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[15.0] Missions To Saturn (2)

v2.0.0 / chapter 15 of 20 / 01 jul 16 / greg goebel

* 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.

Saturn's rings & Mimas


[15.1] SATURN IN THE SPACE AGE
[15.2] SATURN STATISTICS

[15.1] SATURN IN THE SPACE AGE

* 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.

moons of Saturn

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.

rings of Saturn

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."

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[15.2] SATURN STATISTICS

* 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.

Saturn scale diagram

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.
  __________________________________________________________________________

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