Neptune's atmosphere

Neptune’s atmosphere exhibits surprisingly rapid changes, large cloud systems, extremely fast winds, and large-scale vertical motions associated with heating from outside and inside. These features were quite unexpected for a planet so far away from the Sun, where astronomers expected to find a cold, featureless atmosphere.

Overview

Models for Earth’s atmosphere assume that the primary source of energy is solar radiation. Some basic features of a planetary atmosphere can be predicted from the planet’s mass and surface temperature. The mass has to be large enough that the escape velocity of gas at the surface is higher than the random thermal motion speed of most of the molecules. The temperature must be neither too high (as at Mercury) nor so low that everything condenses and freezes. Atmospheric pressure at any level can then be related to the weight per unit area of the gas above that level.

Neptune’s mass is 102.42x10²⁴ kilograms, which is roughly seventeen times that of Earth. It radiates nearly twice as much energy as it receives from the Sun, so that its internal heat is a major factor in the atmospheric dynamics.

The “surface” of Neptune is arbitrarily defined as being the level where the gas pressure reaches one bar or 101.325 kilopascals (or 101,325 newtons per square meter), this being the value of standard sea-level pressure on Earth. The planet’s radius is defined at this level. At the equator of Neptune, this radius is 24,764 kilometers. Here, the acceleration due to gravity is eleven meters per second squared, close to the Earth's surface value of 9.78 meters per second squared.

However, the gaseous atmosphere extends down to levels where the pressure exceeds one thousand bars and temperatures are increasingly higher. The boundary between the condensed (solid and liquid) and gas-state substances is ill-defined but is believed to occur at about the same radius as the Earth’s surface radius, which is only about one-quarter of the radius of Neptune. Substances such as hydrogen, helium, and methane exist in the atmosphere as gases and as solids called “ices.”

The effective temperature of Neptune is fifty-nine kelvins (-214 degrees Celsius). In the troposphere, the atmospheric level above the planet's "surface," the temperature decreases with increasing altitude. At one bar—the arbitrary gaseous surface of zero altitude—the temperature is approximately seventy-two kelvins (-201 degrees Celsius). The temperature is fifty-five kelvins at an altitude of roughly fifty kilometers, where the pressure is 0.1 bar. This level is the tropopause or top of the troposphere. Above the tropopause is the stratosphere, where the temperature increases with altitude. This is attributed to Neptune's core, which, at a temperature of seven thousand kelvins, heats the planet more than the Sun. Temperatures reach up to 275 kelvins (1.85 degrees Celsius) at eight hundred kilometers, where pressure is 1x10^-5 millibars. This level is called the mesopause. After the mesopause is the thermosphere at pressures lower than 1x10^-5 to 1x10^-4 millibars. The thermosphere gradually becomes the exosphere, the farthest level of atmosphere from the surface. At these high levels, the temperature is believed to be due to any of three mechanisms: the “dayglow,” where diatomic hydrogen gets dissociated by collision with low-energy electrons; heating by the production of auroras due to interaction of the magnetic field with ions; or joule heating as ions are accelerated by the magnetic field, relative to the movement of the neutral gases in atmospheric winds.

The average composition of Neptune’s atmosphere is 77 to 83 percent hydrogen, 16 to 22 percent helium, and 1 to 2 percent methane, with about 0.019 percent hydrogen deuteride and traces of ethane. Oxygen is present in water, nitrogen in ammonia, and carbon dominantly in methane, though some carbon monoxide has been detected on Neptune. Other elements, such as sulfur, have also been detected. It is believed that in the molten core of the planet, there is silicate rock.

Condensation clouds are present at various levels in Neptune's atmosphere. Clouds of water, ammonia, and hydrogen sulfide are predicted to form with a base at a level where pressure is more than five bars. Between two and five bars, clouds of solid particles of ammonium hydrosulfide form. Frozen ammonia crystal clouds form at around five bars. Above fifty bars and 273 kelvins (-0.15 degrees Celsius), water-ice clouds are present. Around seventy-five bars, “solution” clouds are present. An ocean of ammonia dissolved in water has been postulated to exist below the five-bar region. Frozen methane clouds occur at around two bars to 0.5 bars, according to predictive models. The fraction of methane in the atmosphere at the one-bar level is about 2 to 3 percent. The temperature at the cloud tops is approximately fifty-five kelvins at 0.1 bars, the bottom of the stratosphere.

Absorption of infrared by methane contributes to the observed light blue color of Neptune. The fraction of methane drops significantly at higher altitudes but is still well above the ratio of methane to hydrogen found in the atmosphere of the Sun. Traces of carbon monoxide (CO) of about one part per million and hydrogen cyanide (HCN) of about one part per billion have been detected. The HCN likely forms through photochemical reactions with molecular nitrogen in the upper atmosphere. The abundance of CO in the stratosphere is greater than what can be explained by photochemical formation. One hypothesis is that CO comes from the lower regions. Another is that CO is formed when ice-bearing meteorites ablate in the upper atmosphere, with the methane participating in the reaction.

