* The observations of Saturn performed by planetary probes have given us new insights into the ringed world, but much more remains to be learned, with the elaborate ring system still mysterious in many ways.
* The gross properties of the Saturn system had been apparent from Earth-based observations, and so the space probes that passed by revealed little dramatically new about the planet itself. The spacecraft did confirm the existence of a strong magnetic field; Saturn's magnetic field is 1,000 times stronger than the Earth's, but only about 1/20th as strong as Jupiter's. The magnetic poles are aligned to within 1 degree of the axis of rotation. Such a close alignment is unusual.
Images of the cloud features returned from the planet were interesting, but also revealed nothing that was particularly surprising. Saturn's cloud activity was known to be subdued compared to that of Jupiter, due to lower temperatures and gravity.
The sunlight falling on Saturn is relatively faint and weak, and so the average difference between polar and equatorial temperatures there is only about five degrees Kelvin. This means that most of the weather patterns are driven by the planet's internal energy and its fast rate of rotation. The end result is the planet's banded appearance, which is a consequence of zonal winds. These winds are very fast: the equatorial belt has a velocity of 500 meters per second. The zonal winds are symmetrical with respect to the hemispheres, demonstrating the relative unimportance of solar radiation on Saturn.
The probes did observe spots, the most spectacular being "Anne's Spot", a cyclonic storm observed by the Voyagers with a width of 5,000 by 3,000 kilometers, much smaller than Jupiter's Great Red Spot. In 2006, the Cassini probe observed a particularly interesting cyclonic storm at the planet's south pole, about 8,000 kilometers across. The storm was unusual in that it featured an "eye" like that of Earthly hurricanes, which hasn't been observed in other large cyclonic storms on the gas giant planets. Another Great White Spot appeared in 2010, about ten years ahead of the expected schedule, with this appearance leading to a spectacular cloud trail across the planet.
* Although observations of Saturn itself were more interesting than revolutionary, of course the unprecedented close-up images of the moons and particularly the ring system provided by spacecraft flybys provided plenty of surprises.
The probes, as well as exacting Earth-based observations performed during the missions and afterward, discovered some new small moons, and provided more detailed observations of the "classic" moons of Saturn. In order from outermost to innermost:
Voyager's additions included:
* The probes provided very significant revelations about Saturn's ring system. They discovered three two more main rings, including the broad and faint outermost "E Ring" (which had been tentatively spotted by Feibelman in the 1960s), the faint but smaller "G Ring" inside the E Ring, and the strandlike "F Ring", which circled the A Ring. The rings were determined to have a typical depth of about 200 meters.
The discoveries led to a modern breakdown of the ring system as follows, arranged from outermost to innermost:
One of the big surprises was that each of the "main" rings consists of hundreds of very narrow rings. One of the outer rings has an oddly twisted or braided appearance, the reason for which is still not fully understood. The spokelike features that O'Meara had vaguely seen were observed in the B Ring by Voyager 1, and still remain mysterious. They may be due to an interaction with Saturn's magnetic field.
Cassini returned an even bigger surprise by providing evidence that the rings are much more stable than previously thought, and in fact may have been around for billions of years. The conventional wisdom was that impacts of ring particles would gradually pulverize them and the rings would eventually dissipate, but it appears instead that the particles will either clump together or break apart, and so the rings are in a good approximation of a steady state.
* Nine new moons of Saturn were thought to have been discovered in 1981 from analysis of Voyager 2 data; seven more were thought to have been discovered in 1995 from Hubble Space Telescope imagery. All of these were in the vicinity of or associated with the ring system, but they were not confirmed and are now generally suspected to have been at best just temporary clumpings of ring material. They have now been generally discounted, though Cassini observations should provide conclusive proof one way or another. However, over 30 irregular moonlets were discovered by Earth-based telescopes in the 21st century:
The irregular moonlets occur in three groups:
Trying to catalog the ever-growing number of ring moonlets is proving burdensome. Sooner or later the search for moonlets around the gas giant planets is certain to run into diminishing returns. Astronomers believe that many more moons will be found as telescope technology improves, until eventually nobody finds it interesting any more. As one astronomer put it: "At some point, you have to decide when to quit."BACK_TO_TOP
* Statistics for Saturn:
__________________________________________________________________________ mean distance from Sun 9.54 AU (1427.0 x 10^6 kilometers) orbital period (sidereal) 29.46 years orbital eccentricity 0.056 orbital inclination 2.5 degrees equatorial diameter 120,000 km (9.41 Earth) mass (relative to Earth) 95.159 mean density (relative to water) 0.70 gravity (relative to Earth) 1.08 escape speed 35.6 kilometers per second rotation period 0.44 days oblateness 1/9 inclination of equator 26.7 degrees albedo 0.47 max surface temperature -180 degrees Celsius (cloud tops) atmosphere (major constituents ) H, He, CH4 clouds of NH3, NH4SH, H2O atmospheric pressure at surface not applicable number of moons > 10 km in size 9, plus dozens of known moonlets __________________________________________________________________________
* Regular moons of Saturn, from outermost to innermost, with pronunciations in parenthesis.
