89 Exploring the Outer Planets
Learning Objectives
By the end of this section, you will be able to:
- Provide an overview of the composition of the giant planets
- Chronicle the robotic exploration of the outer solar system
- Summarize the missions sent to orbit the gas giants
The giant planets hold most of the mass in our planetary system. Jupiter alone exceeds the mass of all the other planets combined (Figure 1). The material available to build these planets can be divided into three classes by what they are made of: “gases,” “ices,” and “rocks” (see Table 1). The “gases” are primarily hydrogen and helium, the most abundant elements in the universe. The way it is used here, the term “ices” refers to composition only and not whether a substance is actually in a solid state. “Ices” means compounds that form from the next most abundant elements: oxygen, carbon, and nitrogen. Common ices are water, methane, and ammonia, but ices may also include carbon monoxide, carbon dioxide, and others. “Rocks” are even less abundant than ices, and include everything else: magnesium, silicon, iron, and so on.
| Table 1. Abundances in the Outer Solar System | ||
|---|---|---|
| Type of Material | Name | Approximate % (by Mass) |
| Gas | Hydrogen (H2) | 75 |
| Gas | Helium (He) | 24 |
| Ice | Water (H2O) | 0.6 |
| Ice | Methane (CH4) | 0.4 |
| Ice | Ammonia (NH3) | 0.1 |
| Rock | Magnesium (Mg), iron (Fe), silicon (Si) | 0.3 |
In the outer solar system, gases dominate the two largest planets, Jupiter and Saturn, hence their nickname “gas giants.” Uranus and Neptune are called “ice giants” because their interiors contain far more of the “ice” component than their larger cousins. The chemistry for all four giant planet atmospheres is dominated by hydrogen. This hydrogen caused the chemistry of the outer solar system to become reducing, meaning that other elements tend to combine with hydrogen first. In the early solar system, most of the oxygen combined with hydrogen to make H2O and was thus unavailable to form the kinds of oxidized compounds with other elements that are more familiar to us in the inner solar system (such as CO2). As a result, the compounds detected in the atmosphere of the giant planets are mostly hydrogen-based gases such as methane (CH4) and ammonia (NH3), or more complex hydrocarbons (combinations of hydrogen and carbon) such as ethane (C2H6) and acetylene (C2H2).
Exploration of the Outer Solar System So Far
Eight spacecraft, seven from the United States and one from Europe, have penetrated beyond the asteroid belt into the realm of the giants. Table 2 summarizes the spacecraft missions to the outer solar system.
| Table 2. Missions to the Giant Planets | |||
|---|---|---|---|
| Planet | Spacecraft[1] | Encounter Date | Type |
| Jupiter | Pioneer 10 | December 1973 | Flyby |
| Pioneer 11 | December 1974 | Flyby | |
| Voyager 1 | March 1979 | Flyby | |
| Voyager 2 | July 1979 | Flyby | |
| Ulysses | February 1992 | Flyby during gravity assist |