General Astronomy I Saturn (& Voyagers) Chapter 23

Rotation of Jovians Jupiter solar system's fastest slightly less than ten hours  creates equatorial bulge easily seen from Earth  centripetal acceleration at equator ~1.67 m/s²  equatorial surface gravity ~24.79 m/s²  net acceleration at equator ~23.12 m/s²  equatorial diameter greater than polar  equatorial diameter is 9275 km longer

Rotation of Jovians Jupiter not solid – differential rotation in upper atmosphere rotation of polar atmosphere ~5 minutes longer three systems are used  System I applies from the latitudes 10° N to 10° S  9 h 50 m 30.0 s  System II elsewhere  9 h 55 m 40.6 s  System III – first defined by radio astronomers  corresponds to the rotation of magnetosphere  “official” rotation

Rotation of Jovians Saturn System I – equatorial zone 10 h 14 min 00 s System II – all other latitudes 10 h 39 min 24 s System III – based on radio emissions 10 h 39 min 22.4 s  very close to System II, it has largely superseded it.

Rotation of Jovians Saturn System III – based on radio emissions from the planet 10 h 39 min 22.4 s  very close to System II, it has largely superseded it. While approaching Saturn in 2004, Cassini found radio rotation period had increased slightly  to ~10 h 45 m 45 s (± 36 s)  cause unknown  thought due to movement of radio source to different latitude inside Saturn (with a different rotational period)

Rotation of Jovians Saturn March 2007, announced rotation of the radio emissions did not actually trace the rotation of planet produced by convection of the plasma disc, independent of rotation  variance may be caused by geyser activity on Enceladus  water vapor emitted into Saturn's orbit becomes charged and “weighs down” Saturn's magnetic field If true, no known method of determining actual rotation of core

Rotation of Jovians Saturn

Rotation of Jovians Saturn

Rotation of Jovians Saturn

Rotation of Jovians Saturn

Jupiter Missions Pioneer 10 – Dec 1973 Pioneer 11 – Dec 1974 Voyager 1 – Mar 1979 Voyager 2 – Jul 1979 Galileo – Dec 1995* Ulysses – Feb 1992/Feb 2004 Cassini – Dec 2000 New Horizons – Feb 2007

Voyager Voyager 1 launched Sept 1977 currently operational longest-lasting farthest from Earth

Voyager Voyager 1 to be Mariner 11 alignment of planets Grand Tour gas giants in 12 yrs  instead of 30 Jupiter Jan 1979 Saturn November 1980 Grand Tour terminated

Voyager Voyager 1 As of May 22, AU (9.90 billion miles) from Sun  more distant than any known solar-system object  light takes over 14.6 hours to reach spacecraft from Earth  Moon ~1.4 light seconds  Sun ~8.5 light minutes  Pluto averages ~5.5 light hours

Voyager Voyager 1 as of May km/s relative to the sun 3.6 AU/year 38,400 miles per hour

Voyager Voyager 1 most distant man-made object most distant known Solar System object

Voyager Voyager terminate scan platform ~2010 terminate gyro ops >2020 power won’t operate any single instrument RTG Plutonium-238

Voyager Voyager 1 estimated sufficient power for radio transmitters until at least years after launch

Voyager Web site

Voyager Voyager 2 was to be Mariner 12 launched Aug 1977 most prolific probe ever completed Grand Tour problems ground crews forgot to send important activation code  caused the probe to shut down its main high-gain antenna  crews established contact through low-gain antenna and activated it

Voyager

Jupiter – Galileo probe Galileo launched Oct shuttle Atlantis STS-34 arrived Dec 7, 1995 firsts discovered asteroid moon orbit jupiter launched probe into atmosphere mission terminated Sep 14, 2005

Jupiter – Galileo probe

Malfunctions high-gain antenna 134,000 bits/sec low-gain 8-16 bits/sec data compression  160 bps tape recorder radiation anomalies parachute near-failure

