Climate of Uranus

• 1. Introduction

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  The climate of Uranus defies easy explanation. When Voyager 2 made its historic flyby in 1986, scientists examining the data found something startling: across an entire planet larger than Earth, they could count the cloud features on two hands. Ten. That was it. Every other giant planet in our solar system churned with storms and bands and atmospheric drama. Uranus looked, in the words scientists reached for, like a dynamically dead planet.

  Yet within a few years, the Hubble Space Telescope and ground-based observatories began catching something Voyager had missed. Clouds were appearing in the northern hemisphere. A dark spot materialized where none had been seen before. And in the second half of 2004, a sudden burst of activity produced record-breaking wind speeds of 824 km/h and a persistent thunderstorm that researchers nicknamed the "Fourth of July fireworks".

  So which is the real Uranus? The placid, featureless world Voyager photographed? Or the turbulent, storm-wracked planet those later observations hinted at? The answer turns out to depend on where you look, when you look, and a peculiarity of Uranus's structure that makes it unlike any other planet we know.

• 2. A Planet Tipped on Its Side

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  Uranus orbits the Sun with its axis tilted so extremely that its poles spend large portions of each year pointing almost directly at the Sun or directly away from it. One full Uranian year lasts 84 Earth years, which means scientists have had reliable atmospheric data for less than one complete orbit. That constraint alone has made it genuinely difficult to understand what counts as seasonal change versus long-term variation.

  Photometry going back to the 1950s, covering roughly half a Uranian year, has shown that Uranus brightens near its solstices and dims near its equinoxes. Microwave measurements of the deep troposphere, begun in the 1960s, show the same pattern. Stratospheric temperature readings begun in the 1970s showed peak values near the 1986 solstice. Three independent measurement types, all pointing in the same direction.

  Part of this brightness variation comes simply from geometry. Uranus is an oblate spheroid, and its visible area grows when one of its poles faces Earth, at the solstices. The south polar region is also considerably brighter than its equatorial bands, so more pole means more brightness. But the real picture is more complicated than geometry alone can explain. During its previous northern solstice in 1944, Uranus showed elevated brightness, which suggests the north pole was once bright. By the time modern telescopes could study it properly, the north pole had dimmed. That asymmetry between hemispheres demands a physical explanation, not just a geometric one.

• 3. Bands, Collars, and the Voyager Snapshot

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  The earliest recorded hints of weather on Uranus go back to 1884, when observers noted a white band circling partially around the planet's equator. That was only two years after Uranus had passed its northern spring equinox.

  Voyager 2 arrived in 1986 at the height of Uranus's southern summer, meaning only the southern hemisphere was fully illuminated. What it found there was a visible structure split into two regions: a bright polar cap and darker equatorial bands, with their boundary sitting at roughly -45 degrees of latitude. Straddling the narrow range from -45 to -50 degrees was the single brightest large feature on the planet, a zone researchers called the southern collar. Both the cap and the collar are believed to be dense concentrations of methane clouds, sitting within a pressure range of 1.3 to 2 bar.

  Because Voyager arrived at southern summer, the northern hemisphere was turned away and could not be studied. When the Hubble Space Telescope and the Keck telescope eventually got their first good look at the north, they found neither a collar nor a polar cap there. Uranus appeared asymmetric: bright near the south pole, uniformly dark to the north of the southern collar. Then in 2007, when Uranus passed its equinox, the situation shifted. The southern collar nearly vanished, and a faint northern collar appeared near 45 degrees of latitude. A specific prediction had been made; a specific prediction came true.

• 4. The Uranus Dark Spot

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  Dark spots had been a signature feature of Neptune for years, but Uranus had never shown one. Then in 2006, observations from both the Hubble Space Telescope and the Keck Telescope captured something new: a small dark feature in the northern hemisphere of Uranus, in the hemisphere that had been in darkness for many years.

  The feature, which became known as the Uranus Dark Spot, or UDS, sat at a latitude of about 28 degrees north, give or take 1 degree. It measured roughly 2 degrees (around 1300 km) in latitude and 5 degrees (around 2700 km) in longitude. It moved in the prograde direction relative to Uranus's rotation at an average speed of 43.1 meters per second, which put it almost 20 m/s faster than the clouds at that same latitude.

  A bright white companion cloud, called the Bright Companion, moved alongside UDS at nearly the same speed. This pairing closely echoed Neptune's Great Dark Spots and their own bright companions. The leading hypothesis for Neptune's dark spots is that they are anticyclonic vortices, while the bright companions are orographic clouds forming where air rises. UDS is thought to operate similarly, though the two kinds of dark spots behave differently at certain wavelengths. Neptune's Great Dark Spots show their strongest contrast at 0.47 micrometres; UDS was invisible at that wavelength but showed its highest contrast at 1.6 micrometres. That difference implies the Uranian feature lies at a somewhat deeper pressure level than its Neptunian counterparts, probably near 4 bar. The very appearance of UDS in a hemisphere that had been dark for so long suggested that Uranus was entering a period of elevated activity as it approached equinox.

• 5. Zonal Winds

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  By tracking individual cloud features, scientists were able to map the wind patterns in Uranus's upper troposphere. The structure they found is both orderly and surprising.

  At the equator, winds blow retrograde, meaning opposite to the planet's rotation, at speeds ranging from -100 to -50 m/s. Moving away from the equator toward either pole, wind speed climbs until it crosses zero near plus or minus 20 degrees of latitude, which is also where the troposphere reaches its temperature minimum. Beyond that point, winds flip to prograde, now moving with the planet's rotation. Wind speeds continue rising, reaching peak values near plus or minus 60 degrees of latitude, before falling back to zero at the poles themselves.

  At -40 degrees of latitude, wind speeds range from 150 to 200 m/s. In the north, maximum speeds as high as 240 m/s have been measured near +50 degrees of latitude. That raw number can suggest northern winds are stronger, but latitude for latitude, the northern midlatitudes are actually slightly slower than their southern counterparts, particularly in the range from plus or minus 20 to plus or minus 40 degrees. Whether wind speeds have changed since Voyager's 1986 flyby remains an open question; scientists have not reached agreement on that point. The much slower meridional winds, which blow north-south rather than east-west, are essentially unknown.

• 6. The Cold Interior Problem

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  Neptune and Uranus are close in size and composition, near-twins by most measures. But when it comes to internal heat, they behave very differently. Neptune radiates 2.61 times as much energy into space as it receives from the Sun. Uranus radiates almost none. Its measured heat flux is only 0.042 plus or minus 0.047 watts per square meter, which is actually lower than Earth's internal heat flux of about 0.075 W/m2. The total power Uranus radiates in the infrared part of the spectrum is only 1.06 plus or minus 0.08 times the solar energy it absorbs.

  The lowest temperature recorded in Uranus's tropopause is 49 K, which is -224 degrees Celsius. That makes Uranus the coldest planet in the Solar System, colder even than the more distant Neptune.

  Why so little internal heat? Two hypotheses compete. The first ties back to Uranus's axial tilt: the same supermassive impactor that knocked the planet onto its side may also have expelled most of its primordial heat, leaving its core cold ever since. The second proposes a barrier of some kind in the upper layers of the planet, a set of compositionally distinct layers where convection happens separately in each layer, blocking heat from rising toward the surface. Neither hypothesis has been confirmed. Whatever the cause, that cold interior is believed to be the primary reason Uranus's atmosphere looked so lifeless to Voyager's cameras in 1986.