Planetary atmospheres

The gaseous envelopes that surround the planets are known as their atmospheres. Scientists believe that planetary atmospheres developed in one of two ways— when the solar system as a whole formed, or later, from materials released by the planets well after their own formation in a process called outgassing. One task of continuing spacecraft investigations is to seek answers to lingering questions about planetary atmospheres.

Overview

Atmospheres are an attribute shared by seven of the eight planets in the solar system. (In 2006, Pluto was classified by the International Astronomical Union as a dwarf planet.) Crater-covered Mercury is the only planet that does not have a gaseous envelope, though it does have a tenuous exosphere. Even Titan, a large satellite of Saturn, possesses a significant atmosphere. Nevertheless, these atmospheres differ considerably in chemical composition, structure, and density, and comparative planetologists’ study of these differences is critical to understanding Earth’s atmosphere and future.

Planetary scientists believe the planets acquired their atmospheres in one of two ways. In some cases, the atmosphere formed with the planet around the Sun out of the solar nebula around 4.5 billion years ago. In such cases, a planet is said to have a primordial, or captured, atmosphere. In other examples, an atmosphere appears to have been created from gaseous material released from within the planet after it formed. These are called secondary or outgassed atmospheres.

The atmospheres of the Jovian planets (Jupiter, Saturn, Uranus, and Neptune) are primordial. Like the Sun itself, they are composed mainly of hydrogen and helium. On the other hand, three terrestrial planets (Venus, Earth, and Mars) may have secondary rather than primary atmospheres. Characteristics and compositions of these atmospheres vary considerably. The dwarf planet Pluto, which is neither Jovian nor terrestrial, does have a very thin primordial atmosphere that freezes on its surface during the part of Pluto’s highly eccentric orbit when the dwarf planet is far from the Sun.

An essential determinant of a planet’s atmospheric composition and density is its ability to retain atoms and molecules of gases with its gravity. According to kinetic molecular theory, gases are composed of discrete atoms and molecules traveling in random directions at various speeds. Collisions between these atoms and molecules occur, transferring energy and altering the velocities of individual atoms or molecules. According to the physics of Maxwell-Boltzmann statistics, the distribution of those molecular speeds for the gas as a whole is governed by the temperature of the atmosphere. Should the speed of any individual atom or molecule at some instant exceed the planet's escape velocity, that atom or molecule will leak away into space. The root-mean-square speed of molecular motion in the gas is a function of the composition and temperature of the gases, while a planet’s escape velocity is a function only of its mass.

The likelihood of collision with other molecules also limits the ability of a molecule to escape the atmosphere. This likelihood is so high in the lower atmosphere as a virtual certainty. However, with increasing distance above the surface, the atmosphere's density diminishes until a point is reached where a molecule moving upward and away from the planet has a very high probability of not encountering any other molecule. At that point, if its velocity exceeds the escape velocity, it will leave the planet. This region of a planet’s atmosphere is called the exosphere.

Although by far the greatest amount of material in the early solar system was hydrogen and helium, in the modern world, these gases are relatively rare in the atmospheres of the terrestrial planets. In part, this results from the fact that proximity to the Sun raises atmospheric temperatures of these inner planets to the point that molecular activity of hydrogen and helium atoms exceeds the low escape velocities of small planets. Some theorists argue, however, that as the solar system formed, most of the interplanetary hydrogen and helium in the inner solar system was pushed away from the region in which the terrestrial planets would later form by the pressure of radiant energy coming from the young Sun as soon as it began to shine.

Whichever theory is true, Earth’s atmosphere is approximately 78 percent nitrogen and 21 percent oxygen. Most of the remaining 1 percent is argon, and only 0.033 percent is carbon dioxide. At sea level, the atmosphere has a 0.001 grams per cubic centimeter density. This atmosphere is divided into layers according to the prevailing characteristics at various altitudes. The lowest layer is the troposphere. It contains 90 percent of the atmosphere’s mass and almost all the water vapor and dust. What sets the troposphere apart is the process of convection. Warm air currents rise, and cool air currents sink. This distributes the heat from the Sun and establishes the dynamics that create weather. The upper limit of this layer, called the tropopause, is defined by a temperature of 333 kelvins.

Above the troposphere is the stratosphere, which extends to an altitude of fifty-five kilometers. In the stratosphere, temperature steadily increases with altitude. Within this layer, between twenty-five and forty-five kilometers high, lies a sublayer in which ozone molecules occur in a density of ten parts per million. This trace concentration is enough to absorb ultraviolet radiation efficiently, thereby protecting life. The ozone layer also prevents photochemical reactions that would deplete the planet of its water.