Neptune orbits the Sun once every 164.79 years but rotates on its axis once every 16.11 hours. The axis is inclined at 28.32 degrees, so there are noticeable seasonal changes. Since Voyager 2's flyby in 1989, the south pole has been closer to the Sun and is, therefore, warmer. The cloud cover has become noticeably brighter, and there is evidence of strong updrafts.

Heidi Hammel and other researchers concluded from analyses of narrow-angle images from Voyager 2 and from radio signals based on Neptune's internal rotation that large-scale cloud features in the equatorial and tropical regions move fast enough against the direction of rotation that their period of rotation about the axis is as high as 18.4 hours, compared to the planet’s 16.11-hour day. This means wind speeds of 325 meters per second or 1,170 kilometers per hour. A large spot, called the Great Dark Spot, seen in the southern atmosphere by Voyager 2 in 1989, had disappeared by 1994; other dark spots have since appeared in the northern and southern hemispheres. These spots appear to be storms that form and dissipate. In the dark spots, wind speeds up to 2,400 kilometers per hour have been postulated. Several bright spots seen in infrared images appear to indicate warmer rising plumes of air that come from warm layers deep down but reach the upper atmosphere.

Knowledge Gained

Much of what is known about Neptune comes from the Voyager 2 spacecraft, which transmitted images from an approach distance of 4.48 million kilometers in 1989. The Hubble Space Telescope (1994–1995), mid-infrared data from the Keck telescope in Hawaii, the National Aeronautics and Space Administration (NASA) Mauna Kea telescope, and the Very Large Array Radio telescope in New Mexico have added to our understanding of Neptune.

Predictive models use data from the observed emission and absorption through the atmosphere at various wavelengths of radiation. Sunlight reflected off clouds in the upper troposphere and lower stratosphere shows bright bands between twenty and fifty degrees south. Distinct bright clouds are seen around seventy degrees south. A bright south polar “dot” is visible. Observations in narrow bands corresponding to the absorption wavelengths of methane and ethane show that in the south polar regions, these substances appear to be less abundant. Scientists argue that this indicates a subsiding flow in the south-polar region that draws the light gases down into warmer regions of the atmosphere and prevents them from freezing, as would happen if they rose into the upper atmosphere. The subsiding flow heats the lower levels adiabatically, while the lack of frozen crystals in the “dry” upper atmosphere above the south polar region makes the atmosphere transparent to much greater depths and allows the warmer regions below to be detected. It appears that air is rising more in the mid-southern and northern latitudes, and it is sinking at the equator and the south pole. This global circulation pattern is partially attributable to solar heating. Smaller clouds, such as the bands observed near seventy degrees south latitude, are attributed to local weather.

A controversial theory for the intense atmospheric activity seen on this distant planet comes from Glenn Orton and his team's electric universe theory. They reported temperatures near Neptune’s south pole that are high enough to allow gaseous methane from lower levels to escape into the upper atmosphere without freezing. They attribute this level of energy input to Neptune to an electrical connection with the rest of the solar system and out to the surrounding interstellar environment. Thus, the heating is associated more with perturbations of the magnetic field of Neptune than with the direct optical flow of thermal energy from the faraway Sun. According to this theory, Neptune is part of the “electric circuit” and hence can experience levels of energy input that are impossible to predict using just the distance from the Sun. Orton and his colleagues have argued that conventional models of atmospheric dynamics based on solar heating fail when dealing with faraway planets.

Context

Expected to be much colder and less active than Uranus because it is 30.1 astronomical units (AU) from the Sun, Neptune has surprised scientists with its profound seasonal changes in cloud cover and atmospheric circulation. Hammel and her colleagues have concluded that cloud-top wind speeds are roughly the same order for all planets ranging from Venus to Neptune, even though solar energy inputs to their atmospheres differ by three orders of magnitude. Estimates of Neptune’s internal energy and the amount of solar radiation that reaches it do not yet explain this level of activity. Radical theories of electric current flows from the Sun to the outer limits of the solar system have been advanced as alternative explanations. Neptune demonstrates, therefore, how the study of an extreme situation can challenge models based on Earth’s atmospheric behavior.

Continued observations of Neptune with both ground-based telescopes and the Hubble Space Telescope must suffice until a Neptune orbiter might be dispatched from Earth. In early 2002, a Neptune orbiter probe was investigated but not funded, in part, because of its projected cost and in part because it would have required nuclear propulsion to make a trip to Neptune in a reasonable amount of time; such propulsion technology was not yet available. Nuclear-powered New Horizons— launched in 2006 and which reached Pluto in 2015—had not been programmed to observe Neptune during its flyby. Plans are in place, however, for New Horizons to capture images of Uranus and Neptune, though because the probe has already passed the planets’ orbit, those images will be from behind. When the James E. Webb Space Telescope, an infrared observatory, was launched into space in 2021, it joined the instruments used to scrutinize the dynamics of Neptune’s atmosphere. In 2022, the Webb Space Telescope captured the clearest images of Neptune’s rings taken in over thirty years.

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