Moons, Rings, & Dwarf Planets

Solar System AstronomyChapter 11

In the Solar System: Dozens of worlds

Rock & ice Diverse properties, only partially understood Offers insight into our ideas/theories of planet

formation

Moons Most larger moons formed with planets

Regular moons Some are captured objects

Usually in retrograde orbits Irregular moons

Major Moons Four gas giants Earth

Some are geologically active Others show evidence of past activity

Many Moons

Formation of Large Moons The largest moons formed the same way

the terrestrial planets did Major processes: accretion and differentiation Biggest difference with the terrestrials:

composition more ices (frozen water, methane, &c) less rock (silicates)

Rings All four gas giants have ring systems

Swarms of tiny moons Saturn’s are the largest and brightest Particles orbit according to Kepler’s laws

orbits are circular collisions or ring gravity keep them that way

Saturn’s Rings Very complicated system

thousands of ringlets bright and dark rings, and “gaps” gaps are not empty brightness/darkness indicates amount of

material in each ring system is extremely thin

Rings Rings don’t contain much material

mass of all the ring particles is about the same as a small, icy moon

Can be distorted by the gravity of nearby moons

Origin of Rings Ring material is from disrupted moons

Large moons are broken up within Roche limit Tidal forces

Other sources of ring material Volcanoes Impacts

Saturn’s rings formed from an icy moon Uranus’ and Neptune’s are very dark

Body rich in carbon

Moons & Rings Rings don’t last forever

Collisions and sunlight would destroy them Small, nearby shepherd moons can help

stabilize Moons also create gaps

orbital resonance: orbital period is in a ratio with the moon period

Titan Saturn’s largest moon

deep, nitrogen-rich atmosphere Currently being explored by the Cassini

spacecraft Huygens lander revealed icy “rocks” and a soil

rich with organic compounds

Titan Saturn’s largest moon

deep, nitrogen-rich atmosphere methane and ethane can condense and lead to rain

of methane and ethane Methane is gradually converted to ethane in the

atmosphere Renewed in active geology

Then there’s poor Pluto… Very small

1/6 the mass of Moon Binary planet: moon Charon

properties like comets eccentric orbit icy composition (probably) member of the Kuiper Belt Objects (KBO)

Probably not even the largest

Discovery of Pluto 1930 Clyde Tombaugh

Existence predicted from disturbances of Neptune

Though it’s too small…

Discovery of Pluto

Discovery of Pluto Venetia Burney

1930 2007

Venetia Burney Phair died April 30, 2009

Pluto Orbit

2:3 resonance with Neptune Comes closer to Uranus than Neptune

Dwarf Planets Small numbers, but increasing

Pluto Charon, Nix, Hydra

Eris Dysnomia

Ceres Haumea

Hi’iaka, Namaka Makemake