Climate of Uranus

Uranus' southern hemisphere in approximate natural colour (left) and in higher wavelengths (right), showing its faint cloud bands and atmospheric "hood" as seen by Voyager 2

The climate of Uranus is heavily influenced by both its lack of internal heat, which limits atmospheric activity, and by its extreme axial tilt, which induces extreme seasonal variation. Uranus' atmosphere is remarkably bland in comparison to the other gas giants, even to Neptune, which it otherwise closely resembles. When Voyager 2 flew by Uranus in 1986, it observed a total of ten cloud features across the entire planet.

One proposed explanation for this dearth of cloud features is that Uranus' internal heat appears markedly lower than that of the other giant planets; in astronomical terms, it has a low thermal flux. Why Uranus' internal temperature is so low is still not understood. Neptune, which is Uranus' near twin in size and composition, radiates 2.61 times as much energy into space as it receives from the Sun. Uranus, by contrast, radiates hardly any excess heat at all. The total power radiated by Uranus in the far infrared (i.e. heat) part of the spectrum is 1.06 ± 0.08 times the solar energy absorbed in its atmosphere. In fact, Uranus' heat flux is only 0.042 ± 0.047 W/m², which is lower than the internal heat flux of Earth of about 0.075 W/m². The lowest temperature recorded in Uranus' tropopause is 49 K (−224 °C), making Uranus the coldest planet in the Solar System, colder than Neptune.

Hypotheses for this discrepancy include that when Uranus was "knocked over" by the supermassive impactor which caused its extreme axial tilt, the event also caused it to expel most of its primordial heat, leaving it with a depleted core temperature. Another hypothesis is that some form of barrier exists in Uranus' upper layers which prevents the core's heat from reaching the surface. For example, convection may take place in a set of compositionally different layers, which may inhibit the upward heat transport.

Banded structure, winds and clouds

Zonal wind speeds on Uranus. Shaded areas show the southern collar and its future northern counterpart. The red curve is a symmetrical fit to the data.

In 1986 Voyager 2 found that the visible southern hemisphere of Uranus can be subdivided into two regions: a bright polar cap and dark equatorial bands (see figure on the right). Their boundary is located at about −45 degrees of latitude. A narrow band straddling the latitudinal range from −45 to −50 degrees is the brightest large feature on the visible surface of the planet. It is called a southern "collar". The cap and collar are thought to be a dense region of methane clouds located within the pressure range of 1.3 to 2 bar (see above). Unfortunately Voyager 2 arrived during the height of the planet's southern summer and could not observe the northern hemisphere. However, at the beginning of the twenty-first century, when the northern polar region came into view, Hubble Space Telescope (HST) and Keck telescope observed neither a collar nor a polar cap in the northern hemisphere. So Uranus appears to be asymmetric: bright near the south pole and uniformly dark in the region north of the southern collar. Details of possible seasonal variations in the planet's appearance will be discussed later. The visible latitudinal structure of Uranus is different from that of Jupiter and Saturn, which demonstrate multiple narrow and colorful bands.

In addition to large-scale banded structure, Voyager 2 observed ten small bright clouds, most lying several degrees to the north from the collar. In all other respects Uranus looked like a dynamically dead planet in 1986. However in 1990s the number of the observed bright cloud features grew considerably. The majority of them was found in the northern hemisphere as it started to become visible. The common explanation of this fact is that bright clouds are easier to identify in the dark part of the planet, whereas in the southern hemisphere the bright collar masks them. Nevertheless there are differences between the clouds of each hemisphere. The northern clouds are smaller, sharper and brighter. They appear to lie at a higher altitude, which is connected to fact that until 2004 (see below) no southern polar cloud had been observed at the wavelength 2.2 micrometres, which is sensitive to the methane absorption, while northern clouds have been regularly observed in this wavelength band. The lifetime of clouds spans several orders of magnitude. Some small clouds live for hours, while at least one southern cloud has persisted since Voyager flyby. Recent observation also discovered that cloud-features on Uranus have a lot in common with those on Neptune, although the weather on Uranus is much calmer. The dark spots common on Neptune had never been observed on Uranus before 2006, when the first such feature was imaged.

The first dark spot observed on Uranus. Image obtained by ACS on HST in 2006.

