written by: George Garza•edited by: RC Davison•updated: 9/7/2011
If you were to land on a gas giant you would sink to the center. There is no solid surface, except for the core, several thousands of miles down. The gas giants lack rock or solid matter. The core is believed to contain liquid metals. The Solar System
has four different gas giants.
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What Planets Make Up the Gas Giants?
Gas giants at their core may have a rocky, metallic, or liquid metal structure. In fact, astronomers believe that such a core is necessary for a gas giant to form. The main constituents of a gas giant will be hydrogen and helium. It will also have traces of water, methane, ammonia, and other compounds. Unlike solid planets, which will have a well defined separation between an atmosphere and the surface, gas giants do not; their atmospheres tend to become denser toward the core.
In our Solar System the gas giantsappear outside of the asteroid belt, unlike the rocky planets, Mercury, Venus, Earth, and Mars, which are inside the asteroid belt. Finally, in the Solar System there are four gas giants: Jupiter, Saturn, Uranus, and Neptune. They are also called the Jovian planets.
The first of the gas giants that we willlook at is Jupiter, the largest planet. It takes 11 years to orbit the Sun, but it takes just under 10 hours to rotate once around its axis. Since it rotates so fast, it has winds that moveat about 150 m/s. Jupiter's atmosphere consists of about 75% hydrogen and 25% helium; there are also trace amounts of ammonia, methane, and other light substances.
Jupiter is a "hot" planet because it gives off about 2½ times more heat than it takes in from the Sun. Astronomers believe that Jupiter has a solid metal-rock core at its center, similar to Earth, but much larger. It has an equatorial diameter of about 142,984 km, by contrast Earth's diameter is 12,756 km. Jupiter has a mass density around 318 Earth-masses.
Other note worthy features: The surrounding atmosphere is a metallic mixture of hydrogen and helium. On Earth, hydrogen and helium behave as gases, but the pressure is so high on Jupiter's interior that the hydrogen behaves like a metal. There is a shell of liquid molecules that surrounds this metallic hydrogen area. Other parts of the atmosphere has concentrations of ammonia and methane. This zone is about 1,000 (km) deep. Another interesting feature is temperature; it varies widely. For example, it goes from -130°C at the top of the clouds, to 30°C about 70 km (43 miles) below, then to about 20,000°C at the planet's center.
The image below points out some features of the atmosphere. Notice the trace compounds at the top of the cloud layer. As you go deeper, there is a zone of fluid molecular hydrogen that extends about 20,000 km (12,427 mi). There is a transition zone that is about 40,000 km (24,854 mi). In this zone, the liquid hydrogen, in molecular form at these levels is compressed until it reaches a fluid metallic state.
Finally, 60,000 km (37282 mi) down, at the center of Jupiter is a small rocky core (about 15 times the mass of the Earth). It may have been residue from the icy-dust particles that collected in an early solar nebula. It has a temperature about 20,000°C, 36,000°F.
Saturn is the second largest gas giant and also the second largest planet in the Solar System. It has an atmospheric composition of 96.3% hydrogen, and 3.25% helium, but it also has traces of methane, ammonia, ethane, ethylene, and phosphine.
It has been observed that Saturn's atmosphere has alternate jet streams going from east-west and west-east. Sometimes speeds reach up to about 1,800 km/h. The atmospheric conditions are responsible for the banded appearance of the planet. There are electrical processes and heat deep within Saturn, which modify the layered chemical mixture of the atmosphere and may be responsible for the transitions that occur from superheated water near the core to the extremes of ammonia ice at the observed cloud tops.
The Solar System has four gas giants planets, but two of them, Uranus and Neptune, are also called "ice giants." Uranus has an atmosphere, like the other gas giants, composed of hydrogen (83%), helium (15%), methane (2%), and other trace amounts of acetylene and other hydrocarbons. It has a bluish tint, which comes from an upper-methane haze that strongly absorbs red wavelengths.
At the core of the ice giants the ice that might exist may in fact be more like a supercritical fluid, instead of like the ice on Earth. Normally liquids are distinguished from gases on the basis of density; liquids are more dense than vapors. But at the critical point both have become identical and this comes from raising the temperature and pressure of a liquid so it turns into a vapor and it has the same density as the liquid. If there is a substance where its temperature and pressure are above its critical pointit is called supercritical. The supercritical fluid can dissolve materials like a liquid or diffuse through solids like a gas. This supercritical fluid is believed to be at the core of Uranus.
The image shows the suspected breakdown of the atmosphere.
The last of the gas/ice giants is Neptune. It has a deep atmosphere, which goes down to between 80 and 90% of the center of the planet. At high altitudes it consists mostly of hydrogen (80%) and helium (19%). There are increasing concentrations of methane, other hydrocarbons (including acetylene, diacetylene, and ethane), plus ammonia, and water vapor, which occur in the dark, warmer, and lower regions of the atmosphere. Finally, the atmosphere transitions toasuperheated liquid interior.
Where does the blue color come from? It is mainly due to methane in the atmosphere, because it absorbs red light. Another aspect about thecolor is that there is a difference between the bright blue of Neptune and the blueish-green of Uranus. Both planets are affected by the atmospheric methane. However, astronomers believe that an additional component must also be responsible for Neptune's distinctive hue, but it has not been identified yet.
The Great Dark Spot
Neptune also has a two dark spots, a great dark spot and the small dark spot, which are oval-shaped anticyclones. They are not as prominent as the Jupiter Spot, but are fairly notecable nonetheless. The cause of the Neptune spots is the difference in temperature between the heat-producing core and the frigid cloud tops.
There are four rings around Neptune. They are vey dark which explains why they cannot be easily seen. There is one broad and three narrow rings.They are located betwen 40,000 and 63000 km (25000 to 39000 mi).
Neptune is about 4.5 billion kilometers from the Sun (2.8 billion miles). It has a diameter of 49,400km (30,700 mi). It has a volume that is 72 time greater than earth, but it's mass is only 17 time larger. It rotates once around its axis every 16 hours, and orbits the Sun every 164.79 Earth years.
This guide to what are the gas giant planets discusses the four largest planets in the Solar System. They have high amounts of hydrogen and helium in their atmospheres. They are unlike the rocky planets, which are smaller and have a well defined solid surface. Gas giants and the ice giants do not have a well defined surface. Their atmospheres can be thousands of miles thick. The behavior of the planet is more dynamic than that of the rocky planets, because the atmosphere changes the appearance, the temperature, and even the rotational speed of the planet.
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What are Gas Giant Planets
Image 1: Jupiter credit Nasa
Gas Giants Of the Solar System - Jupiter
Image 2: Giant Balls of Gas: http://www.solstation.com/stars/jovians.htm