006	Son of Eurytus, Jason's host during his consultation with the Oracle at Delphi	Template:WGPSN
Jason	Template:IPAc-en	Ἰάσων	Template:Coord	101	2006	The leading Argonaut, son of the Thessalian king Aeson, delivered the Fleece	Template:WGPSN
Mopsus	Template:IPAc-en	Μόψος	Template:Coord	37	2006	Prophesying son of Apollo	Template:WGPSN
Nauplius	Template:IPAc-en	Ναύπλιος	Template:Coord	24	2006	Son of Neptune and Amymone, or of Klytoneos	Template:WGPSN
Oileus	Template:IPAc-en	Ὀϊλεύς	Template:Coord	56	2006	King of the Locrians, renowned for his courage in battle	Template:WGPSN
Peleus	Template:IPAc-en	Πηλεύς	Template:Coord	44	2006	Son of Aeacus, father of Achilles	Template:WGPSN
Phlias	Template:IPAc-en	Φλίας	Template:Coord	14	2006	Son of Dionysus	Template:WGPSN
Talaus	Template:IPAc-en	Ταλαός	Template:Coord	15	2006	Son of Teleon, or of Bias and Pero	Template:WGPSN
Telamon	Template:IPAc-en	Τελαμών	Template:Coord	28	2006	Son of Aeacus, took part in the Calydonian boar hunt	Template:WGPSN
Zetes	Template:IPAc-en	Ζήτης	Template:Coord	29	2006	Son of Boreas, the north wind	Template:WGPSN

MapsEdit

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

FormationEdit

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.<ref name=planetlike/>

Observation and explorationEdit

File:Phoebe-2010Mar09.jpg

Phoebe (with NGC 4179 in the lower right corner) as imaged with a 24" telescope

File:Saturn's outer satellite - Phoebe PIA02229.jpg

Images of different hemispheres of Phoebe by Voyager 2

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.<ref>Template:Cite journal</ref>

Cassini passed Template:Convert from Phoebe on 11 June 2004,<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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<ref name="Martinez"/> 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 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.<ref>Roth et al., AAS Paper 05-311</ref>