Outer Solar System

weather report
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: October 14, 2008

In three different reports, scientists unveil the seasons on Uranus, a giant cyclone on Saturn and the mechanism driving the powerful jet streams on all four gas giants. In a fourth report, the exotic weather experienced on Jupiter-like exoplanets is revealed.

All four giant planets – Jupiter, Saturn, Uranus and Neptune – play host to powerful jet streams that whip around the planets’ atmospheres between speeds of 650 to 1,500 kilometres per hour. Jet streams are narrow rivers of air that flow around the gas planets, but the question of what causes them and what controls their structure has long been debated. On Earth, jet streams form at the boundaries between cold polar air and warmer air towards the equator, such as the major jet stream that flows west to east in the northern hemisphere and which controls much of the large-scale weather experienced by the United States and other countries outside of the tropics.

New images of Saturn's poles reveal powerful cyclones in better detail than ever. The north polar image (left) shows a curious hexagonal shaped wind feature populated with fast moving clouds reaching 500 kilometres per hour. Image: NASA/JPL/University of Arizona.

Speaking at the Division of Planetary Sciences Meeting in Ithaca this week, Yuan Lian and Adam Showman of The University of Arizona revealed how thunderstorms could be the missing link for the gas planet jet streams. "Thunderstorms have been known to exist on Jupiter and Saturn since the early 1980s, and it has repeatedly been proposed that they drive the jet streams on these planets, but before now this idea had never been adequately tested," says Lian. "We showed that such storms can indeed drive jet streams similar to those observed."

But there is more to the story since Jupiter and Saturn both have about 20 jet streams, whereas Uranus and Neptune have just three.  Another conundrum is that the jet stream on the equator flows eastward on Jupiter and Saturn but westward on Uranus and Neptune. In Lian and Showman's computer simulations, these differences are controlled by the abundance of water vapour in the atmosphere, which is modest on Jupiter and Saturn but expected to be large on Uranus and Neptune.

"Previous investigations generally predicted that the equatorial jet stream would have the same direction on all four planets, inconsistent with observations," says Lian. "Our study is among the first to provide an explanation for these differences."

In a separate study, new images from the Cassini spacecraft reveal giant cyclones at Saturn’s north and south poles. Cassini mapped a new-found cyclone at the north pole in infrared, discerning features down to 120 kilometres in size. Time-lapse movies show the clouds spinning round the cyclone at 530 kilometres per hour, more than twice as fast as the highest winds seen in similar features on Earth. Moreover, the cyclone is surrounded by an odd, honeycombed-shaped hexagon, which itself does not seem to move, and neither the fast-moving clouds inside the hexagon nor this new cyclone seem to disrupt the hexagon pattern. Furthermore, Saturn's hurricanes are locked to the planet's poles, whereas terrestrial hurricanes drift across the ocean.

This detailed Cassini view of the monstrous vortex at Saturn's south pole provides valuable insight about the mechanisms that power the planet's atmosphere. Image: NASA/JPL/Space Science Institute.

"These are truly massive cyclones, hundreds of times stronger than the most giant hurricanes on Earth," says Kevin Baines, Cassini scientist on the visual and infrared mapping spectrometer. "Dozens of puffy, convectively formed cumulus clouds swirl around both poles, betraying the presence of giant thunderstorms lurking beneath. Thunderstorms are the likely engine for these giant weather systems.”

But unlike Earth-bound hurricanes, which are powered by the ocean's heat and water, Saturn's cyclones have no body of water at their bases to control them. Instead, the heat released from the condensing water in Saturnian thunderstorms deep down in the atmosphere is thought to be the primary power source energizing the raging storm.

New high resolution images of the south polar vortex show the entire region is marked by hundreds of dark cloud spots resulting from convective, thunderstorm-like processes extending some 100 kilometers below the clouds, and dredging up materials from depth. In one example, a deep convective structure has punched through to a higher altitude, creating its own vortex surrounded by an outer ring of higher clouds. "It's like seeing into the eye of a hurricane," says Andrew Ingersoll, a member of Cassini's imaging team. "It's surprising. Convection is an important part of the planet's energy budget because the warm upwelling air carries heat from the interior. In a terrestrial hurricane, the convection occurs in the eyewall; the eye is a region of downwelling. Here convection seems to occur in the eye as well."

