General Astronomy I

General Astronomy ISaturn (& 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 9h50m30.0s

System II elsewhere 9h55m40.6s

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very close to System II, it has largely superseded it.


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10 h 39 min 22.4 s very close to System II, it has largely superseded it.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)


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





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, 2008 106.4 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 2008 17.1 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 1

2003 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 2025

48 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

Voyager

Jupiter – Galileo probe Galileo

launched Oct 18 1989 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




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 2002 14 yrs in space/8 yrs around Jupiter crashed into Jupiter’s atmosphere final mission: measure Amalthea’s mass

Hitchhiker’s Guide to the Jovians

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

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

9.537 AU 1.427 million km

eccentricity 0.054 orbital inclination 2.48° orbital period 29.45 years period of rotation 10h47m6s axial tilt 26.73°

Saturn – physical data Characteristics

Equatorial diameter 120,536 km 9.449 Earths

Mass 5.685x1026 kg 95.162 Earth density 0.6873 g/cm3 surface gravity 0.914 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

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

10-30 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 1850 5 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 1 2005

Saturn Keeler Gap

shepherd moon Daphnis

Saturn

Saturn Shepherd Moons

F-ring shepherds Prometheus (102 km) Pandora (84 km)

image 15 Aug 1981 Voyager 2


F-ring shepherds Prometheus (102 km) Pandora (84 km)

image 1994 Cassini


Prometheus 14.7 hr orbit periodic ‘gores’


Pandora (84 km) Janus (181) Prometheus (102 km)


Prometheus (102 km)


Keeler Gap Keeler moon (7 km

across) Enke Gap (at right)

Pan


Pan (26 km across) Spiral density waves

propagate outward

Pan


Keeler Gap


Pandora in the F-ring 84 km


Pandora in the F-ring 84 km

Saturn’s Moons

Saturn’s Moons Enceladus

6th largest water ice surface albedo ~1 terrain old cratered to

tectonically deformed non-global

atmosphere 2:1 resonance with

Dione

Saturn’s Moons

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

3rd largest peculiar shape

neither spherical nor ellipsoid leading hemisphere dark

albedo ~ 0.03-0.05 slight reddish-brown

poles & trailing hemisphere bright albedo 0.5-0.6

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


Enceladus non-global

atmosphere


Dione (1118 km)


Dione (1118 km)

Rings edge-on dominated by F-ring


Telesto (24 km)


Hyperion too small to pull into

spherical shape


Tethys (1071 km) Craters

Penelope Antinous


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 2nd 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 (NH3).

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!

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General Astronomy I