Astronomy 10: Lecture 10

# Lecture 10: The Jovian Planets

Reading Assignment: Arny: Chapter 9, Cosmos Chapter 6

Summary of the Jovian Worlds: Far from the Sun, many moons, large and massive, low density, mostly gaseous, rocky/iron core.

 Planet dSun(AU) OrbitalP (yr) Radius(R) Mass(M) Density(g/cm3) RotationP (days) Axistilt moons symbol Jupiter 5.2 11.9 11.2 318 1.3 0.41 3.1° 16 Saturn 9.5 29.5 9.4 95.2 0.7 0.45 26.7° 20 Uranus 19.2 84.1 4.0 14.5 1.3 0.72 97.9° 15 Neptune 30.1 165 3.9 17.2 1.6 0.67 29.6° 8

## Jupiter: King of the Gods

Planet #5 from the Sun, at least 16 known moons.

Its radius is 11 times Earth's (R = 11R), and one-tenth the size of the Sun (R = 0.1R). Jupiter's mass is 320 times that of Earth (M = 320M) and one-thousandth that of the Sun (M = 10-3M).

The planet is composed of mostly hydrogen and helium just like the Sun.

Colorful bands parallel to the Equator. Lot's of swirls, spots: stormy. One storm, the "Great Red Spot" is 2 times the size of Earth, and has lasted for at least 300 years (Galileo saw it in his telescope). It's a gigantic hurricane.

The planet appears oblate (squashed) this is due to rapid rotation (once every 10 hours!).

Planet has very thin rings seen for the first time by the Voyager spacecraft, they lie at a radius of 1.8R. The spacecraft also witnessed lightening storms in the clouds.

Spacecraft visits: Voyager 1 and 2 (1979), and Galileo (1996/1997 - today). Beautiful close-up views of the planets and its moons.

The Galilean Satellites: the 4 largest moons of Jupiter each is larger than the Moon.

• Ganymede: The largest moon in the Solar System, it has an old, cratered surface.
• Callisto: Heavily cratered like Ganymede, lots of ice on the surface and frozen into the crust.
• Europa: Smooth surface with narrow, dark stripes running across, few craters. The surface seems to be fractured ice (water ice). Recent information from the Galileo spacecraft suggests that there is world-wide ocean of liquid water beneath the frozen surface. Life?
• Io: Erupting volcanos! The most geologically active body in the Solar System. The interior is molten, this is due to tidal forces which change throughout its elliptical orbit. A tortured world.
The 12 other known moons are all rather small; many are captured asteroids.

## Saturn: Father of the Gods (Father Time)

Planet #6, 20 known moons.

Saturn has a remarkably low density (0.7 g/cm3): it could float on an ocean of water!

Like Jupiter and the Sun it is mostly composed of Hydrogen and Helium, with an Earth-like core. It also has atmospheric bands of clouds like Jupiter, but they are less colorful.

It's most outstanding feature are its magnificent rings. The rings are mad of chunks of rock and ice. It is material that failed to coalesce into a moon, due to tidal forces of Saturn (inside the "Roche Limit"). The rings lie in the planet's equatorial plane (which is inclined 27°) to its orbital plane. This makes the rings appear "edge-on" twice per 29-year orbital period (e.g. in 1995). Only 4-5 of the rings are generally visible from Earth. There is a dark division between the 2 main rings ("Cassini's Division").

Spacecraft visits: Voyager 1 and 2 (1980): spectacular details! More than 100,000 "ringlets" only about 20 meters thick! Cassini spacecraft on its way...

Saturn has more than 20 moons (new ones being discovered all the time). The largest of the Moons (visible from Earth in a small telescope) is Titan. Titan has a thick Nitrogen atmosphere (like Earth!). It also has the same chemical in smog pervading its atmosphere. There is some greenhouse effect that helps warm it. It's possible that there are methane lakes and a rain of organic materials onto the surface. Life?

## Uranus: God of Heaven

Planet #7, 15 known moons

This planet was not known to the ancients. It lies at the threshold of the human eye's brightness detection limit. It was discovered in 1781 by William Hershal a musician and part-time astronomer.

It is smaller that Jupiter and Saturn, but still much larger than Earth. It is composed mostly of Hydrogen and Helium, plus some ammonia and methane clouds. The methane clouds absorb red light and reflect blue giving the planet its blue color.

Uranus is very cold: 60 K. Its axis of rotation is tipped by 98°! It almost lies in its orbital plane. Thus the planet has extreme season in terms of Sun variability.

Approximately 10 very narrow rings were discovered in 1977 when Uranus passed in from of a bright star (the light blinked out many times). The rings are very thin (less than 10 km wide). This is believed to be caused by "Shepard Moons".

Spacecraft: Voyager 2 (1986 - just a few days before the Space Shuttle Challenger disaster). It discovered the Shepard Moons around some rings, as predicted. They keep the rings narrow. The atmosphere of Uranus is remarkably featureless. The uneven heating is smoothed out.

The moon, Miranda, has quite an amazing surface. It's carved with grooves.

## Neptune: God of the Sea

Planet #8, 8 known moons

It is approximately the same size as Uranus (brother planets). It is also mostly composed of Hydrogen and Helium with some methane mixed in for good measure.

Neptune was discovered in 1846 (J. Galle), as a result of the analysis of Uranus' orbit: Neptune perturbs Uranus. In 1613 Galileo actually saw it and recorded it in his notebook, but he didn't realize what it was, he just thought it was a star.

Spacecraft: Voyager 2 (1989) made an approach within 5000 km of the planet! Discovered complete, but clumpy rings. Atmosphere much more dynamic than Uranus'. Winds up to 1,100 km/hr!. Many white and dark spots (storms). Wispy clouds of methane skirt around.

Magnetic field is tilted 50°, and offset from the center (like Uranus').

Triton: is Neptune's largest moon. It has a thin methane atmosphere. Fascinating and varied terrain. Dark plumes - active, icy volcanos! Fault blocks, collapsed basins, craters.

The end of Voyager's 12-years Odyssey...