Saturn, the ringed planet, and its strange moons

Neptune

Uranus

Size comparison of the Outer Planets

Earth

Early telescopic observations of Saturn

a. Galileo, 1610; b. Huygens 1655; c. Cassini 1676

Spacecraft that have visited Saturn

• Pioneer 11 flew within 21,000 km of Saturn on 1 September 1979, discovered two new moons and an additional ring, charted Saturn's magnetosphere and magnetic field and found its planet-size moon Titan, to be too cold for life.

• The Voyager 1 and 2 Saturn encounters occurred nine months apart, in November 1980 and August 1981. Discovers larger of internal to external heat: 2-to-1 compared to 1-to-1 for Jupiter.

• Cassini orbital insertion on July 1, 2004. Took first image of a moon (Phoebe) on June 11, 2004. Still working in April 2006:

http://photojournal.jpl.nasa.gov/mission/Cassini• Cassini dropped the Huygens probe into atmosphere of

Titan on January 14, 2005.

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Cassini spacecraft;

launched on Oct. 15, 1997 from

KSC

7 Year cruise on Venus-Venus-Earth-JupiterGravity Assist Trajectory

The Cassini Spacecraft

Interior Structure of Saturn

Like on Jupiter, there are latitudinal bands of clouds and storms

Ribbon clouds in atmosphere – high-speed turbulence

Wind speeds at top of Saturn’s Atmosphere

1700 km/hr

Oppositedirection

Multiple Rings!

CC AABB

Encke Gap

FF

Cassini Division

E Ring extends to 7.5 Saturn radii from cloud top

DD

Structure: Non-symmetric on each side of Saturn

Braided F-Ring of Saturn due to co-orbiting “shepherd” moons

Two shepherd satellites confine Saturn’s narrow F ring. The outer shepherd gravitationally deflects ring particles inward, and the inner shepherd

deflects ring particles outward.

Prometheus

<R> = 47 km, Density = 0.42

<R> = 41 km, Density = 0.54

Pandora

Ring Spokes – Dust above the ring plane

The Smaller Moons of Saturn

Atlas

Prometheus

Pandora

Janus

Epimetheus

Telesto

Calypso

Helene

<R> = 90 km, Density = 0.61

Janus

Epimetheus<R> = 58 km, Density = 0.64

Mimas, 392 km in diameter Herschel Crater is 130 km wide and 10 km deep!

Enceladus – 500 km in diameterDiverse surface, with some tectonics

Enceladus

Tethys – 1,060 km in diameter. Heavily cratered, with some lineaments

Dione – 1,120 km in diameter. Bright surface swirls, many craters

Rhea – 1,530 km in diameter. Very heavily cratered

Pioneer 11

Voyager 1

Cassini visible light image

Various spacecraft views of Titan – 5,150 km in diameter

Cassini near-IR image

What do we know about Titan?

• It is the biggest of Saturn’s moons. • Titan’s diameter of 5,150 km makes it larger than the

planet Mercury with a diameter of 4,880 km and Pluto (2,350 km).

• Only Jupiter's moon Ganymede is larger. • Three-and-a-quarter Earth Moons could fit inside

Titan. • Titan is the only moon known to have a large

atmosphere – maybe thicker than Earth’s (>1,000 mb)! • Its atmosphere is mostly nitrogen, just like the

atmosphere surrounding Earth, but thicker. • Its atmosphere contains methane and tiny amounts of

oxygen.

Comparison of Titan’s atmosphere with Earth’s

Important gases in Titan’s atmosphere

1. Ethane-methane lakes 2. Moist convection in methane clouds3. Sedimentation of aerosols 4. Elevated terrain washed by rain

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January 14, 2005 – Huygens Probe sent to Titan’s surface

Huygens scientific instruments • Aerosol Collector and Pyrolyzer: collects Titan's aerosols for

chemical composition analysis;• Descent Imager/Spectral Radiometer: makes spectral

measurements and takes pictures of Titan's surface and atmospheric hazes;

• Doppler Wind Experiment: uses radio signals to deduce wind speeds on Titan;

• Gas Chromatograph/Mass Spectrometer: identifies and quantifies various atmospheric constituents on Titan;

• Surface Science Package: determines the physical properties of Titan's surface.

Radar Image of Titan’s surface – valley systems

Few impact craters seen on Titan – implies young surface

Horizon at 66 m

15 cm diameter

30 cm diameter

Range 85 cm

Range 240 cm

Surface image from Huygens probe

Iapetus – 1,460 km in diameter. Striking albedo differences

Cassini Views of IapetusIapetus is 1,460 km diameterOrbits 59.1 Saturn radii from planet

100 km

What’s Interesting about Phoebe?

• Phoebe is roughly spherical and is ~220 km in diameter.• Phoebe rotates on its axis every 9 hours and orbits Saturn in ~18

months.• Its irregular, elliptical orbit is inclined ~30 degrees toward

Saturn’s equator.• Phoebe’s orbit is retrograde.• Phoebe’s average distance from Saturn is 13 million km, which is

~4 times farther away from Saturn than its nearest neighbor, the moon Iapetus.

• Phoebe and Iapetus are the only major moons in the Saturn region that do not orbit close to the plane of Saturn’s equator.

• Unlike most major moons orbiting Saturn, Phoebe is very dark and reflects only 6 percent of the sunlight it receives.

Saturn’s moon Phoebe

Identification of surface materials on Phoebe

Saturn Summary