And it’s not just Jupiter that has raging storms. The weather on Uranus is throwing up a few surprises too, with observations from the Keck II telescope led by University of Wisconsin-Madison researcher Lawrence Sromovsky revealing wind speeds of up to 900 kilometres per hour. The new observations also charted changes in the brightness of cloud bands in the planet's northern and southern hemispheres as well as changes in two previously observed and apparently long-lived discrete cloud features. One is a massive vortex that had been oscillating in Uranus's southern hemisphere for several decades between 32 degrees and 36 degrees latitude. In 2004, the feature began drifting north, and according to the new report, may soon dissipate.

"For two decades, it seemed like it was behaving in a very reliable way," Sromovsky says. "It may be that a change in the seasons has triggered it into a new dynamical state."

With the aid of new imaging technologies and telescopes, scientists had their best chance to observe the change of seasons on the distant planet and to look for seasonal effects on some of the solar system's most mysterious weather features. Image: Imke de Pater, University of California, Berkeley; Heidi Hammel, Space Science Institute; Lawrence Sromovsky and Patrick Fry, University of Wisconsin-Madison. Obtained at the Keck Observatory, Kamuela, Hawaii.

With an 84-year orbit around the Sun, it isn't often that
planetary scientists have an opportunity to observe the change of seasons on Uranus. But in 2007, the planet reached equinox, the point in time where the Sun is directly over the planet's equator and what little sunlight the planet gets is distributed evenly over its northern and southern hemispheres. This gave scientists their best opportunity to probe the seasonal dynamics of the ringed planet, which is also tilted by 98 degrees from its orbital plane.

"The last time this happened, there were no instruments that could resolve any features on the planet. Now we can see what's going on,” says Sromovsky. "This tilt gives it the largest seasonal forcing of any planet in the Solar System. On an annual average basis, the poles get more sunlight than the equator."

Seasonal forcing is the change in the distribution of solar heating caused by the tilt of a planet on its axis. On Earth, the seasons change in relation to a hemisphere's orientation to the Sun as determined by the 23.5 degree tilt of the planet's axis of rotation. If the latitudinal distribution of solar energy input varies over the planet's orbit, that forces changes in the weather. But for Uranus, changes in weather due to seasonal forcing seem to lag behind the forcing.

"Although both hemispheres were symmetrically heated by sunlight at equinox, the atmosphere itself was not symmetric, implying that it was responding to past sunlight instead of current sunlight, a result of Uranus's cold atmosphere and long response time," says Sromovsky. Uranus is 19 times further from the Sun than the Earth, resulting in 400 times less heat than is received at the Earth.

By studying the temperatures of hot-jupiters, and combining this data with detailed computer simulations, planetary scientists have predicted the weather of alien worlds. Image: NASA/JPL-Caltech.

By studying the weather systems of well-known planets, particularly the gas giants which mimic the so-called ‘hot-jupiters’ discovered in extrasolar systems, and combining this data with computer simulations, planetary scientists can make predictions about the weather systems of alien worlds. Adam Showman of The University of Arizona led a study explaining how a global atmospheric circulation driven by the dayside heating and nightside cooling can drive weather on the hot-jupiters – giant gaseous planets that orbit up to 20 times closer to their star than the Earth is to the Sun. This results in extreme daytime temperatures of 1,600 degrees Celsius.

"Because these planets are so close to their stars, we think
they're tidally locked, with one side permanently in starlight
and the other side permanently in darkness," says Showman. "So, if there were no winds, the dayside would be extremely hot and the nightside would be extremely cold."

Observations conducted with the Spitzer Space Telescope showed, however, that for one such planet at least – HD 189733b – the nightside temperature exceeds 700 degrees Celsius, much warmer than expected for a wind-free planet. Showman and colleagues performed detailed computer simulations which showed that exoplanets, just like the gas giants in our own Solar System, also have jet streams with speeds reaching at least 1000 kilometres per hour to carry heat from the dayside to the nightside. "According to the observations, the hottest region on the planet is not 'high noon' but eastward of that by maybe 30 degrees of longitude," says Showman.

So to sum up the weather forecast for exoplanets: "Hot Jupiters
are pretty crazy places," says Showman. "Expect supersonic winds and dayside temperatures hot enough to melt lead and rocks. Only problem is, if you tried to visit, you'd be fried to a crisp before you could enjoy the view."