The stratosphere ends, and the mesosphere begins when the temperature reaches about 283 kelvins and begins dropping again. The mesosphere continues to an altitude of eighty kilometers, where the temperature is about 163 kelvins. Above the mesosphere lies the ionosphere, so named because many of the atoms of gas in this layer have been split into positively charged ions and negatively charged electrons by the Sun’s intense X-ray and ultraviolet radiation. Earth’s exosphere—the layer that marks the transition to outer space—begins at 400 kilometers and extends for thousands of kilometers. The determination by the National Aeronautics and Space Administration (NASA) that the atmosphere starts at an altitude of 400,000 feet, the point where reentry of spacecraft begins, is, therefore, somewhat arbitrary.

The presence of free oxygen in abundance is unique to the atmosphere of Earth. Whether Earth’s original atmosphere was of the primordial or outgassed type, free oxygen would have resulted in part from the photochemical dissociation of water vapor (until the development of an ozone layer) and then from photosynthesis, the process whereby early microorganisms, such as cyanobacteria and later plants generate energy-supplying carbohydrates from water vapor and carbon dioxide in the presence of sunlight and chlorophyll. There is fossil evidence of photosynthetic plants existing on Earth going back at least two billion years ago. It is probably not coincidental that rocks 1 to 1.5 billion years old are the earliest to show strong evidence of forming an oxygen-rich atmosphere.

The atmosphere of Venus is ninety times more massive than Earth’s and is composed of 97 percent carbon dioxide. The remaining 3 percent is almost entirely nitrogen, with trace amounts of water vapor, sulfur dioxide, and other gases. The atmosphere of Venus is very hot, resulting from the fact that the dense envelope of carbon dioxide is an excellent absorber of infrared energy and very effectively traps heat radiated by the surface. This phenomenon, called the greenhouse effect, causes the temperature of the atmosphere near the surface to reach 733 kelvins. Because of this high temperature, any water on the planet should exist as water vapor in the atmosphere. Its very low abundance (150 parts per million) presented a major puzzle for planetologists. The high atmospheric temperature causes water vapor to rise to a great height, where it is then subjected to photochemical dissociation by intense ultraviolet radiation.

The dense atmosphere of Venus is efficient at circulating heat. The day and night temperatures of the troposphere do not differ appreciably. Huge, slow-moving convective air currents called Hadley cells span the planet’s temperate latitudes from the poles to the equator and are not affected much by the planet’s slow rotation. At elevations above fifty kilometers, where the atmospheric density is much lower than at the surface(actually about the same as the density of Earth’s lower atmosphere), a strong westward current prevails over all but the polar regions and produces 360-kilometer-per-hour winds that whip the clouds into enormous swirling patterns.

Astronomers have long realized that the brilliance of Venus in the night sky is caused by the high albedo of the planet’s perpetual cloud cover. Seventy-five percent of the sunlight reaching Venus bounces off the tops of a nearly featureless and visually impenetrable deck of sulfuric acid-laden clouds suspended between forty-seven to seventy kilometers above the surface. The clouds result primarily from photochemical reactions driven by strong ultraviolet energy from the Sun; hence, they have more in common with Earth’s ozone layer than Earth’s clouds of condensed water vapor.

Mars has an atmosphere that is only 1/150 the mass of Earth, but carbon dioxide is the dominant component, accounting for 95.3 percent of its mass. Nitrogen accounts for an additional 2.7 percent and argon 1.6 percent of the total, with small fractions of a percentage each of oxygen, carbon monoxide, and water vapor. Apart from the occasional planet-enveloping dust storms, Martian skies are generally clear, but several water vapor clouds form in localized areas. Clouds of carbon dioxide are also noted occasionally.

Like Venus, Mars has a very low water vapor content in its atmosphere, but for a different reason. The Martian atmosphere's very low pressure and temperature are not conducive to holding moisture at any elevation. A few hundredths of a percent water vapor content constitutes high relative humidity on Mars. This results in the appearance of water vapor clouds and fog at lower elevations.

Mars exhibits seasonal wind patterns similar to Earth's, with prevailing westerlies, high-altitude jet streams, and cyclonic disturbances. Strong localized surface winds are common, sometimes reaching 150 kilometers per hour. These are believed to be associated with powerful convection currents generated by the rapid heating of the surface during the daytime and are similar to dust devils, an atmospheric phenomenon common in the desert regions of the Earth. Dust devils on Mars were imaged from the surface by the Spirit and OpportunityMars exploration rovers; those dust devils had the coincidental benefit of blowing dust off these rovers’ solar panels, thereby increasing the electrical power available to the rovers. Once such a wind pattern forms, it may lift Martian dust high into the atmosphere, where the dust grains have a self-propagating effect, retaining heat and intensifying the wind disturbance. Periodically, such dust devils grow into planet-encompassing storms that may last for months. The Martian atmosphere also exhibits a strong seasonal wind flow caused by carbon dioxide exchange between the atmosphere and the polar caps. The Mars Curiosity rover has helped paleontologists observe patterns of wind and natural radiation, which has furthered their understanding of the Martian atmosphere and environment.

