Jupiter's atmosphere

Jupiter’s atmosphere differs significantly from that of Earth. It is composed mainly of hydrogen and helium and is far enough from the Sun that the temperature of the visible cloud deck is only 153 kelvins. Voyager spacecraft data revealed details concerning chemical composition, heat transport, and wind patterns within the atmosphere. The Galileo spacecraft’s atmospheric entry probe sampled that atmosphere directly and forced a rethinking of the physical model of Jupiter.

Overview

Observed through an Earth-based telescope, Jupiter’s most striking aspects are a pearly glow reflected from the planet, a series of east-west bands of pastel yellows, whites, browns, and blues, and the oblate aspect of its disk. The equatorial diameter is 6 percent larger than the polar diameter, a fact that is readily apparent to the observer. Closer inspection will reveal that distinctly bright and darker individual cloud features are visible within the banded structure of the atmosphere. Throughout the evening, an observer will notice that cloud features rotate from west to east at about 36° per hour, indicating that Jupiter rotates on its axis in less than ten hours. Careful observations of Jupiter will show that the planet’s visible cloud deck does not rotate as a solid body, but eastward winds at the equator sweep the clouds past those at midlatitudes at a rate that displaces them 7° eastward each day. This type of motion indicates that the cloud deck is opaque. One must know how fast the planet's core is rotating to understand the atmospheric winds.

Radio astronomers collected data from Jupiter and realized that variations in the radio signals from Jupiter could be attributed to the interaction of the charged particles ejected from the Sun with the planet's magnetic field. Although the signal that the radio astronomers were measuring was generated above the atmosphere of the planet, the signal varied as Jupiter’s magnetic field rotated. Astronomers’ understanding of magnetic fields led them to believe that the radio astronomers were measuring the rotation of Jupiter’s core. They determined that one rotation took 9 hours, 55 minutes, and 29.771 seconds.

A ground-based observer with an eyepiece outfitted with a crosshair can center the planet and record the time that a selected cloud feature takes to rotate past the crosshair. Because the planet rotates about its axis in approximately ten hours, while the observer is constrained to observe within a twenty-four-hour time frame, the feature will be visible on alternate nights. The rate of rotation of the feature across the visible disk of the planet, however, is such that the observer will find that the cloud rotates five times in slightly more than two days.

Accurate rotation periods were determined by the British Astronomical Association and the American Lunar and Planetary Institute during the first half of the twentieth century. Careful measurements of photographic data by Elmer Reese from 1960 to 1974 refined these data. An alternating pattern emerged when the data were related to the planetary core using the radio period of rotation. Strong eastward winds near the equator, as swift as 150 meters per second (more than 450 kilometers per hour), decreased poleward. Near 15° latitude, the prevailing wind in both hemispheres was westward. Between 10° and 35° latitude, the displacement of clouds revealed two westward and two eastward peaks in the horizontal wind speeds. Highly reflective regions called zones were bracketed on the equatorward side by westward winds and on the poleward side by eastward winds. The less reflective browner regions, or belts, were nested between the zones. Horizontal wind flow of this type in Earth’s atmosphere would generate conditions that would cause rising air in the zones, forming ice clouds at high altitudes. Air would descend in the belts and cause ice to melt, allowing a longer line-of-sight through the atmosphere and more light absorption—hence, less reflection. Recognition of this general circulation pattern in the 1960s led to questions concerning the nature of Jupiter’s ices.

As light travels outward from the Sun, it spreads out equally; thus, its ability to heat a surface decreases rapidly via an inverse square relationship in all radial directions. By the time sunlight reaches Jupiter, at a distance five times greater than Earth’s distance from the Sun, the intensity is diluted by a factor of twenty-five. This dilution leads to temperatures too low to allow the melting of ice formed from water; therefore, the visible cloud deck must contain another kind of ice.

Astronomers began to make infrared measurements of Jupiter’s atmosphere to refine their understanding of the temperature regime. They determined that the planet radiates one and a half times more heat than it absorbs from the Sun. These results imply that the interior of Jupiter is hotter than the cloud deck and that convection from the interior transports heat outward. The picture of a deep atmosphere dominated by east-west winds emerged. If Jupiter were composed of the same chemical mixture as the Sun, its atmospheric gas would be so strongly compressed that deep below the visible cloud deck, it would form a sea of liquid hydrogen and helium. There would be an indistinct change between the surface of the cryogenic fluids and the atmosphere. It also became apparent that it was essential to understand the atmosphere's chemistry.

Calculations carried out by American astronomers John Lewis and Ronald Prinn led to a model of the atmosphere that posited the existence of an upper cloud layer of ammonia ice, underlain by an ammonium hydrosulfide layer and a cloud layer composed of water ice (where the pressure is greater than ten times Earth’s atmospheric pressure at sea level). All these ices are white; therefore, the calculations yielded no information about coloring agents in the Jovian atmosphere. Above the topmost cloud deck, the atmosphere is predominantly hydrogen and helium gas, with traces of ammonia and methane.

