Uranus's atmosphere

Uranus is the seventh planet from the Sun. It shares much in common with Jupiter and Saturn, but it is also significantly different from the larger Jovian, or gas giant, planets. Its atmosphere is composed mainly of hydrogen and helium, but its color is governed by selective absorption of light by methane, which is abundant in greater measure in Uranus’s atmosphere than in either Jupiter’s or Saturn’s.

Overview

The planet Uranus was the first to be discovered with a telescope. Its existence was declared by Sir William Herschel on March 13, 1781. After several proposed names, the most curious of which was a proposed reference to the King of England, George III, the planet was named Uranus. From mythology, Uranus is the father of Saturn and grandfather of Jupiter.

Uranus is the third largest planet in the solar system. With an orbit that varies from 18.4 to 20 astronomical units (AU, or the mean distance from the Earth to the Sun, namely 150 million kilometers), it takes Uranus eighty-four years to complete one revolution about the Sun. Naturally, at this greater distance from the Sun, Uranus receives far less solar radiation than Jupiter and Saturn. Nevertheless, its location suggested to early researchers that the composition and nature of Uranus were similar to those of Jupiter and Saturn.

Superficially, that is true. Uranus is composed largely of hydrogen and helium. However, the atmosphere of Uranus has been determined to be colder than that of Jupiter and Saturn and has a less dynamic structure than the turbulent atmosphere of Jupiter or the pastel banding of Saturn. Uranus’s atmospheric temperature can drop to forty-nine kelvins. In addition to the preponderance of hydrogen and helium, the atmosphere has a larger amount of ices and hydrocarbon than the atmospheres of Jupiter and Saturn. Ices include water, ammonia, ammonium hydrosulfide, and methane. Selective absorption of radiation, in good measure by methane, results in the planet’s pale bluish-green appearance.

By compositional abundance, Uranus is 83 percent hydrogen, 15 percent helium, and 2.3 percent methane. The total also includes other low-concentration gases and hydrocarbons and does not add up precisely to 100 percent, given significant uncertainties about the abundance of hydrogen and helium. Hydrocarbons that appear only in trace amounts include ethane, acetylene, methyl acetylene, and diacetylene. These and other hydrocarbons are thought to be produced in the upper atmosphere by photolysis of methane under incident solar ultraviolet light. Carbon monoxide and carbon dioxide have also been detected. Like the planet’s water vapor, carbon dioxide and carbon monoxide must have been acquired by impacting comets and infalling dust. All totaled, Uranus has a carbon content, primarily found in the atmosphere, somewhere between twenty and thirty times that of solar abundance.

One thing that makes Uranus particularly curious is the fact that its rotational axis is tilted 97.8 degrees from the perpendicular to the ecliptic plane. This tilt is why many refer to Uranus as the planet that rotates on its side. There is no universally accepted explanation for this high degree of tilt, but many believe the planet was knocked on its side by a collision, or multiple collisions, with a large body early in the Uranian system’s development. This curious tilt means that for roughly half of each orbit, the north pole receives solar radiation, and for roughly half of the rest of the orbit, the south pole is in sunlight. This makes for unusual seasons and atmospheric dynamics. Although the planet’s interior rotates once every seventeen hours and fourteen minutes, the atmosphere rotates differentially. Features in the upper atmosphere have been clocked at as much as 0.25 kilometers per second and thus may experience a full rotation in less than fourteen hours.

Knowledge Gained

Earth-based telescopic studies of Uranus revealed it to have a bizarre orientation of its rotational axis, several relatively small satellites, and an orbital period of 84.3 years. In 1977, observations made from an aircraft-based telescope as Uranus occulted a star revealed the presence of dark rings around the mysterious seventh planet from the Sun. That same year, the Voyager 2spacecraft launched on an approved mission to fly by both Jupiter and Saturn. The National Aeronautics and Space Administration (NASA) originally proposed sending an armada of sophisticated spacecraft on what had been termed the “Grand Tour.” This “tour” referred to the fact that every 176 years, planetary alignments are such that gravitational slingshot maneuvers in the outer solar system can be used to send spacecraft to investigate all the outer planets from Jupiter to Pluto. Unfortunately, that ambitious plan was not funded, but NASA was given authorization to build two modest Voyager spacecraft for exhaustive investigations of Jupiter and Saturn. When Voyager 1 was successful at both gas giants, the Voyager 2 spacecraft was targeted through the Saturn system in such a way as to make possible a flyby of Uranus and Neptune.