Radii are given from the center of Saturn. The abbreviation "RS" stands for the radius of Saturn, while "RM" stands for the radius of the orbit of the Earth's Moon, and "M" stands for the diameter or mass of the Earth's Moon. Densities are given relative to water, which is equivalent to grams per cubic centimeter. Orbital and rotation periods are given in days and fractions of days, with days-hours-minutes added for periods under two days. Moonlets less than a kilometer in diameter are not listed.
__________________________________________________________________________ PHOEBE ("FEE-bee") / Saturn IX / irregular (Norse group): mean distance from planet 12,952,000 km / 216 RS / 33.71 RM orbital period (sidereal) 550.46 days orbital eccentricity 0.163 orbital inclination 175.3 degrees (retrograde) equatorial diameter 220 km / 0.06 M mass 0.000054 M mean density 0.7 albedo 0.06 year of discovery 1898 (Pickering) __________________________________________________________________________ IAPETUS ("eye-AP-i-tus") / Saturn VIII: mean distance from planet 3,561,300 km / 59.4 RS / 9.26 RM orbital period (sidereal) 79.331 days orbital eccentricity 0.028 orbital inclination 14.7 degrees equatorial diameter 1,460 km / 0.42 M mass 0.026 M mean density 1.2 albedo 0.2 year of discovery 1671 (Cassini) __________________________________________________________________________ HYPERION ("hy-PEER-ee-en") / Saturn VII: mean distance from planet 1,481,000 km / 24.7 RS / 3.85 RM orbital period (sidereal) 21.276 days orbital eccentricity 0.104 orbital inclination 0.4 degrees dimensions 410 x 260 x 220 km mass 0.00024 M mean density 1.4 albedo 0.3 year of discovery 1848 (Bond, Bond, Lassell) __________________________________________________________________________ TITAN ("TY-tun") / Saturn VI: mean distance from planet 1,221,850 km / 20.4 RS / 3.18 RM orbital period (sidereal) 15.945 days orbital eccentricity 0.029 orbital inclination 0.33 degrees equatorial diameter 5,150 km / 1.48 M mass 1.83 M mean density 1.88 albedo 0.21 year of discovery 1655 (Huygens) __________________________________________________________________________ RHEA ("REE-a") / Saturn V: mean distance from planet 527,040 km / 8.77 RS / 1.37 RM orbital period (sidereal) 4.517 days / 4 days 12 hours 10 minutes orbital eccentricity 0.001 orbital inclination 0.35 degrees equatorial diameter 1,530 km / 0.44 M mass 0.034 M mean density 1.3 albedo 0.7 year of discovery 1672 (Cassini) __________________________________________________________________________ DIONE ("dy-OH-nee") / Saturn IV: mean distance from planet 377,400 km / 6.3 RS / 0.98 RM orbital period (sidereal) 2.737 days orbital eccentricity 0.002 orbital inclination 0.02 degrees equatorial diameter 1,120 km / 0.32 M mass 0.014 M mean density 1.4 albedo 0.7 year of discovery 1684 (Cassini) HELENE / Saturn XII: CO-ORBITAL with Dione, at leading libration point. dimensions 36 x 32 x 30 km mass UNKNOWN albedo 0.7 year of discovery 1980 (Laques, Lecacheux) __________________________________________________________________________ POLYDEUCES ("polly-deuces") / Saturn XXXIV (S/2004 S5) mean distance from planet 377,396 km / 6.29 RS / 0.99 RM orbital period (sidereal) 2.737 days orbital eccentricity 0 orbital inclination 0 equatorial diameter 3.5 km mass / density: UNKNOWN albedo UNKNOWN year of discovery 2004 (Cassini) __________________________________________________________________________ TETHYS ("TEE-this") / Saturn III: mean distance from planet 294,660 km / 4.92 RS / 0.77 RM orbital period (sidereal) 1.888 days / 1 day 21 hours 19 minutes orbital eccentricity 0 orbital inclination 1.1 degrees equatorial diameter 1,060 km / 0.30 M mass 0.010 M mean density 1.2 albedo 0.9 year of discovery 1684 (Cassini) CALYPSO ("ka-LIP-so") / Saturn XIV: CO-ORBITAL with Tethys, at leading libration point dimensions 34 x 22 x 22 km mass / density UNKNOWN albedo 0.6 year of discovery 1980 (Pascu, Seidelmann, Baum, Currie) TELESTO ("ta-LESS-toh") / Saturn XIII: CO-ORBITAL with Tethys, at trailing libration point dimensions 34 x 28 x 26 km mass / density UNKNOWN albedo 0.5 year of discovery 1980 (Smith, Larson, Reitsma) __________________________________________________________________________ ENCELADUS ("en-SELL-a-dus") / Saturn II: mean distance from planet 238,000 km / 3.97 RS / 0.62 RM orbital period (sidereal) 1.370 days / 1 day 8 hours 53 minutes orbital eccentricity 