Molecules of methane and ammonia can absorb enough energy from incident ultraviolet light to break bonds, which allows the hydrogen atoms to escape. Darrel Strobel calculated that ionized molecules combine to form more complex molecules, possibly aerosols or hazes. Laboratory work by other investigators has shown that many of the compounds that are formed are yellow and brown. Much of the color variation in the Jovian atmosphere may arise from variations in the heights of the underlying clouds.

Removal of the smog occurs in two ways. Either the particles grow large and fall to lower levels, or convective clouds of ammonia are carried upward like thunderheads, and the ammonia ice encases the smog particles and again causes them to fall to lower levels in the atmosphere. Computer modeling of scattering and transmission of light through the layers of hazes, gas, and clouds, carried out by Martin Tomasko, Robert West, and others, supports the theory that colorization is dependent on the varying heights of underlying clouds.

These calculations cannot explain Jupiter’s Great Red Spot—neither its colors nor its long life. This feature is the largest Jovian cloud system. North to south, it spans a distance slightly larger than the diameter of Earth, and it extends farther than two Earth diameters in the east-west direction. This large, unique cloud system is trapped between a westward wind on the equatorward side and an eastward wind on the poleward side. The winds are diverted around its perimeter, rotating it in a counterclockwise direction. In Earth’s atmosphere, a weather system with similar motion would rise in the center, spiral outward at the top of the cloud deck, and descend around its perimeter. The degree of redness of the Great Red Spot varies with time. A unique property of the Red Spot is its ability to absorb ultraviolet, violet, and blue light. Apparently, some constituent that has been carried up from lower, warmer depths absorbs the blue light, causing the Spot to appear redder than the surrounding clouds.

Observations of Jupiter in the infrared indicate that brown and blue-gray regions are warmer than white areas. Thus, the white zones are colder than the brown belts. The clouds above the Red Spot are cold, but there is a warmer region around its perimeter. The general heat loss from the planet indicates that the interior is warmer than the upper cloud deck that radiates to space; hence, infrared maps allow astronomers to determine the relative heights of clouds. A desire to obtain high-resolution maps of the infrared data and the structure of the cloud deck led to the development of the instruments on board the Pioneer and Voyager spacecraft. The Galileo spacecraft carried an atmospheric probe dispatched into Jupiter's upper atmosphere to relay data until the tremendous atmospheric pressure destroyed it.

Knowledge Gained

The Pioneer 10 and Pioneer 11 spacecraft, which arrived at Jupiter in November 1973 and November 1974, respectively, each carried three instruments that sampled the Jovian atmosphere. The fact that the Pioneer spacecraft were spin-stabilized limited the types of instruments that could be placed on board. Voyager 1 and Voyager 2 passed through the Jupiter system four months apart in 1979. Ultraviolet and infrared instruments and a spin-scan camera were trained on Jupiter’s atmosphere. The Galileo probe also sampled the atmosphere beginning in 1995. The Cassini spacecraft, in December 2000, and the New Horizons spacecraft, in February 2007, conducted in-depth studies of Jupiter as they flew past on their way to Saturn and Pluto, respectively.

Infrared data revealed little temperature variation between the equator and the pole at the cloud-top level. Data indicated there are limits to the role that equatorial solar heating plays in driving the zonal winds. Andrew Ingersoll proposed that solar heating at the equator could bring about a cloud structure that would act as an insulating blanket, causing the heat from the interior to emerge near the poles and resulting in slight temperature variation at the level of the ammonia cloud deck. This hypothesis implies that the outward heat transport from the interior may dominate the atmospheric wind patterns.

Pioneer spin-scan cameras were equipped with blue and red filters and polarizers. The nature of the camera did not allow many images to be obtained. Nevertheless, the collected data provided valuable material for studying the scattering properties of the atmospheric smog and haze layers. Although a series of images with sufficiently high resolution to map cloud motions could not be obtained, images of the Red Spot and north polar regions confirmed information previously gained from ground-based observation. They also provided data on the scale of the cloud structures.

Voyager 1 and Voyager 2 carried five instruments that were used to observe Jupiter’s atmosphere: two television cameras (one with a wide-angle view and the other with higher resolution and a narrow field of view), infrared and ultraviolet spectrographs, and a photopolarimeter. Multicolor high-resolution mapping of the visible cloud deck could be obtained at three-month intervals with each spacecraft. Near-encounter infrared measurements resolved temperature variations as a function of latitude and longitude on the planet. The ultraviolet spectrometer obtained data concerning high altitudes in the Jovian atmosphere. This extensive data set has been combined with the Pioneer and historical ground-based data sets in an effort to shed light on both short-term and long-term atmospheric variations. Cloud displacements were measured by Reta Beebe, Ingersoll, and others. Eastward winds near the equator were as powerful as 160 meters per second. Westward wind speeds at 15° north and 17.5° south latitude were both retrograde at 40 and 70 meters per second, respectively. Eastward wind maximums at 20° north and 24.5° south latitude were 170 and 60 meters per second, respectively. Voyager scientists found considerable differences between the magnitudes of wind jets in the northern hemisphere and those in the southern hemisphere; no change in the average zonal wind was detected at any latitude, however, during the five-month interval between the two encounters.

