Jupiter Atmospheric characteristicsplanet

Prior to the deployment of the Galileo probe, astronomers had relied upon studies of the planet’s spectrum to provide information about the composition, temperature, and pressure of the atmosphere. In the particular version of this technique known as absorption spectroscopy, light or thermal radiation from the planet is spread out in wavelengths (colours, in visible light, as in a rainbow) by the dispersing element in a spectrograph. The resulting spectrum contains discrete intervals, or lines, at which energy has been absorbed by the constituents of the planet’s atmosphere. By measuring the exact wavelengths at which this absorption takes place and comparing the results with spectra of gases obtained in the laboratory, astronomers can identify the gases in Jupiter’s atmosphere.

The presence of methane and ammonia in Jupiter’s atmosphere was deduced in this way in the 1930s, while hydrogen was detected for the first time in 1960. (Although 500 times more abundant than methane, molecular hydrogen has much weaker absorption lines because it interacts only very weakly with electromagnetic waves.) Subsequent studies led to a growing list of new constituents, including the discovery of the arsenic compound arsine in 1990. The table includes a list of Jupiter’s atmospheric constituents and their abundances as determined by Earth-based, spacecraft, and atmospheric probe observations as of 2002.

If the condition of chemical equilibrium held rigorously in Jupiter’s atmosphere, one would not expect to find molecules such as carbon monoxide or phosphine in the abundances measured. Neither would one expect the traces of acetylene, ethane, and other hydrocarbons that have been detected in the stratosphere. Evidently, there are sources of energy other than the molecular kinetic energy corresponding to local temperatures. Solar ultraviolet radiation is responsible for the breakdown of methane, and subsequent reactions of its fragments produce acetylene and ethane. In the convective region of the atmosphere, lightning discharges (observed by the Voyager and Galileo spacecraft) contribute to these processes. Still deeper, at temperatures around 1,200 K (1,700 °F, 930 °C), carbon monoxide is made by a reaction between methane and water vapour. Vertical mixing must be sufficiently strong to bring this gas up to a region where it can be detected from outside the atmosphere. Some carbon monoxide, carbon dioxide, and water in the atmosphere come from icy particles bombarding the planet from space.

Galileo’s probe carried a mass spectrometer that detected the constituent atoms and molecules in the atmosphere by first charging them and then spreading them out with a magnetic field according to their masses. This technique had the advantage that it could measure noble gases like helium and neon that do not interact with visible and infrared light. As the probe descended through the atmosphere on its parachute, its spectrometer also studied variations in abundance with altitude. This experiment finally detected the previously missing hydrogen sulfide, which was found to be present even lower in the atmosphere than anticipated. Evidently this cloud-forming gas, like ammonia and water vapour, was depleted in the upper part of the hot spot by the aforementioned downdraft. It was not possible to measure oxygen, because this element is bound up in water, and the probe did not descend into the hot spot deeply enough to reach the atmospheric region where this condensable vapour is well-mixed.

The elemental abundances in Jupiter’s atmosphere can be compared with the composition of the Sun (see the right two columns of the table). If, like the Sun, the planet had formed by simple condensation from the primordial solar nebula that is thought to have given birth to the solar system, their elemental abundances should be the same. A surprising result from the Galileo probe was that all the globally mixed elements that it could measure in the Jovian atmosphere showed the same approximately threefold enrichment of their values in the Sun, relative to hydrogen. This has important implications for the formation of the planet (see below Origin of the Jovian system). Spectroscopy from Earth reveals a large spread in the values of other elements (phosphorus, germanium, and arsenic) not measured by the probe. The abundances of the gases from which these elemental abundances are derived depend on dynamical phenomena in Jupiter’s atmosphere—principally chemical reactions and vertical mixing. The significance of the helium and neon depletions is discussed in the section The interior, below.

Another difference with solar values is indicated by the presence of deuterium on Jupiter. This heavy isotope of hydrogen has disappeared from the Sun as a result of nuclear reactions in the solar interior. Because no such reactions occur on Jupiter, the ratio of deuterium to hydrogen there should be identical to the ratio of those isotopes in the cloud of interstellar gas and dust that collapsed to form the solar system 4.6 billion years ago. Since deuterium was made in the big bang that is postulated to have begun the expansion of the universe, a still more accurate measurement of the deuterium/hydrogen ratio on Jupiter would allow the calibration of expansion models.