Atmospheric structure is often discussed in terms of pressure, temperature, or both. If one defines Uranus’s “surface” as the site where the pressure is 1 bar (1 Earth atmosphere, or 10⁵ pascals), then that atmosphere can be described as follows. Uranus has a troposphere found from -300 to fifty kilometers above the surface, where the pressure varies from 100 to 0.1 bar respectively. A stratosphere exists between fifty and four hundred kilometers, where the pressure varies from 0.1 to 10^-10 bar. Then, from four hundred kilometers out to as much as two planet radii, or roughly fifty thousand kilometers, is the thermosphere and corona, where the pressure dwindles down to near vacuum from the upper stratospheric level of 10^-10 bar.

One might think that because its poles receive more solar illumination than the equatorial region, Uranus would be warmer at the pole presently facing toward the Sun, but that is not the case. Near the equator is the planet’s only portion to experience fairly rapid day-night variation due to the excessive tilt of Uranus. Near the equator, the warmest temperatures are recorded. Upper atmospheric temperatures near the equator can rise to fifty-seven kelvins. Why the equatorial region is warmer than the illuminated polar region is currently unknown.

The Hubble Space Telescope routinely was used by planetary scientists to examine Uranus for features and changes in those features within the planet’s atmosphere. In 1998, an image credited to NASA and Erich Karkoschka of the University of Arizona revealed the order of twenty clouds in Uranus’s atmosphere. That was rather remarkable; prior to that time, in the entire history of Uranus observations, there had been fewer than that number of clouds seen in the planet’s usually unremarkable-looking atmosphere. This Hubble image was taken in infrared and clearly showed the planet’s rings and many of its known satellites. In addition to the clouds, it revealed a bright band circling the planet. Wind speeds of clouds near the band were determined to be in excess of five hundred kilometers per hour. One of the clouds seen in this infrared image was the brightest Uranian cloud ever observed.

In 2006, Hubble images, in concert with near-infrared observations made using the ground-based Keck telescope, revealed a new, more dynamic picture of Uranus. The planet was seen to have a great dark spot and some degree of banding. These observations were summarized by astronomer Heidi Hammel at a Hubble science overview briefing held in 2002, about a month in advance of what was then proposed to be the final shuttle servicing mission to the Hubble. Hammel used her appearance at that briefing as an opportunity to stress how much Hubble had already done to change the picture of an inactive Uranian atmosphere as presented by Voyager 2 in 1986. Hammel and other astronomers had detected an increase in the number and scope of clouds in the ice giant’s atmosphere in addition to finding that great dark spot. She expressed enthusiasm about extended Hubble operations advancing understanding of Uranus, perhaps the planet about which the least is known. This increased activity appears related to seasonal changes as Uranus orbits the Sun. Scientists continued to search for answers about Uranus’s atmosphere as the twenty-first century progressed. Launched in 2021, the James T. Webb Space Telescope has imaged incredibly clear pictures of Uranus’s rings, storm clouds, and polar caps, providing increasing data about the workings of the planet.

Context

Uranus was the first planet for which clear records of discovery exist. Although Herschel was not the first to note Uranus in astronomical records, he was the first to identify it correctly as a planet and not a comet or unidentified star, as others had done previously. From the time of its discovery to the dawn of the space age, little could be learned about Uranus from ground-based telescopes. However, observations of Uranus’s orbit around the Sun led to the recognition that there was good reason to believe that it was not the last planet to be discovered in the solar system. Based on gravitational perturbations in the orbit of Uranus, Neptune was discovered by and large by mathematical analysis. Observations verified the correctness of those calculations. Uranus and Neptune were believed to be very similar. Both displayed a bluish-green tint in telescopic views. Spectroscopic analysis indicated that both planets had atmospheres different from those of Jupiter and Saturn. Like their larger gas giant cousins, Uranus and Neptune were known to have atmospheres rich in hydrogen and helium, but their bluish-green color was identified as due to extensive absorption of red light by methane.

Voyager 2 provided the greatest portion of current understanding about the Uranian system. Planetary scientists interested in Uranus await a return mission, most likely an orbiter, perhaps with a lander probe for one of the icy satellites and an atmospheric probe to ram through the upper atmosphere of Uranus and conduct measurements until it is crushed. In 2022, NASA announced plans to make its Uranus Orbiter Probe (UOP) a priority within the space program. This probe would not only orbit Uranus but also dive into its atmosphere, producing valuable information about the planet. The UOP is slated to launch in the mid-twenty-first century. Ground-based observations and imaging by the Hubble Space Telescope and the James T. Webb Space Telescope continued in the meantime. The biggest question about the detection and observation of clouds in Uranus’s atmosphere centers on the source of energy driving those storms since Uranus’s internal heat flow appears to be insufficient to cause such airflow.

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