Differential gravitational forces between near and far sides can overcome internal strength and self-gravitation of a body, which is then torn apart. (That is the reason planetary rings exist close to a planet). Called tidal forces since similar effect raises tides on Earth.

Comets have low tensile strength, about one ten millionth that of steel wire, so comets can be ripped apart easily-when approaching a large mass like Jupiter or the Sun. Comet Shoemaker - Levy 9 orbited Jupiter, fragmented to 21 or more pieces, about 1 kilometer apart, when the comet came within 20,000 kilometers of Jupiter's cloud tops on July 7, 1992.

Return visit July 16-22, 1994 fragments of ruptured comet collide with Jupiter's backside, releasing energy that far exceeds that of the world's entire nuclear arsenal.

• Low density-1.34 grams per cubic centimeter
• Main ingredient hydrogen-lightest element
• Outer atmosphere-frozen clouds
• Deep interior-hot, planet radiates about twice the heat it receives from the Sun.
• Mostly heat left over from Jupiter's creation.
• Long year = 12 Earth years.
• Rapid rotation period = 9 hours ,55 minutes, 26 seconds.
• Rotation stretches clouds into dark belts and light zones that move in opposite directions.
• Cyclonic storms = eddys between zones and belts.
• Great red spot = seen for three centuries and three Earth diameters in size.
• Layered clouds = amononia ice = light ammonia hydrosulfide ice = dark water ice -lower zones.

• Youngest surface in solar system.
• Active volcanoes turn satellite inside out.
• No water - volcanoes of sulfur and sulfur dioxide.
• Io's elliptical orbit causes tidal flexing and internal heat that melts rock.
• Io most volcanically active object in solar system.
• Io has no substantial atmosphere, but may have a thin one.

New Galileo fly by - from gravitational pull on spacecraft, Io has a dense, massive center, probably an iron core, and since inside it is at least partially melted to make volcanoes, Io probably generates a magnetic field.

Ganymede - largest moon in solar system

• roughest surface in solar system

New Galileo - Ganymede has an intrinsic magnetic field about one thousandth the strength of Earth's magnetism. Due to electric currents in saltwater sea (inside) or molten core. Surface of frozen ice, with mountains ridges, valleys due to expansion and contraction of water-ice crust.

• Ice flows, lubricated from below.
• Cracked ice let liquid water in.
• Ice surface few impacts so relatively young - smoothest surface in the solar system.
• Europa completely covered by frozen ocean at 128 ^oK.
• Europa tidal flexing less than Io, but gravitational interaction with Jupiter and neighboring moons Io and Ganymede creates internal friction that heats Europa inside.
• Subsurface liquid water and energy source make life possible on Europa.
• Internal waters could seethe with alien life forms.

Satellites with atmospheres = Io, Titan, Triton, Europa

Europa only satellite with molecular oxygen in its atmosphere. Europa's atmosphere very thin and tenuous - one hundred billionth of Earth's surface pressure - oxygen molecule has 3 to 4 collisions before escaping Europa - atmosphere confinously replenished from surface.

Charged particles trapped by Jupiter's magnetism constantly bombard Europa's surface, knocking water molecules free. Once suspended, the water is split by sunlight into oxygen, which stays for a little while, and hydrogen, that escapes as fast as it can.

• Ancient battered surface, only craters.
• Ice as rigid as steel.
• Probably 4.6 billion years old.

Composed of fine dust particles knocked off tiny moons Metis and Adrastea, and ash from Io's volcanoes. Dust sand-blasted off these nearby satellites rains into planet. Dusty ring seen when lit from behind, like smoke in a movie theatre. (new) - shape of ring distorted by Jupiter's magnetic field-some ring dust charged. Eight outer moons-captured asteroids maybe, eccentric tilted orbits.