Composition and Atmospheric Structure
Because of its great mass, Jupiter has been undiscriminating in its composition. All gases and solids available in the early solar nebula were attracted and held by its powerful gravity. Thus it is expected that Jupiter has the same basic composition as the Sun, with both bodies preserving a sample of the original cosmic material from which the solar system formed.
Jupiter is a gas giant, composed of the same elements as the Sun and stars—primarily hydrogen and helium. Its internal structure is dominated by the properties of hydrogen, its most abundant constituent and by the high temperatures in the deep interior that remain from its luminous youth. Most of the interior is liquid: metallic hydrogen at great depths and high pressures, and normal hydrogen nearer the surface. In the upper few thousand kilometers, the hydrogen is a gas. The primary known or suspected cloud layers are, from the top down, thin hydrocarbon “smog”; ammonia; ammonium hydrosulfide; water-ice, and liquid water. [260-828]
Cloud top-aerosols Ammonia crystals Ammonium hydrosulfide clouds Ice crystal clouds Water droplets Trace compounds Fluid molecular hydrogen Transition Zone Fluid metallic hydrogen Possible core
The primary constituents of Jupiter have long been suspected to be hydrogen and helium, the two simplest and lightest atoms. However, it has proved impossible to derive accurate measurements of the abundance of these two elements from astronomical observations. On the basis of a rather simple infrared measurement, Pioneer investigators found He/H₂ = 0.14 ± 0.08. On Voyager, IRIS was able to obtain much improved infrared spectra, yielding an initial value of He/H₂ = 0.11 ± 0.3. Voyager scientists expect that further analysis will reduce the uncertainty to about ± 0.01. The ratio of 0.11 is in excellent agreement with the solar value of about 0.12, supporting the idea that Jupiter and the Sun have similar elemental compositions.
Astronomers have known for a long time that, in addition to hydrogen and helium, the compounds methane (CH₄) and ammonia (NH₃) are present in the visible atmosphere of Jupiter. In the 1970s, additional spectra in the infrared resulted in the discovery of water (H₂O), ethane (C₂H₆), germane (GeH₄), acetylene (C₂H₂), phosphine (PH₃), carbon monoxide (CO), hydrogen cyanide (HCN), and carbon dioxide (CO₂). All these are trace constituents, with two of them, ethane and acetylene, apparently formed at high altitudes by the action of sunlight on methane.
A total of approximately 100 000 infrared spectra, many of small regions on the disk, were obtained by IRIS. These spectra generally show hydrogen, helium, methane, ammonia, phosphine, ethane, and acetylene. In addition, excellent spectra were obtained in “hot spots,” regions in which breaks in the upper clouds permit radiation from deeper layers to escape. (The hot spots generally correspond to dark brown regions on photographs of the planet.) IRIS measured temperatures in the hot spots up to -13° C but no higher; apparently this temperature corresponds to the top of a deeper cloud deck. Spectral features indicative of the presence of water vapor and germane were clearly seen in the hot spots.
Further analysis of the IRIS spectra will be required to derive the abundances of the gases detected. However, even the preliminary data showed how variable Jupiter can be, especially in its upper atmosphere. The two hydrocarbons, ethane and acetylene, vary in relative abundance with latitude; there is less acetylene near the poles. In addition to this planetwide trend, smaller variations were seen from place to place and between the observations in March and July. All the variations will eventually provide information on the processes of formation, transportation, and destruction of hydrocarbons in the upper atmosphere.
| ELEMENTS DETECTED IN THE JOVIAN MAGNETOSPHERE | ||
|---|---|---|
| Element | Atomic Number | Instruments |
| Hydrogen (H) | 1 | UVS, Plasma, LECP, CRS |
| Helium (He) | 2 | Plasma, LECP |
| Carbon (C) | 6 | LECP |
| Nitrogen (N) | 7 | CRS |
| Oxygen (O) | 8 | UVS, Plasma, LECP, CRS |
| Neon (Ne) | 10 | CRS |
| Sodium (Na) | 11 | LECP, CRS |
| Magnesium (Mg) | 12 | CRS |
| Silicon (Si) | 14 | CRS |
| Sulfur (S) | 16 | UVS, Plasma, LECP, CRS |
| Iron (Fe) | 26 | CRS |
Voyager did not make any direct measurements of the chemical composition of the clouds, but theorists generally agree that the uppermost clouds are ammonia cirrus, and that layers of ammonium hydrosulfide (NH₄SH) and water exist at deeper levels. All these clouds are formed in the troposphere, the layer of the atmosphere in which convection takes place. The top of the ammonia cloud deck is thought to have a pressure of about 1 atmosphere and a temperature of about -113° C.