Jupiter – Galileo probe

End of mission cameras deactivated 17 Jan, 2002 irrecoverable radiation damage  Io damaged tape recorder electronics recovered deorbited 21 Sept yrs in space/8 yrs around Jupiter crashed into Jupiter’s atmosphere final mission: measure Amalthea’s mass

Hitchhiker’s Guide to the Jovians

much larger than terrestrial planets thick atmospheres, but very inhospitable no solid surfaces mostly hydrogen and helium strong atmospheric circulation (winds/storms) cloud belt patterns rings multiple moons

Saturn

Cassini-Huygens NASA/ESA/ASI joint mission named for Giovanni Domenico Cassini French-Italian Christiaan Huygens Dutch astronomer launched 15 Oct 1997 reached Saturn Orbit 1 July 2004

Saturn

eclipsing Sun

Changing views

Saturn Named for Roman god Saturn greek Kronos symbol is stylized sickle known for magnificent rings 56 confirmed satellites

Saturn – physical data Orbit AU million km eccentricity orbital inclination 2.48° orbital period years period of rotation 10 h 47 m 6 s axial tilt 26.73°

Saturn – physical data Characteristics Equatorial diameter 120,536 km Earths Mass 5.685x10 26 kg Earth density g/cm 3 surface gravity gee mean cloudtop temperature: 93 K albedo 0.47

Saturn comparison ~1/3 mass of Jupiter ~16% smaller rotates as rapidly, but twice as oblate no large heavy-element core radiates ~1.8 x energy received from Sun likely heated by liquid He droplets falling toward center

Saturn Magnetosphere ~20 x weaker than Jupiter’s weaker radiation belts not inclined against rotation axis  aurorae centered around poles

Saturn

Atmosphere cloud-belt structure not as distinct as Jupiter’s colder: less energy to drive weather

Planetary Atmospheres

Saturn Atmosphere Three-layered cloud structure, just like on Jupiter Fewer wind zones much stronger winds winds up to ~500 m/s near the equator

Saturn

Cassini images northern hemisphere is changing colors now appears a bright blue similar to Uranus unobservable from Earth – rings in the way one theory: shadows cast by rings cause yellow clouds to cool & sink, revealing deeper blue atmosphere Saturn’s colors

Saturn’s neighborhood

Saturn

Saturn’s Rings Composition ice & dust observed 1610 Galileo 1655 Christiaan Huygens proposed ring several gaps

Saturn’s Rings A ring outermost bounded by Cassini division & Atlas meter thick outer edge 7:6 resonance with Janus

Saturn’s Rings B ring innumerable ringlets some eccentric orbits spokes… 5-10 m thick resonance with Mimas

Saturn’s Rings spokes Voyager 2 22 Aug 1981

Spokes Until 1980, the structure of rings explained exclusively as the action of gravitational forces Voyager spacecraft found radial features in the B ring, called spokes not explainable in this manner persistence and rotation around the rings were inconsistent with orbital mechanics

Spokes Until 1980, the structure of rings explained exclusively as the action of gravitational forces spokes assumed connected to electromagnetic interactions rotate almost synchronously with the magnetosphere of Saturn precise mechanism remains unknown

Spokes imaged by Cassini 2005 appear to be a seasonal phenomenon disappear Saturnian midwinter/midsummer reappear as Saturn comes closer to equinox not visible when Cassini arrived early 2004

Spokes imaged by Cassini 2005 some scientists speculated that the spokes would not be visible again until 2007, based on models attempting to describe spoke formation reappeared in images taken September 5, 2005

Saturn’s Rings C ring wide & faint discovered meter thick

Saturn’s Rings Cassini Division 4800 km across 1675 Giovanni Cassini full of tiny rings

Saturn’s Rings Encke Gap gap in A ring Pan Cassini observes 2 knotted ringlets

Saturn’s Rings Keeler Gap 42 km gap in A ring Daphnis discovered May

Saturn Keeler Gap shepherd moon Daphnis

Saturn

Shepherd Moons F-ring shepherds Prometheus (102 km) Pandora (84 km) image 15 Aug 1981 Voyager 2

Saturn Shepherd Moons F-ring shepherds Prometheus (102 km) Pandora (84 km) image 1994 Cassini

