A. Why the difference between the terrestrial and Jovian planets?

Recall that, as the solar nebula formed, we would expect that in the inner solar system the solar radiation would not allow various ices to form, but farther out they could condense onto dust particles, etc. We will see that the icy parts of comets tend to boil off to form the coma and tail at a distance from the Sun typically somewhere between Jupiter's and Mars' orbits. So we can imagine the small planetismals in the early inner solar system as being like rocks and those in the early outer solar system as being like dirty snowballs (but containing not only water ice, but CO₂ ice, etc.) The outer planets not only grew from different materials, but grew to masses such that they could accrete and retain gaseous material directly from the solar nebula. Thus Jupiter, for example, has a "rocky" core only slightly higher in mass than the Earth; most of its mass is hydrogen, and, in fact, its composition closely mirrors that of the Sun.

B. A quick and selective tour through the outer Solar System

The outer planets have been explored by various space probes: Voyager 1 (Jupiter and Saturn), Voyager 2 (Jupiter, Saturn, Uranus, Neptune), the current Galileo mission (Jupiter and its moons), and the Cassini mission, now on its way to Saturn.

Jupiter

• No "surface" as such; we see clouds at different layers in the atmosphere. At lower levels, calculations indicate a gradual transition to liquid hydrogen.
• In the deep interior, the liquid hydrogen has properties of a metal, conducting electricity and giving rise to a dynamo effect, creating a strong magnetic field: aurora, etc.
• Clouds in atmosphere divided into rising "zones" and sinking "belts", with cyclonic storms intermixed (e.g., great red spot).
• Jupiter has a ring, much weaker and darker than Saturn's.
• Jupiter has a strong internal heat source from its continued contraction.

Jupiter's satellites

Jupiter has many satellites; most are small--probably captured asteroids. We discuss only the four large "Galilean" moons, which probably formed with Jupiter.

• Callisto and Ganymede are larger than Mercury. Callisto has a surface of water ice, heavily cratered and contaminated by debris from meteorite falls; little evidence of any recent activity.
• Ganymede has a similar surface, but shows some regions with closely spaced parallel grooves. These apparently are places where the icy crust fractured and liquid water covered the region in repeated episodes. These areas show fewer craters and are younger.
• Europa has an icy crust, probably over a global liquid water ocean. There are no significant craters, indicating that the crust is constantly being renewed.
• Io is the strangest moon. Entirely covered with compounds of sulfur, it is also the only place in the Solar System besides Earth where volcanoes have been seen in action. Too small to generate sufficient heat on its own, it is flexed like a rubber ball by the tidal action of Jupiter, and this generates the heat that drives the volcanoes. Similar flexing is probably responsible for the heat source that renews the ice crust of Europa. One of the largest volcanoes on Io is named Pele (the advantage of having Hawaiian astronomers on the team!).

Saturn

• Famous for its ring (but now we know that all of the giant planets have rings: Saturn's is just the largest and brightest). Ring is less than 1 km thick; perhaps only a few meters. The ring cannot have a lifetime as long as the age of the Solar System, so it has to be renewed, most likely by impact erosion of small satellites.
• Smaller and less dense than Jupiter; weaker magnetic field; surface features (i.e., clouds) less evident.
• Distribution of rings controlled by resonances with satellites.
• Largest moon, Titan, a little larger than Mercury, has a dense atmosphere (surface not visible), organic molecule "smog". Target of Cassini probe/lander in 2004. Why can Titan, which is actually less massive than Mercury, have a dense atmosphere, while Mercury has none?

Uranus

• Blue-green in color, due to traces of methane (absorbs red light) in its mostly hydrogen and helium atmosphere; few distinctive surface features, probably because there is little heat flow from its interior.
• Tilted on its side, so axis of rotation is almost in the plane of its orbit around the Sun. What does this mean for seasons on Uranus?
• System of dark, narrow rings. Outermost ring controlled by two "shepherd" satellites.
• System of 5 major moons and at least 10 more minor ones, all "dirty iceballs".

Neptune

• Blue color, again due to methane in atmosphere (but Neptune has more than Uranus). More "weather" than on Uranus, including oval storms (similar to those seen on Jupiter, but less colorful). Evidently, Neptune has more heat flow from its interior than does Uranus.
• Narrow rings; outermost has lumpy structure, apparently due to resonances with one of the inner moons.
• Largest satellite, Triton, is mixture of rock and ice and in a reverse orbit from most other satellites in the solar system; few craters, indicating a young surface, but long fault lines where the icy crust has cracked. Dark regions near south pole seem to be due to liquid nitrogen jets, and resurfacing may be due to "volcanic" activity from radioactive warming of ices in the mantle by the rocky interior.

Large impacts in the early solar system?

The tilted axis of Uranus and the retrograde rotation of Neptune's large satellite Triton are difficult to understand on the solar nebula model, unless these objects were disturbed by major collisions early in their formation. Recall that similar collisions were invoked to explain the Earth--Moon system and the high ratio of core to crust in Mercury. Such interactions of large bodies may have been rather common in the early Solar System.

C. Return to Planet Earth

(See your textbook, Chapter 11.1--11.3)

Now that we have visited a number of other planets, we can return to look at our own home planet with a sharpened awareness of what makes it distinctive.

Kind of tectonic activity--So far as we can tell, Earth is the only planet for which the crust is currently broken up into large plates that move with respect to each other because of slow convective currents in the mantle. This feature allows Earth to have mountains and land masses in spite of the large amount of erosion from water and wind.

Water--Earth is the only terrestrial planet that has been able to retain large amounts of liquid water. Venus started out with lots of water (probably almost all in vapor form), but lost it to dissociation into hydrogen (which leaked off into space) and oxygen (which reacted with minerals and other gasses). Mars seems to have had liquid water early in its history, when the atmosphere was denser, but its low mass means that most of the atmosphere has leaked away, including much of the water; what remains is frozen out at the poles and probably below the surface. As we have seen in our exercise on Venus and Earth, the oceans are essential in transporting carbon dioxide out of the atmosphere and preventing Earth from having a runaway greenhouse effect.

Life--Because Earth has life--and, in particular photosynthesis--it has a large amount of oxygen in its atmosphere. This also allows it to have an ozone layer, which prevents the short-wavelength (high-energy) ultraviolet radiation from reaching the surface, allowing more delicate forms of life (including multicellular animals, like us) to exist.

End lecture 18 3/18/03

Lecture 19: Class Activity: Killer Asteroids

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Last updated 20 March 2003

Alan Stockton (stockton@ifa.hawaii.edu)