The Universe in the Classroom

The topsy-turveying of planets, stars, and lava lamps

The Yearn to Churn

The temperature profile isn't the only requirement for convection. The force driving convection -- the weight difference between hot fluid and cold fluid -- must overcome the fluid's natural resistance to motion, known as viscosity.

Heinz ketchup commercials used to compare Heinz to other, runny brands. If you believe those ads, Heinz is more viscous than its competitors. It's harder to get Heinz to flow out of the bottle. It would be harder to start convection in a vat of Heinz than in a vat of another ketchup.

Air, too, is viscous. Its resistance to motion is what slows raindrops and parachutists down to reasonable speeds. If the viscosity of air were smaller, raindrops would fall so fast that they could smash car roofs and kill small dogs. But the air's viscosity is small enough that it doesn't stop convection from occurring in the atmosphere.

Earth and Jupiter
Figure 6
Planetary paisley. Like cream in coffee, clouds on the giant planet Jupiter bend and twist and curl into psychedelic patterns. These storm systems dwarf the Earth, superimposed for scale. Such turbulence is what happens when convection gets carried away. The Great Red Spot is the tilted oval in the upper right corner. Photo courtesy of NASA.

In some cases, the viscosity is too small, and convection gets out of control. It goes haywire, and the nice orderly patterns of Figure 1 degenerate into unsteady, unpredictable turbulence. In turbulent convection, the fluid gets bunched up in spiral eddies. These eddies pop up without warning, as airplane passengers who've been on a bumpy flight will testify. The chaotic curlicues in rising cigarette smoke, or projected on the wall behind an electric heater, are turbulent convection. So are the swirls in the clouds of Jupiter (see Figure 6).

Deep underground, the problem isn't too little viscosity, but too much. The continents and ocean floor of the Earth float on a gargantuan sea of hot rock, known as the mantle (see Figure 2). The viscosity of the mantle is enormous, so enormous that the mantle seems not to move at all. But over thousands and millions of years, the mantle moves. Its slow slithering creates many of the geologic landforms we see.

One way we know the mantle moves is by watching how the ground has responded since the last Ice Age ended. During the Ice Age, Scandinavia and other northern areas were covered by glaciers miles thick. The enormous weight of the glacier pushed down on the land. When the ice melted, the land wanted to pop back up. But for the land to bounce up, the mantle underneath it must be able to flow. It takes 150 years for the land to pop up by one foot, and by measuring this, geologists have calculated the viscosity of the mantle.

Because the mantle is fluid, and because it is hot, it can convect. To overcome the high viscosity takes a lot of heat, supplied by radioactive uranium, thorium, and potassium in a sort of slow-simmering nuclear reactor. The mantle churns in giant convection cycles 450 miles deep and 900 miles wide. In so doing, it drags along the plates that form the Earth's surface. When these plates rub against each other, they cause earthquakes; when these plates crash into one another, they crinkle into mountains. Earthquakes and volcanoes are just a way for the Earth to cool off.

Other planets also have mantles. On Venus, the mantle doesn't shuffle plates horizontally. Instead, the venusian mantle likes to push the surface vertically. In some areas, this vertical shove has created highlands. Miranda, one of the moons of Uranus, appears to have a mantle made of ice, rather than of rock. Convection in this icy mantle contorts the surface into gnarled patterns of mountain ridges and valleys (see Figure 7).
Figure 7
The Chevron Moon. Miranda, one of the satellites of Uranus, looks more like a military badge than a moon. The little moon, about 300 miles across, is stamped with weird diamond shapes. This Voyager 2 picture shows three such shapes: Elsinore (left), Inverness (center), and Arden (right). The shapes may result from convection inside the tormented moon. Photo courtesy of Jet Propulsion Laboratory.

From Lava Lamps to Lava Flows

The urge of the mantle to lose heat can also cause rock to bubble up toward the surface in blobs. Blobs are smaller and shorter-lived than the gigantic, full-blown convection cycles, but they too help planets to cool off.

Lava lamps work on the same principle. Heat causes blobs to form and float up. If you turn a syrup bottle upside down, you can see blobs of air slowly rise upwards. The same basic thing happens inside planets, except that the blobs are made of hot rock, instead of air. In fact, scientists have watched bubbles in syrup in order to understand the effect of rock blobs on the surface of planets. Because blobs carry hot rock from the interior of the planet toward the surface, they supply volcanoes with lava. This is what happens in the volcanoes of Hawaii and East Africa. Ten percent of the Earth's heat escapes this way.

On Venus, blobs are even more important than on Earth. The venusian surface is peppered with volcanoes and roundish terraces called coronae. Coronae are several hundreds miles across and appear to form when hot blobs push and stretch the surface (see Figure 8).
Figure 8
Blobology on Venus. Rising blobs of hot rock, a hundred or so miles across, can push up the venusian surface and form mountains. When the blob is still far underground, it squeezes the fluid between it and the surface, causing the surface to bow out (top). As the blob gets closer to the surface, it begins to flatten like pizza dough into a pie shape (center). After a while, the blob cools down, and the surface sags (bottom). Diagram courtesy of Steven W. Squyres, Cornell University.

Convection cycles can be tens of thousands of miles tall, as they are in stars; hundreds of miles tall, as they are in planets; or just a few inches tall, as they are in tea kettles. But no matter how big or small, the basic idea is the same. Remember that the next time you boil water.

Activities in the Classroom

Convection currents in water

This demonstration requires a glass jar and a stove, bunsen burner, or other heater. An automatic drip coffee pot is perfect. Fill the jar with water and add sawdust, iron filings, peppercorns, or other small particles to the water. Give the particles time to settle on the bottom, and then turn on the heat. The water will begin to convect, and the particles will follow the convection currents.

If you shine the light from a slide projector through the hot water and project it onto a screen, you can see vivid shadows of the hot water vapor convecting upwards.

Convection currents in air Hold a pinwheel above a candle or burner. The hot flame will set up convection currents in the air, causing the pinwheel to spin.

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