Astronomy Notes

The Four Giant Planets have several similarities

• They are all much larger than the earth (4-10 times the diameter).
• The all are much lower density than the earth, being composed of ices and hydrogen compounds.
• Being so far from the sun, they all are very cold.
• All of them are covered by thick clouds.
• They all have many satellites.
• They all have rings.

Jupiter and Saturn are Similar

• Both are huge: Jupiter is 10 times the diameter of the earth, Saturn is 9.
• Both are about 2/3 hydrogen.
• The clouds of both Jupiter and Saturn are arranged in to dark belts and bright zones.
• Both have day and night cloudtop temperatures about 120 K. (-200° F).
• Both probably do not have a surface, but just get thicker clouds which turn to thick mush as you go down.
• Both rotate rapidly, in about 10 hours.
• Both radiate over twice as much gravitational thermal (infrared) radiation as they receive from the sun.
• Both have a satellite larger than the planet Mercury.
• Both have retrograde motion in the sky for over 4 months every year, when they are rising opposite the sun.

Atmospheric Conditions

• Their atmospheres are composed of about the same hydrogen-helium mixture as the sun and most stars.
• The terrestrial planets (Mercury through Mars) to too small to keep those super light elements.
• The clouds are made of ammonia and water, which are minor constituents of the atmosphere.
• Jupiter has the amazing Great Red Spot, an oval hurricane that has persisted over 300 years.
• Jupiter also has huge thunderbolts of lightning, just as the god Jupiter was depicted.

Jupiter's Magnetic Field

• Jupiter has the strongest magnetic field of any planet, 15 times Earth's, Saturn's or Uranus's.
• This field has trapped many charged particles, similar the Earth's Van Allen belts.
• This strong magnetic field causes those particles to emit radio waves, called synchrotron radiation.
• Jupiter probably has a liquid metallic hydrogen core, which accounts for its large magnetic field.

Other Characteristics

• Jupiter's density is 1.3 times water and Saturn's is only 0.7, so it would float like ice in water.
• Jupiter takes nearly 12 years to orbit the sun, Saturn takes nearly 30.

Rings

• Rings occur when a large object gets too close to the planet and gets broken into pieces.
• Rings are made of tiny particles, gravel to boulder size.
• Jupiter has one single very thin small ring discovered by Voyager photos, made of dark rocks.
• Saturn's Rings are huge, formed almost entirely of golf ball to house-sized ice chunks.
• Cassini's Division, is a gap in the rings, visible in even a six-inch telescope.
• That gap is swept clear because it is at a distance with half the orbital period of the nearest large moon, Mimas.

They have Similar Kinds of Satellites

• Small moons very close to the planet.
• Large moons at a medium distance from the planet, which follow a Bode's Law in a plane.
• Small moons far from the planet which appear in random-looking orbits, as if they have been captured.
• The moons are composed either of bright ice or dark soot, or a combination called dirty ice.
• Jupiter has 16 moons and Saturn has 18 discovered so far.

Jupiter has 16 moons, which are neatly divided into 4 sets of 4.

1. 4 small inner moons, very close to Jupiter.
2. 4 large Galilean satellites, as big or bigger than our moon.
3. 4 tiny moons, all inclined at 27° in a prograde orbit (west to east), all 6 times further than Galilean.
4. 4 more tiny moons, all inclined at 60° in a retrograde orbit (east to west), all twice as far as last four.

Four Galilean Satellites are visible in any telescope, or even high-powered binoculars.

• They are Io, Europa, Ganymede, Callisto (in order of distance from Jupiter).
• They all face Jupiter as our moon faces the earth.
• They are spaced according to Bode's law.
• They all have prograde orbits with nearly zero inclination to the ecliptic.
• They all are cold, from 80 - 150 K (-300 to -180° F) (all 16 moons are).

Io: The Sulfur Volcano Moon

• It is yellow, orange and tan colored from sulfur.
• It has active volcanoes, one of which was erupting when the Voyagers flew by.
• It is heated by tidal flexing caused by the other satellites adding or subtracting from Jupiter's pull.
• Most of the surface is cold like the other moons, but it has local hot spots around the volcanoes.

Europa: The Icy Cue-Ball Moon

• It has almost no features: no craters, no mountains, no markings.
• It is covered with water ice.
• It is more the same density as the moon (same as rocks), so it is not all ice.
• It has little cracks all over it, probably caused by tidal flexing, making it look like a pool cue-ball.

Ganymede: The Largest Moon in the Solar System

• It looks a lot like our moon, with lots of old craters.
• It has an ice cap.

Callisto: The Cratered Moon

• Its the same size as the planet Mercury.
• It has the most ice inside of it (it is lowest density of the moons).
• It has the most craters of any ot the moons.

General Features.

• They show less diversity than Jupiter's moons.
• They are more icy than Jupiter's moon's.
• There are six small moons just outside Saturn's Ring, thought to be fairly pure ice, like the rings.
• Then there are five medium sized moons, again mostly ice.
• Then comes Titan, the second largest moon in the solar system, next to Jupiter's Ganymede.
• Then there are three more moons. The most distant, Phoebe, is very distant and retrograde: captured.

Titan has a Thick, Smoggy Atmosphere

• Titan is the only moon with a thick atmosphere.
• It is reddish orange with the same chemicals as smog, with a lot of methane gas.
• Its surface atmospheric pressure is greater than that on earth.
• The same side always faces Saturn.