Infrared measurements indicated that the winds decrease with height above the deck and that temperatures and abundance of ammonia above the cloud deck are consistent with an atmosphere that is driven by cloud motions at the level of the visible cloud deck.

Galileo’s entry probe hit Jupiter’s upper atmosphere on December 7, 1995, at a speed of as much as 170,000 kilometers per hour. The atmosphere decelerated the probe at an increased g-load of approximately 230 (that means the force was 230 times that of normal Earth gravity at sea level). During the probe’s fifty-seven-minute-long plunge, it successfully relayed its findings to the Galileo orbiter for storage and eventual playback to Earth. Galileo was a little over 200,000 kilometers above the probe at the time.

The probe provided some surprising data. It was hoped that the probe would find considerable amounts of water vapor in the atmosphere and detect extensive electrical activity or lighting. It found very little of either. It did find a new radiation belt just 50,000 kilometers above the cloud tops. The probe registered extreme winds and experienced significant turbulence as it descended through Jupiter's atmosphere. Spectrometers found lower helium, neon, carbon, oxygen, and sulfur abundances than expected. Helium was nearly half as abundant as expected in contemporary atmospheric models for Jupiter. Galileo researchers were expecting the probe to fall through a three-layered cloud structure. The probe did not experience anything like what was predicted. The net flux radiometer on the probe found some high-level ammonia ice clouds, and the nephelometer instrument provided evidence of ammonium hydrosulfide clouds. Water ice was absent, suggesting the probe had entered one of the driest spots on Jupiter.

Wind strengths and atmospheric temperatures varied during the probe’s descent. Winds reached 350 kilometers per hour with gusts up to 525 kilometers per hour. After the probe had plunged 156 kilometers through the atmosphere under its main parachute, the high-temperature and high-pressure environment destroyed it; most likely, it was crushed, vaporized, or both nearly simultaneously. Essentially, the probe’s encounter forced planetary scientists to rethink the current model of Jupiter’s atmosphere.

Context

By the mid-nineteenth century, astronomers realized that Jupiter was unlike Earth. Using the basic laws of motion, the apparent size of Jupiter, and the known distances within the solar system, they determined that even though the volume of Jupiter was more than eleven hundred times larger than Earth’s volume, its mass was only 318 times larger than Earth's. Thus, although this planet is much more massive than Earth, gravity has not compressed Jupiter’s interior to the high densities present in Earth's interior. Nineteenth-century astronomers concluded that Jupiter could not have Earth's chemical composition.

By 1960, spectroscopists had determined that the atmosphere of Jupiter was cold and that temperatures at the level of the visible clouds were near 153 kelvins. Spectra revealed absorption by molecules of methane and ammonia. These observations are consistent with an atmosphere composed mainly of hydrogen and helium with small amounts of carbon and nitrogen. At the observed temperatures, oxygen would combine with hydrogen to form water, which would be trapped below the visible cloud deck. It became apparent that Jupiter was composed of a chemical mixture similar to that of the Sun and that the inner solar system's small silicon- and iron-rich planets were very different from the outer gas-rich planets.

Modern models concerning the formation of a solar system propose that planets the size of Jupiter form first at distances far enough from the parent star that the radiation has not expelled hydrogen and helium from the star. The turbulence this generates in the preplanetary gas and dust cloud leads to the formation of other planets, with the inner ones forming from hydrogen-poor material. The importance of Jupiter-sized bodies in forming planets that could support other life forms has stimulated interest in learning more about the nature of this gas giant. Notably, extrasolar planets have been found to have masses in excess of Jupiter and to be located extremely close to their stars.

Jupiter’s atmosphere is chemically unlike that of Earth. The planet’s depth and lack of irregular landmasses at its lower boundary contrasts with conditions in Earth’s atmosphere. There are, however, some similarities: the main constituents of Jupiter’s atmosphere are hydrogen and helium. Like Earth's atmosphere's nitrogen and oxygen molecules, these particles do not readily absorb sunlight. A large portion of solar energy passes through the upper atmospheres of these planets, and, in the case of Earth, the surface absorbs the energy and is warmed. The atmosphere is heated from the bottom, with trace constituents, carbon dioxide, and water absorbing and reradiating the energy. This leads to decreasing temperatures at increasing altitudes in the lower atmosphere.

Voyager infrared data confirmed that Jupiter has an internal heat source and emits 1.67 times more energy than it absorbs from the Sun. Infrared data indicate that the winds of Jupiter are driven, like those on Earth, by energy input in the lower atmosphere. Jupiter’s dissimilarity to Earth provides checks and challenges in the search to understand Earth and the solar system.

The next major Jupiter mission spacecraft, Juno, launched in August 2011 from Cape Canaveral. After traveling for five years, Juno reached Jupiter on July 4, 2016, and began to orbit the planet. The probe began collecting pictures and data from the planet to uncover more about the nature of Jupiter's atmosphere. Its mission continued in the early 2020s. Additionally, NASA’s James Webb Space Telescope captured very clear images of Jupiter in 2022 that allowed scientists a better understanding of Jupiter's atmosphere.

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