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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.


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

can i know deep about black hole
Monisha Reply
yupp ask questions about whatever you wanna know in black hole?
where does it lead
how black holes are detected in deep space as we are unable to see them.
if black holes are hypothetical then how do they detect them?
who said blackholes are hypothetical?
They are detected by their gravitational pull...
also we can detect them by X-rays as they emit hawking radiation as they 'shrink'
hawking radiations are not detected yet...
to find astronomical units do you divide or multiply?
Bethany Reply
I think it depends what you're converting from. i.e. from km you divide
I tried that for question a in section 0.3 but the answer doesn't seem right
make sure you're using consistent units m/m
i too had the same doubt
hi dear
can you tell me what is ecliptic ?
the great circle formed by the intersection of the plane of the earth's orbit with the celestial sphere; 
OK thanks
who is the father of Astronomy?
subhajit Reply
what happen if earth stop to rotate?
Sijan Reply
well first of all we wouldn't have day and night and it would get cold in the dark side humans and animals would have to get use to the darkness. plant would not he able to grow on the dark side either. Ad for the light side or "day" its the same thing but just...hot.
and contestant datlight such could be harmful beacuase of the constant if rays
Our gravitational field would collapse.
Magnetic field sry. That means no shielding from cosmic rays.
actually we would have day and night...but each day would last a year. We would also still have a magnetosphere unless the Earth's core also stopped rotating. Gravity would increase as we would lose any centripetal effect. We would only have one tide per 29 days as the moon orbits us.
we would still have gravity Hannah because we would still have mass!
pretty sure there would be no more rain or wind because any warm air and water vapour would just rise and fall above wherever it came from.
As we are rotating right now, if the earth suddenly stopped, objects that are not bound to the ground will get flung due east at very high speeds. It would be very catastrophic. That's the first thing that'll happen if such an event were to happen.
what is comets?
Ido Reply
the bottom of the page of chapter 13 thinking ahead describe it
I also don't know but I think it is star
I once watched a show discussing finding dark matter ON planet earth, deep down inside caverns where sound and light are absent. I seem to remember them also saying that the slightest of sound would "charge" the matter up. If the solar system is primarily dark matter, could we use sound to travel?
Jamie Reply
I thought the same thing. If space is a vacuum and sound does not travel in a vacuum, it would be difficult?
What exactly is dark matter?
from what i gathered, dark matter is incredibly motionless atoms. Almost dormant, and they are easily disturbed? i am by NO MEANS an expert on any of this. just a curious mind... if sound doesnt travel in a vacuum, maybe the light then? disturb the matter with light and maybe 'swim' it?
what are whitehole a wormholes
gajanan Reply
***en.m.wikipedia.org/wiki/White_hole ***en.m.wikipedia.org/wiki/Wormhole
is the reason why you can't see the carp nebula in some towns and city's due to other lights?
Ghost Reply
Which of the following transformations is (are) fusion and which is (are) fission: helium to carbon, carbon to iron, uranium to lead, boron to carbon, oxygen to neon? (See Appendix K for a list of the elements.)
Nanea Reply
fission refers to conversion of helium into two hydrogen atoms fusion is combination of hydrogen atoms to helium
but acc to the question helium to carbon is fission after hydrogen
how does a super nova form?
spacegurl Reply
I've found something for you: ***nasa.gov/audience/forstudents/5-8/features/nasa-knows/what-is-a-supernova.html
super novas are the result of the explosion of a star. The star is composed mainly of hidrogen and helium, but due to thermonuclear fusion, the electrons from the atom are forcefully separated from its core thanks to the intense heat
the cores are compressed against each other, and heavier elements are formed. This process is kind of, the fuel of the star. Stars emit radiation because of thermonuclear fusion, but are also kept in their shape due to a strong gravitational force pushing everything in.
when in balance, the star remains healthy. but when the "fuel" is used completely, gravity outruns radiation, and everything is compressed real quick into a tiny space. The star later on implodes, as far as I know. The implosion itself is a supernova. If i'm wrong in anything, please lend me a hand
can You please send me some maths related to astronomy eith answers?
je suis Mathemacien Français
The attraction force between the huge celestrial bodies can be explained by string theory. Can the theory explain the incident of the fall of apple on the ground??? Plz write to me.
Chandrayan Reply
In broad daylight, the size of your pupil is typically 3 mm. In dark situations, it expands to about 7 mm. About how much more light can it gather?
Chris Reply
what is quaser
Babul Reply
How the cosmos evolves?
Ramdas Reply
which planet orbits the closest?
Alastair Reply
Mercury is closest to the sun.
Mercury since it is the closest one to the sun so it orbits faster than Earth.

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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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