The atmospheres of the gas giant Jovian planets contrast sharply with those of Venus, Earth, and Mars. Jupiter, Saturn, Uranus, and Neptune are composed almost entirely of gases, but the pressure deep beneath their cloudy exteriors causes the gaseous mixture to become extremely dense. Their atmospheres grade gradually into regions where the gases exist as liquids.

The atmospheres of Jupiter and Saturn are similar in terms of chemistry, and the abundances of hydrogen, carbon, and nitrogen are, in bulk, close to those of the Sun. These atmospheres are primordial. Both planets display a striking pattern of alternating cloud bands parallel to their equators. Colors are primarily attributable to the photochemical reactions at and near the cloud tops. Since Saturn lies about twice as far from the Sun as Jupiter, it receives significantly less solar radiation and ultraviolet energy. Photochemical reactions are less strong and more varied than they are on Jupiter. The highest cloud level in Jupiter is a reddish color believed to be explained by the presence of phosphorus, which may be photochemically dissociated from phosphine gas known to be in Jupiter’s upper atmosphere. The second level of clouds is white, the third layer is brown (most likely caused by sulfur), and the fourth layer is blue. Ammonia and ammonium hydrosulfide are major cloud-forming molecules present in Jupiter and Saturn. A better understanding of the relationship of the observed colors to the chemistry of the cloud layers must await the descent of instrumented probes into the atmospheres of the gas giants. From late 1995 to 2003, the probe Galileo gathered data on Jupiter’s upper atmosphere; its results did not match the expectations of the contemporary model for Jupiter’s atmosphere. On August 5, 2011, NASA launched its Juno spacecraft to investigate Jupiter's atmosphere, magnetosphere, structure, and origins. Juno reached orbit around Jupiter in 2016 and continued its mission through the early 2020s.

Studies of features moving in the cloud bands show that the bands are driven by alternating east and west wind patterns. Explaining this wind pattern presents a difficult challenge to planetologists. One leading theory holds that the fluid interior of each planet is organized as if it were a series of nested cylinders rotating around axes coincident with the planet’s axis of rotation. If this is the case, the contra-rotating cloud bands are the extremities of the nested structures, visible to Earth at the point where the spherical limits of the planet itself truncate them. Laboratory studies show that such structures can develop in rapidly rotating fluids and that alternating cylinders rotate in opposite directions under certain conditions. Jupiter’s Great Red Spot, Saturn’s white spot, and numerous lesser cyclonic systems visible on both planets support this theory of the origin and motion of the cloud bands. These features do not participate in the rapid westward or eastward motions of the cloud bands adjacent to them but circle the planets more slowly, rolling like ball bearings between surfaces rotating in opposite directions. Moreover, suppose the cloud bands’ motions are driven by fluid dynamics extending deep into the planet. In that case, the systems must have enormous inertia, explaining the observed persistence of the cloudy bands with minimal variation over decades.

Although Uranus and Neptune are Jovian and possess primordial atmospheres rich in hydrogen (84 percent) and helium (14 percent), their characteristics differ somewhat from those of the solar system’s two largest planets. The suggested explanations for these differences are that Uranus and Neptune receive substantially less heat from the Sun and are physically much smaller than Jupiter and Saturn.

In Uranus’s case, the planet’s radical tilt on its axis and the resulting pattern of solar heating must be considered. Uranus displays a nearly featureless pale blue-green exterior in which there is almost no hint of the cloud bands typical of Jovian planets. This is caused by a high haze of methane and its photochemical by-products. Spacecraft imaging has revealed that cloud bands are present beneath this haze, indicating that the currents that drive the clouds must be caused by the planet’s fast rotation rather than by solar insolation. Uranus was examined during an extended period when its northern hemisphere was exposed to continual sunlight and its southern hemisphere to continual night. Yet, the wind flow seemed unaffected by the unequal heating of the two. Nevertheless, Uranian cloud bands all rotate in the same direction, and there is no evidence of any large cyclonic structures being spun between contra-rotating bands.

Neptune’s atmospheric features are more distinct than Uranus’s and more transient than any of the other gas giants. Cloud bands are visible even in the best terrestrial telescope images, and Voyager 2 discovered the Great Dark Spot in the planet’s southern hemisphere and several other smaller dark spots and bright features. These features fade in and out, brighten and darken, and even wander across the latitudinal cloud bands, like cars changing lanes on a highway. The Great Dark Spot, evidently very similar to the big cyclonic features on Jupiter and Saturn, was observed to change size and shape as it rolled along between two contra-rotating cloud bands. Because of the limited amount of sunlight available to produce photochemical reactions at this distance from the Sun, the haze of methane and hydrocarbons is thinner at Neptune than at Uranus. Below the haze are clouds composed primarily of methane ice, and below that are layers of hydrogen sulfide and ammonia clouds.