0.004 orbital inclination 0.02 degrees equatorial diameter 500 km / 0.14 M mass 0.0011 M mean density 1.24 albedo 1.0 year of discovery 1789 (Herschel) __________________________________________________________________________ PALLENE ("pah-LEE-nee") / Saturn XXXIII (S/2004 S2) mean distance from planet 212,280 km / 3.54 RS / 0.56 RM orbital period (sidereal) 1.154 days / 27 hours 42 minutes orbital eccentricity 0 orbital inclination 0.2 equatorial diameter 4 km mass / density: UNKNOWN albedo UNKNOWN year of discovery 2004 (Cassini) __________________________________________________________________________ ANTHE ("AN-thee") / Saturn XLIX (S/2007 S4) mean distance from planet 197,700 km / 3.29 RS / 0.52 RM orbital period (sidereal) 1.036 days / 24 hours 52 minutes orbital eccentricity 0 orbital inclination 0.1 degrees equatorial diameter ~2 km mass / density: UNKNOWN albedo UNKNOWN year of discovery 2007 (Cassini) __________________________________________________________________________ METHONE ("me-THO-nee") / Saturn XXXII (S/2004 S1) mean distance from planet 194,440 km / 3.24 RS / 0.51 RM orbital period (sidereal) 1.010 days / 24 hours 14 minutes orbital eccentricity 0 orbital inclination 0 equatorial diameter ~3 km mass / density: UNKNOWN albedo UNKNOWN year of discovery 2004 (Cassini) __________________________________________________________________________ MIMAS ("MY-mass") / Saturn I: mean distance from planet 185,520 km / 3.12 RS / 0.49 RM orbital period (sidereal) 0.942 days / 22 hours 36 minutes orbital eccentricity 0.020 orbital inclination 1.5 degrees equatorial diameter 392 km / 0.11 M mass 0.00052 M mean density 1.17 albedo 0.5 year of discovery 1789 (Herschel) __________________________________________________________________________ JANUS ("JAY-nus") / Saturn X: CO-ORBITAL with Epimetheus, in unusual "reciprocal" orbit. mean distance from planet 151,472 km / 2.52 RS / 0.39 RM orbital period (sidereal) 0.694 days / 16 hours 39 minutes orbital eccentricity 0.007 orbital inclination 0.14 degrees dimensions 196 x 192 x 150 km mass 0.000003 M? mean density 0.67? albedo 0.8 year of discovery 1966 (Dollfus) EPIMETHEUS ("ep-eh-MEE-thee-us") / Saturn XI CO-ORBITAL with Janus, in unusual "reciprocal" orbit. dimensions 144 x 108 x 98 km mass 0.000008 M? mean density 0.7? albedo 0.8 year of discovery 1966 (Fountain, Larson) __________________________________________________________________________ PANDORA ("pan-DOH-ruh") / Saturn XVII: mean distance from planet 141,700 km / 2.37 RS / 0.37 RM orbital period (sidereal) 0.628 days / 15 hours 4 minutes orbital eccentricity 0.004 orbital inclination 0 dimensions 114 x 84 x 62 km mass 0.000003 M? mean density 0.7? albedo 0.9 year of discovery 1980 (Collins, Carlson) __________________________________________________________________________ PROMETHEUS ("pra-MEE-the-us") / Saturn XVI: mean distance from planet 139,350 km / 2.32 RS / 0.36 RM orbital period (sidereal) 0.613 days / 14 hours 17 minutes orbital eccentricity 0.003 orbital inclination 0 dimensions 145 x 85 x 65 km mass 0.000004 M? mean density 0.7? albedo 0.6 year of discovery 1980 (Collins, Carlson) __________________________________________________________________________ ATLAS / Saturn XV: mean distance from planet 137,640 km / 2.28 RS / 0.36 RM orbital period (sidereal) 0.602 days / 14 hours 27 minutes orbital eccentricity 0 orbital inclination 0 dimensions 40 x 30 km mass / density: UNKNOWN albedo 0.9 year of discovery 1980 (Terrile) __________________________________________________________________________ DAPHNIS ("dafniss") / Saturn XXXV (S/2005 S1) mean distance from planet 136,504 km / 2.28 RS / 0.36 RM orbital period (sidereal) 0.594 days / 14 hours 15 minutes orbital eccentricity 0 orbital inclination 0 equatorial diameter ~7 km mass / density: UNKNOWN albedo UNKNOWN year of discovery 2005 (Cassini) __________________________________________________________________________ PAN / Saturn XVIII: mean distance from planet 133,583 km / 2.23 RS / 0.35 RM orbital period (sidereal) 0.575 days / 13 hours 48 minutes orbital eccentricity 0 orbital inclination 0 equatorial diameter 20 km mass / density: UNKNOWN albedo 0.5 year of discovery 1990 (Showalter) __________________________________________________________________________
Ring moonlets observed so far include Aegaeon, in the G Ring, and S/2009 S1, in the B Ring. More are likely to be found.
