Last week we saw that the atmosphere of Saturn’s moon Titan wasn’t quite as weird as we thought. But there another way it’s really weird, completely unlike Earth but yet very much like Venus. Titan’s a superrotater, a world whose atmosphere circles the planet much faster than its surface does.
Let’s start with a relatively simple Earthside phenomenon, a hurricane. Warm air rises, right? When the warmth comes from bathtub-temperature sea-water, it’s wet warm air. As the air rises it cools and releases the moisture as rain. But the air can’t just keep rising forever or we’d squirt out all our atmosphere. So where does it go?
From a physicist’s perspective, that’s the key question. If we can track/predict the path of a small parcel of air molecules through a weather system, then we’ve got at least a rough understanding of how that system works.
For the past half-century, atmosphere physicists have been engaged on a project grandly entitled the General Circulation Model (GCM), a software mash-up of the Ideal Gas Law, Newton’s Laws of motion, thermodynamic data for solid/liquid/gas transformations, the notoriously difficult Navier-Stokes equations for viscous fluids, and careful data management for input streams from thousands of disparate sources. Oh, and it’s important that the Earth is a rotating spheroid rather than a flat plane.
Kevin Song’s diagram summarizes much of what we know about hurricanes. An air packet rises until it hits the tropopause (the top edge of the troposphere), then expands horizontally. While the packet’s spreading out, the planet’s rotation generates Coriolis “forces” that bend straight-line radial paths into the spirals we’ve seen so often in satellite photos.
A hurricane may look big on your weathercaster’s screen, but it’s less than 0.1% of Earth’s surface area. Nonetheless, many of the same principles that drive a hurricane underlie global weather patterns.
Air warmed by the equatorial Sun rises, only to sink as it heads poleward. Our packet loops between the Equator and about 30ºN (see the diagram).
Actually that loop is a slice through a big doughnut that stretches all the way around the Earth. Another doughnut lies southward just below the Equator. Two more pairs of doughnuts reside polewards of those as indicated by the other arrows in the diagram. The doughnuts act like a set of interlocking gears, each reinforcing and moderating the motion of its neighbors.
Thanks to the same geometric phenomenon that spins a hurricane, air packets in these doughnuts don’t loop back to the points they started from. The Earth turns under the packets as they journey, so each packet takes a spiraling tour around the planet.
Because of all those doughnuts, on average Earth wears a set of cloud-top necklaces. Regions within 15º of the Equator are rain-forested, as are the Canadian and Siberian forested belts near 60ºN. The world’s most prominent deserts cluster beneath the dry downdrafts near the 30º latitudes. Jupiter, “the Easter egg planet,” gets its pink and blue bands from similar doughnuts except that Jupiter has room for many more of them.
Those green circles in the diagram are important, too. They also represent Earth-circling doughnuts, but ones whose winds flow parallel to Earth’s surface rather than perpendicular to it. The ones close to the surface give rise to the trade winds. The high-altitude ones are the jet streams that steer storm systems and give the weathercasters something to talk about, especially in the wintertime.
Jet streams flow briskly — 60 to 200 mph, on a par with a middling hurricane. Here’s a benchmark: Earth’s equatorial circumference is 25,000 miles, so Ecuadorian palm trees circle the planet at (25000 miles/24 hours)=1041 mph. Our jet streams go about 15% of that. Theory and GCM agree that the jets are powered by the Coriolis effect — spiraling air packets in the primary donuts cooperate to push jet stream air packets like oars on a galley ship. That adds up.
Titan and Venus can’t possibly work that way. Both of them rotate much more slowly than Earth (Titan about 30 mph, Venus only 4), so Coriolis forces are negligible. But Titan’s jet streams do 75 mph and Venus’ race at 185. What powers them? The physicists are still arguing.
~~ Rich Olcott