The existence of an atmosphere around Pluto is somewhat speculative. It must be very thin if it truly exists as something more than just a tenuous feature since the planet is so cold. At forty-eight kelvins, almost all gases except hydrogen and helium would form liquids or solids and settle on the planet's surface as ice or snow. Also, the planet is too small to retain hydrogen or helium. A small amount of gaseous methane may exist close to the surface. There is some evidence that the amount increases and decreases as the planet varies its distance from the Sun by 13 percent during its orbit.

The only satellite in the solar system to possess a significant atmosphere is Titan, a satellite of Saturn whose diameter is about the same as that of Mercury. Data from the Voyager and Cassini missions confirmed a nitrogen-rich atmosphere as much as two hundred kilometers deep, in which photochemical and electrochemical reactions have produced a high-altitude haze of complex hydrocarbons that hide all features of the surface and lower atmosphere. Of particular interest is evidence that polymer chains of organic molecules form in this haze and then sink to the surface of the planet. This scenario closely resembles the process proposed to explain the origin of life on Earth. However, the temperature of Titan, ninety-four kelvins, precludes liquid water, which is also believed necessary for life. Besides Titan, some other satellites, especially those that are geologically active (and thus have conditions for outgassing), have tenuous atmospheres.

Methods of Study

Understanding planetary atmospheres beyond Earth has been facilitated greatly by data returned by such spacecraft as MESSENGER, Mariner 10, a host of Venus and Mars probes, Pioneer 10 and Pioneer 11, the two Voyagers, and the Galileo and Cassini orbiters. The Mars Curiosity rover has revealed that what remains of the Martian atmosphere is still active. Before the epic journeys of these robotic laboratories, knowledge of the gaseous envelopes around the planets was severely limited by the difficulties of extracting meaningful data from faint images seen in telescopes.

Techniques for determining the chemical constituents of an atmosphere from afar are not essentially different, whether the measurements are made through a telescope on Earth or from aboard a spacecraft coming within tens of thousands of kilometers of the planet. In both cases, the viewing telescope must be equipped with a spectroscope, an instrument that breaks the reflected light of the planet up into its component colors. It thereby reveals the “fingerprints” of chemicals present in the source of the light. A spacecraft in close proximity to a planet, however, has more reflected light with which to work, and its target appears big enough that the instrument can be very discriminating about what it samples.

Coupled with the accuracy and specificity of the chemical assays provided by planetary space probes are the results of radio science experiments. These can probe the temperature and density of the deeper atmosphere. A wide variety of other instruments can image and sample the planet in many other wavelength regions of electromagnetic energy. In the cases of Venus and Mars, it has also been possible to send instrumented probes through the atmosphere, gathering data as they prepared to land on the surface.

Efforts to explore the solar system fully with sophisticated robotic spacecraft reached a peak during the interval between 1976 and 1989. Some planetologists have termed this the “golden age” because it produced an unprecedented harvest of information about the planets and converted a field of astronomy that previously had relied heavily on guesswork into a sophisticated branch of science, rich in data and theories with relevance to Earth. The revolution in planetary astronomy led to a field of specialization called comparative planetology, which sought to learn more about the features (such as the atmospheres) of similar bodies by comparison to one another. A second age of robust planetary exploration began with advanced spacecraft in orbit around the planets, such as Magellan and Venus Express (Venus), Galileo (Jupiter), Cassini (Saturn), and MESSENGER (Mercury). This period also included a sustained series of landers and orbiters sent to Mars on a fairly regular basis. Galileo and Cassini shed light on the atmospheric dynamics of the gas giants Jupiter and Saturn, respectively. They were follow-ons to the Voyagers, which provided tantalizing glimpses of these two planets as they flew past Jupiter and Saturn.

Context

Accompanying the great increase in data about the solar system has been a quantum improvement in understanding how each of the planets formed. This knowledge rests heavily on information concerning the planetary atmospheres, particularly as the planets with primordial atmospheres retain vestiges of the chemical makeup of the solar nebula from which the Sun and all its planets were born.

Some fundamental questions regarding the atmospheres of the other planets remain unanswered. This is particularly true of the Jovian planets. The depth to which the visible patterns extend is not generally known, nor is it clear whether and to what depth the atmospheres are layered below the cloud tops. Whether heat from the Sun or heat generated in the planet's interior has the primary role in driving the circulation of the atmosphere has also not yet been determined. The next generation of planetary probes will seek to answer these questions.

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