* Irregular moons of Saturn:
_____________________________________________________________ preliminary moon designation group _____________________________________________________________ YMIR Saturn XIX S/2000 S1 Norse group PAALIAQ Saturn XX S/2000 S2 Inuit group SIARNAQ Saturn XXIX S/2000 S3 Inuit group TARVOS Saturn XXI S/2000 S4 Gallic group KIVIUQ Saturn XXIV S/2000 S5 Inuit group IJIRAQ Saturn XXII S/2000 S6 Inuit group THRYMR Saturn XXX S/2000 S7 Norse group SKATHI Saturn XXVII S/2000 S8 Norse group MUNDILFARI Saturn XXV S/2000 S9 Norse group ERRIAPUS Saturn XXVIII S/2000 S10 Gallic group ALBIORIX Saturn XXVI S/2000 S11 Gallic group SUTTUNGR Saturn XXIII S/2000 S12 Norse group NARVI Saturn XXXI S/2003 S1 Norse group - S/2004 S7 Norse group FORNJOT Saturn XLII S/2004 S8 Norse group FARBAUTI Saturn XL S/2004 S9 Norse group AEGIR Saturn XXXVI S/2004 S10 Norse group BEBHIONN Saturn XXXVII S/2004 S11 Gallic group - S/2004 S12 Norse group - S/2004 S13 Norse group HATI Saturn XLIII S/2004 S14 Norse group BERGELMIR Saturn XXXVIII S/2004 S15 Norse group FENRIR Saturn XLI S/2004 S16 Norse group - S/2004 S17 Norse group BESTLA Saturn XXXIX S/2004 S18 Norse group HYRROKKIN Saturn XLIV S/2004 S19 Norse group - S/2006 S1 Norse group KARI Saturn XLV S/2006 S2 Norse group - S/2006 S3 Norse group GREIP Saturn LI S/2006 S4 Norse group LOGE Saturn XLVI S/2006 S5 Norse group JARNSAXA Saturn L S/2006 S6 Norse group SURTUR Saturn XLVIII S/2006 S7 Norse group SKOLL Saturn XLVII S/2006 S8 Norse group TARQEQ Saturn LII S/2007 S1 Inuit group - S/2007 S2 Norse group - S/2007 S3 Norse group _____________________________________________________________
* Rings of Saturn:
__________________________________________________________________________ E RING 480,000 - 180,000 km / 8.0 - 3.0 RS / 1.25 - 0.47 RM G RING 173,800 - 165,800 km / 2.9 - 2.76 RS / 0.45 - 0.43 RM F RING 140,350 - 140,200 km / 2.34 RS / 0.37 RM A RING 136,000 - 122,200 km / 2.27 - 2.04 RS / 0.35 - 0.32 RM cassini division B RING 117,500 - 92,000 km / 1.96 - 1.53 RS / 0.31 - 0.24 RM maxwell division C RING 92,000 - 74,500 km / 1.53 - 1.24 RS / 0.24 - 0.19 RM guerin division D RING 74,500 - 67,000 km / 1.24 - 1.12 RS / 0.19 - 0.17 RM __________________________________________________________________________ The Encke division bisects the A Ring at 2.23 RS. __________________________________________________________________________BACK_TO_TOP