The stark shift may owe to seasonal change or possibly even shadows cast by Saturn's rings, according to a study led by Agustín Sánchez-Lavega at the Universidad del País Vasco in Spain and reported in the June 5 issue of the journal Nature.

"The big surprise is that the winds at the equator have changed so much over such a short time period," said Wellesley College's Richard French, co-author of the paper. "If the winds are the surface manifestation of motions that extend very deep into the planet, we would not expect them to be able to change very rapidly.

Little is known about how surface winds interact with deeper regions. But if internal heat drives the wind, by causing gas to rise in one area and fall in another, then there should be a strong connection from top to bottom.

French likened the wind changes, if they're tied to process below the visible surface, to an enormous flywheel that slows down rapidly -- not an easy thing to do.

So instead, the new study suggests the equatorial winds do not extend very deep into Saturn's gaseous envelope.

On Earth, winds change with the seasons. Seasons, in turn, are caused by 23.5-degree tilt in Earth's rotational axis and the planet's yearly trip around the Sun. For six months of the year, the Northern Hemisphere leans toward the Sun and receives sunlight more directly. During the other six months of Earth's orbit, the Northern Hemisphere leans away from the Sun, fostering winter there while Australia basks in summer's glow.

Saturn is tipped about 25 degrees.

But on Saturn, as was recently noted at Neptune, seasonal change would take much longer and so might have gone undetected until now.

"Over the course of its 31-year trip around the Sun, we might expect to see some seasonal changes in the winds," French said in an e-mail interview. "However, these changes are not instantaneous -- just as on Earth, June 21 is not the hottest day of the year, even though it is the longest day. It takes awhile for the atmosphere to heat up or cool down."

Variations in heat, and the resulting different pressure pockets, are what makes wind. Air is constantly moving from regions of high pressure to fill low-pressure voids. Motion can be up-and-down or sideways. Winds can be localized, as with a tornado, or planet-wide, as with jet streams that race around the globe.

More interesting is the possible effects of Saturn's rings, diffuse collections of small and large bits of rock and ice.

"The rings act like giant sunshades," French said. "In spring and fall, the rings are edge-on as seen from the Sun, but in summer and winter, the rings block quite a bit of sunlight near the equator. This is one way of accentuating the effects of the seasons on Saturn."

The puzzle is not solved.

"The wind patterns are very similar in the northern and southern hemispheres -- the pattern is very symmetrical -- and that is one reason that people have thought that the winds really aren't affected strongly by seasons," French said.

Researchers will soon get a chance to study the situation up close. NASA's Cassini spacecraft is due to reach Saturn next year.

"Cassini is going to be crucial in helping us figure out what is happening," French said. "Over the course of Cassini's 4-year orbital tour, we'll have a chance to monitor the winds in incredible detail. We'll be able to see changes in the wind speeds, in cloud patterns, and in the detailed shapes of storm systems.

Further, accurate temperature measurements in Saturn's atmosphere will help researchers sort out what makes the wind blow.

"We're in for a real feast," French said.

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