The tracking of numerous cloud features allowed determination of zonal winds blowing in the upper troposphere of Uranus. At the equator winds are retrograde, which means that they blow in the reverse direction to the planetary rotation. Their speeds are from −100 to −50 m/s. Wind speeds increase with the distance from the equator, reaching zero values near ±20° latitude, where the troposphere's temperature minimum is located./> Closer to the poles, the winds shift to a prograde direction, flowing with the planet's rotation. Windspeeds continue to increase reaching maxima at ±60° latitude before falling to zero at the poles. Windspeeds at −40° latitude range from 150 to 200 m/s. Since the collar obscures all clouds below that parallel, speeds between it and the southern pole are impossible to measure. In contrast, in the northern hemisphere maximum speeds as high as 240 m/s are observed near +50 degrees of latitude. These speeds sometimes lead to incorrect assertions that winds are faster in the northern hemisphere. In fact, latitude per latitude, winds are slightly slower in the northern part of Uranus, especially at the midlatitudes from ±20 to ±40 degrees. There is currently no agreement about whether any changes in windspeed have occurred since 1986, and nothing is known about much slower meridional winds.

Seasonal variation

Uranus in 2005. Rings, southern collar and a bright cloud in the northern hemisphere are visible.

For a short period in Autumn 2004, a number of large clouds appeared in the Uranian atmosphere, giving it a Neptune-like appearance. Observations included record-breaking wind speeds of 824 km/h and a persistent thunderstorm referred to as "Fourth of July fireworks". On August 23, 2006, researchers at the Space Science Institute (Boulder, CO) and the University of Wisconsin observed a dark spot on Uranus' surface, giving astronomers more insight into the planet's atmospheric activity. Why this sudden upsurge in activity should be occurring is not fully known, but it appears that Uranus' extreme axial tilt results in extreme seasonal variations in its weather.

Determining the nature of this seasonal variation is difficult because good data on Uranus' atmosphere has existed for less than 84 years, or one full Uranian year. A number of discoveries have however been made. Photometry over the course of half a Uranian year (beginning in the 1950s) has shown regular variation in the brightness in two spectral bands, with maxima occurring at the solstices and minima occurring at the equinoxes. A similar periodic variation, with maxima at the solstices, has been noted in microwave measurements of the deep troposphere begun in the 1960s. Stratospheric temperature measurements beginning in 1970s also showed maximum values near 1986 solstice.

The visible magnitude of Uranus in two spectral bands (upper graph) adjusted for the distance and effective microwave temperature (lower graph). Blue band is centered at 470 nm, yellow at 550 nm.

The majority of this variability is believed to occur due to changes in the viewing geometry. Uranus is an oblate spheroid, which causes its visible area to become larger when viewed from the poles. This explains in part its brighter appearance at solstices. Uranus is also known to exhibit strong meridional variations in albedo (see above). For instance, the south polar region of Uranus is much brighter than the equatorial bands. In addition, both poles demonstrate elevated brightness in the microwave part of the spectrum, while the polar stratosphere is known to be cooler than the equatorial one. So seasonal change seems to happen as follows: poles, which are bright both in visible and microwave spectral bands, come in to the view at solstices resulting in brighter planet, while the dark equator is visible mainly near equinoxes resulting in darker planet. In addition, occultations at solstices probe hotter equatorial stratosphere.

HST image of Uranus showing clouds in the northern hemisphere.

However there are some reasons to believe that seasonal changes are happening in Uranus. While the planet is known to have a bright south polar region, the north pole is fairly dim, which is incompatible with the model of the seasonal change outlined above. During its previous northern solstice in 1944, Uranus displayed elevated levels of brightness, which suggests that the north pole was not always so dim. This information implies that the visible pole brightens some time before the solstice and darkens after the equinox. Detailed analysis of the visible and microwave data revealed that the periodical changes of brightness are not completely symmetrical around the solstices, which also indicates a change in the meridional albedo patterns. In addition, some microwave data show increases in pole–equator contrast after the 1986 solstice. Finally in the 1990s, as Uranus moved away from its solstice, Hubble and ground based telescopes revealed that the south polar cap darkened noticeably (except the southern collar, which remained bright), while the northern hemisphere demonstrates increasing activity, such as cloud formations and stronger winds, bolstering expectations that it should brighten soon. In particular the bright polar collar present in the southern hemisphere at −45° is expected to appear in the northern part of the planet.

The mechanism of physical changes is still not clear. Near the summer and winter solstices, Uranus' hemispheres lie alternately either in full glare of the Sun's rays or facing deep space. The brightening of the sunlit hemisphere is thought to result from the local thickening of the methane clouds and haze layers located in the troposphere. The bright collar at −45° latitude is also connected with methane clouds. Other changes in the southern polar region can be explained by changes in the lower cloud layers. The variation of the microwave emission from the planet is probably caused by a changes in the deep tropospheric circulation, because thick polar clouds and haze may inhibit convection. Now that the spring and autumn equinoxes are arriving on Uranus, the dynamics are changing and convection can occur again.