Ammonia cirrus is white, yet Jupiter’s clouds display a spectacular range of colors. Voyager did not determine the nature of the coloring agents; they may be minor constituents—trace impurities in a sea of white clouds. Perhaps organic polymers, formed from atmospheric chemicals such as methane and ammonia that have reacted with lightning, are responsible for the oranges and yellows. The color of the Red Spot could be caused by red phosphorus (P₄). According to this theory, phosphine (PH₃) from deep in Jupiter’s atmosphere is brought to high altitudes by the upwelling of the Great Red Spot. Ultraviolet light, penetrating the upper reaches of the Red Spot, splits the phosphine molecules, and, through a series of chemical reactions, converts the phosphine into pure phosphorus. However, this theory fails to explain the existence of the smaller red spots on Jupiter; these spots are not at such high altitudes as the Great Red Spot (which is the highest and coldest of Jupiter’s visible clouds), so it is unlikely that ultraviolet light could react with any phosphine in these areas to produce red phosphorus.
Although the Voyager spacecraft never flew over the poles of Jupiter, it is possible to reconstruct from several images the View that would be seen from directly above or below the planet. Note the absence of a strong banded structure near both poles. The regular spacing of cloud features is obvious. In the Southern hemisphere, the three white ovals are 90 degrees apart in longitude, but a fourth oval at the other quadrant is missing. The irregular black areas at each pole are places for which no Voyager data exist. The resolution of the original pictures from which these polar projections were made was about 600 kilometers.
North pole. [P-21638C]
South pole. [P-21639C]
Various forms of elemental sulfur might be responsible for the riot of color we see on Jupiter. Sulfur forms polymers (S₃, S₄, S₅, S₈,) that are yellow, red, and brown, but no sulfur in any form has been detected on Jupiter. “We never promised you we were going to identify the colors on Jupiter with this mission,” one of the atmospheric scientists remarked, “but we will have a probe that is going into the atmosphere in the mid-1980s—Galileo.” Perhaps the mystery of the Jovian clouds will have to wait till then.
Temperature maps of Jupiter were obtained by IRIS in radiation arising at different levels above the clouds. Maps show temperatures at pressures of 0.8 atmosphere near the clouds, and 0.2 atmosphere near the top of the troposphere. In addition to the low temperatures over the bright zones and the higher temperatures over dark belts, there is a great deal of smaller scale structure. It is interesting that a cold area corresponding to the Great Red Spot is clearly visible even near the top of the troposphere, indicating that this feature disturbs the atmosphere to very high altitudes.
The structure of the atmosphere of Jupiter above the troposphere was investigated through the radio occultation experiment as well as by IRIS. The level in which the minimum temperature of about -173° C occurs has a pressure of 0.1 atmosphere. Above this point lies the stratosphere, in which temperatures increase with altitude as a result of sunlight absorbed by the gas or by aerosol particles resembling smog. At 70 kilometers above the ammonia clouds, the temperature is about -113° C. Above this level, the temperature stays approximately constant, although at extreme altitudes the temperature again rises in the ionosphere.
If one could “unwrap” Jupiter like a map, views such as these would be obtained. The comparison between the pictures shows the relative motions of features in Jupiter’s atmosphere. It can be seen, for example, that the Great Red Spot moved westward and the white ovals eastward during the time between the acquisition of these pictures. Regular plume patterns are equidistant around the northern edge of the equator, while a train of small spots moved eastward at approximately latitude 80° S. In addition to these relative motions, significant changes are evident in the recirculating flow east of the Great Red Spot, in the disturbed region west of the Great Red Spot, and as seen in the brightening of material spreading into the equatorial region from the more southerly latitudes. [P-21771C]
The planet as it appeared about March 1.
As it was in early July.