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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 ( [link] ). The material available to build these planets can be divided into three classes by what they are made of: “gases,” “ices,” and “rocks” (see [link] ). 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.

Jupiter.

Photograph of Jupiter. Taken from the Cassini spacecraft, the alternating light and dark cloud bands are visible over the entire planet. The Great Red Spot is at lower right. Also seen is the shadow of the moon Europa at lower left.
The Cassini spacecraft imaged Jupiter on its way to Saturn in 2012. The giant storm system called the Great Red Spot is visible to the lower right. The dark spot to the lower left is the shadow of Jupiter’s moon Europa. (credit: modification of work by NASA/JPL)
Abundances in the Outer Solar System
Type of Material Name Approximate % (by Mass)
Gas Hydrogen (H 2 ) 75
Gas Helium (He) 24
Ice Water (H 2 O) 0.6
Ice Methane (CH 4 ) 0.4
Ice Ammonia (NH 3 ) 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 H 2 O 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 CO 2 ). As a result, the compounds detected in the atmosphere of the giant planets are mostly hydrogen-based gases such as methane (CH 4 ) and ammonia (NH 3 ), or more complex hydrocarbons (combinations of hydrogen and carbon) such as ethane (C 2 H 6 ) and acetylene (C 2 H 2 ).

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. [link] summarizes the spacecraft missions to the outer solar system.

Missions to the Giant Planets
Planet Spacecraft Both the Ulysses and the New Horizons spacecraft (designed to study the Sun and Pluto, respectively) flew past Jupiter for a gravity boost (gaining energy by “stealing” a little bit from the giant planet’s rotation). 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
Galileo December 1995 Orbiter and probe
Cassini December 2002 Flyby
New Horizons February 2007 Flyby during gravity assist
Juno July 2016 Orbiter
Saturn Pioneer 11 September 1979 Flyby
Voyager 1 November 1980 Flyby
Voyager 2 August 1981 Flyby
Cassini July 2004 (Saturn orbit injection 2000) Orbiter
Uranus Voyager 2 January 1986 Flyby
Neptune Voyager 2 August 1989 Flyby

Questions & Answers

which planet orbits the closest?
Alastair Reply
What is the angle between Earth's equator and the Celestial equator? In the drawing they seem pretty similar. Thank you for this study resource.
Chuck Reply
Describe the spectrum of each of the following: starlight reflected by dust, a star behind invisible interstellar gas, and an emission nebula
shakila Reply
If the Oort cloud contains 1012 comets, and ten new comets are discovered coming close to the Sun each year, what percentage of the comets have been “used up” since the beginning of the solar system?
Day Reply
what is spectral type of sun
Akshat Reply
what everyone asking here? and who answers for them?
Shashi Reply
highest frequency wavelengh
Kathy Reply
may I know which Kingdom shows largest diversity
Arpita Reply
or students should post tough likely questions
Adepitan Reply
for example questions on demand functions and etc
Adepitan Reply
are there no ways we can get tough questions to answer
Adepitan Reply
What do you mean?
Amman Reply
excellent book bro.... keep it up.... if I find any query i will ask u.... thanks
rao Reply
Hi! I'm a bit confused, what is this
Sizakele Reply
They hsve new set of questions every time they test us. i do revision with the tutorials they give us answer extra questions from their moodle site but every time i write exams there will be few not even 10% of the questions are they. most of the time i guess. They give us three lectures who do not
Phumza Reply

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Source:  OpenStax, Astronomy. OpenStax CNX. Apr 12, 2017 Download for free at http://cnx.org/content/col11992/1.13
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