Saturn Shepherd Moons Prometheus 14.7 hr orbit periodic ‘gores’

Saturn Shepherd Moons Pandora (84 km) Janus (181) Prometheus (102 km)

Saturn Shepherd Moons Prometheus (102 km)

Saturn Shepherd Moons Keeler Gap Keeler moon (7 km across) Enke Gap (at right) Pan

Saturn Shepherd Moons Pan (26 km across) Spiral density waves propagate outward

Pan

Saturn Shepherd Moons Keeler Gap

Saturn Shepherd Moons Pandora in the F-ring 84 km

Saturn Shepherd Moons Pandora in the F-ring 84 km

Saturn’s Moons

Enceladus 6 th largest water ice surface albedo ~1 terrain old cratered to tectonically deformed non-global atmosphere 2:1 resonance with Dione

Saturn’s Moons

Cryovolcanism

Saturn’s Moons Tethys icy heavily cratered regions some less cratered active in past

Saturn’s Moons Tethys Odysseus crater 400 km ~2/5 of Tethys

Saturn’s Moons Dione mostly ice rocky interior varied terrain

Saturn’s Moons Rhea ice small rocky core evidence of cryovolcanism

Saturn’s Moons Iapetus 3 rd largest peculiar shape neither spherical nor ellipsoid leading hemisphere dark albedo ~ slight reddish-brown poles & trailing hemisphere bright albedo

Saturn’s Moons Iapetus no bright craters on dark side indicates recent deposits equatorial ridge Cassini Dec 2004 some parts 20 km high

Saturn’s Moons

Saturn Resonances orbital period of a moon is a small-number fractional multiple (e.g., 2:3) of the orbital period of material in the disk resonance material is cleared out  division!

Saturn Resonances That’s no moon – that’s a battle station… Mimas

Saturn’s Moons Mimas mostly water ice small rock component Herschel crater 130 km ~1/3 diameter Cassini division 2:1 orbital resonance

Saturn’s Moons Smaller moons Mimas

Saturn Smaller moons Mimas

Saturn Smaller moons Enceladus non-global atmosphere

Saturn Smaller moons Dione (1118 km)

Saturn Smaller moons Dione (1118 km) Rings edge-on dominated by F-ring

Saturn Smaller moons Telesto (24 km)

Saturn Smaller moons Hyperion too small to pull into spherical shape

Saturn Smaller moons Tethys (1071 km) Craters  Penelope  Antinous

Saturn Smaller moons Tethys & Mimas

Saturn Smaller moons Atlas (32 km) between broad A & F

Saturn Origin of Moons no evidence of common origin, as for Jupiter’s moons probably captured icy planetesimals moons interact gravitationally, mutually affecting each other’s orbits Co-orbital moons (orbits separated by only 100 km) periodically exchange orbits Small moons are also trapped in Lagrange points of larger moons Dione and Tethys

Saturn’s Moons Titan & Enceladus icy jets at south pole (down)

Saturn’s Moons

Saturn Titan Explored in detail by the Cassini 2005 Huygens probe about size of Ganymede 2 nd largest in solar system rocky core/large amount of ice

Saturn Titan thick atmosphere hides surface from direct view

Saturn Titan Atmosphere mostly nitrogen, methane and ethane Surface pressure 50% greater than Earth’s Surface temperature: 94 K (–290° F)

Saturn Titan Atmosphere methane and ethane can condense and lead to rain of methane and ethane Methane is gradually converted to ethane in the atmosphere

Saturn Titan Atmosphere Methane must be constantly replenished, probably through breakdown of ammonia (NH 3 ).

Saturn’s Moons Epimetheus (116 km across) as it passes in front of Titan (5,150 km) and then Dione (1,126 km)

Saturn’s Moons Janus and Epimetheus swinging around Saturn's rings and past Dione

Saturn’s Moons crater-covered moon Rhea slips between the moons Mimas (top) and Enceladus (bottom)

Saturn’s Moons far-off Dione slips behind Rhea

Wow!