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7.2 Atmospheres of the Gas Giant Planets

The atmospheric composition of the giant planets is mostly hydrogen, ranging from 63% to 93% hydrogen by mass. Most of the rest is helium, with only tiny traces of other compounds. This is very different from the terrestrial planets’ atmospheres. The rocky planets have only a thin sheath of gases, most of which are the heavier ones like carbon dioxide, nitrogen, and oxygen.

How do we know the composition of giant planets? Their mean densities are important clues to the internal compositions of these worlds. Density is mass divided by volume. We measure the volume by knowing the distance to the planet, and converting its angular size into a true size using the small angle equation. We measure the mass by using the law of gravity and analysis of the orbits of their moons. The giant planets are much less dense than the terrestrial planets. The mean density of the giant planets is 700 to 1600 kg m-3 (kilograms per meter cubed), compared to 3900 to 5500 kg m-3 for the terrestrial planets. Remember that the density of water, a useful benchmark, is 1000 kg m-3. Saturn, at only 700 kg m-3, would float like an ice cube in water, if we could find a big enough ocean! The density of all four worlds is much lower than rock. This proves that the giant planets are made not of rock, but largely of low-density ices, liquids, and gases.

A visible spectrum of the sun. The black lines in the spectrum are absorption lines. These lines can tell us about the chemical composition of the sun. Similar spectra can be gathered from planets, which allow us to determine their composition. Click here for original source URL.

A diagram of the electromagnetic spectrum, showing various properties across the range of frequencies and wavelengths. Click here for original source URL.

Astronomers can make more direct measurements of the planets’ compositions using spectrometers on telescopes and on spacecraft. Each element or compound absorbs light with a specific set of wavelengths. The pattern of absorption is a direct guide to the chemical composition of the atmospheres. We find a hydrogen-helium mix of roughly the same basic composition as that found in the Sun, as well as all the other stars and gas in our region of the galaxy. This is not a coincidence. This was the composition of the thin gaseous material that filled the solar system as the planets formed. As giant planets grew to massive sizes, their gravitational attraction became strong enough to capture and hold some of the surrounding gaseous material. This is how they acquired massive, hydrogen-rich atmospheres. None of the terrestrial planets grew big enough to do this, so their atmospheres consist mostly of gases added by internal processes like volcanism, and external sources such as comets.

The Galileo spacecraft right before it was loaded into the cargo bay of the Space Shuttle Atlantis. Click here for original source URL.

The best evidence on planet composition comes from a direct probe. Imagine the awesome cloud scapes that might confront a spacecraft flying into the atmosphere of one of the giant planets. In 1995, such a flight was made for the first time when the Galileo spacecraft parachuted a probe into the atmosphere of Jupiter. Unfortunately, the probe had no camera to send back images, but it did measure cloud composition and other environmental conditions. Because the probe got hotter than expected when it slammed into the atmosphere, scientists required several months to interpret the data from the instruments, which were operating beyond the range of their usual temperatures. Eventually, the data revealed Jupiter's atmosphere had roughly the same percentage of hydrogen and helium as the Sun. Jupiterappears to have more carbon, nitrogen, sulfur, and other heavy elements, than the Sun - these may come from interplanetary bodies that strike Jupiter. Few organic molecules were found.

The image shows the vertical structure of the atmosphere of Jupiter. The temperature is shown as a function of pressure (black line). The approximate altitudes of atmospheric transitions troposhere? stratopshere (tropopause) and stratosphere?thermosphere are shown, as well as tropospheric cloud layers. Click here for original source URL.

Investigators were surprised that the probe did not detect clear signs of the three main cloud layers hypothesized from earlier studies. It detected no more than thin mists at the levels of the ammonia and ammonia hydrosulfide clouds and observed no thick layer at the lower level where water clouds were predicted. It also measured less water vapor in the air than expected. Scientists are still puzzling over whether the probe happened to enter a particularly dry area of thin clouds, or whether all of Jupiter is drier than had been thought. The strong winds of up to 400 mph measured at still deeper levels were also a surprise. As the probe descended deeper, it measured higher temperatures. By the time of the last contact with the probe, it was bouncing on its parachute shroud lines, buffeted by the high winds. It had plunged into a region below the clouds, where the air pressure was about 20 times that on Earth (equivalent to the pressure around 230 meters below the Earth's ocean surface) and the temperature was about 420 K (300 °F). The Galileo probe to Jupiter confirmed some earlier ideas, but it also raised further questions.

While the gas giants all have large atmospheres that are similar in the Hydrogen to Helium ratios, they differ when other elements are considered. The compositions of each atmosphere are listed below:

• Jupiter: Gases - 89.8±2.0% Hydrogen (H2); 10.2±2.0% Helium; ~0.3% Methane; ~0.026% Ammonia; ~0.003% Hydrogen deuteride (HD); 0.0006% Ethane; 0.0004% water. Ices - Ammonia, water, ammonium, hydrosulfide(NH4SH).

• Saturn: Gases - ~96% Hydrogen (H2); ~3% Helium; ~0.4% Methane; ~0.01% Ammonia; ~0.01% Hydrogen deuteride (HD); 0.000 7% Ethane. Ices - Ammonia, water, ammonium, hydrosulfide(NH4SH).

• Uranus: 83 ± 3% Hydrogen (H2); 15 ± 3% Helium 2.3% Methane; 0.009% Hydrogen; (0.007–0.015%) deuteride (HD). Ices - Ammonia, water, ammonium, hydrosulfide (NH4SH), methane (CH4).

• Neptune: Gases - 80±3.2% Hydrogen (H2); 19±3.2% Helium; 1.5±0.5% Methane; ~0.019% Hydrogen deuteride (HD); ~0.00015% Ethane. Ices - Ammonia, Water, Ammonium hydrosulfide(NH4SH), Methane (CH4).