One Universe: At Home in the Cosmos

Energy | Pages 124-125 | (back to unlinked version)

Greenhouses on Planetary Scales

Gases such as water vapor , carbon dioxide , and methane can form an insulating layer in the atmospheres of planets. Such a layer allows visible light to penetrate easily but would be more opaque to infrared radiation, trapping the heat near the planet's surface. Since this resembles what happens in a gardener's greenhouse--the glass walls and roof let visible light pass through but trap the warmed air inside--we call it the greenhouse effect . This phenomenon has been good to Earth: Without it our world would be on average many degrees cooler, possibly just enough to prevent life from forming here the way it did. Other worlds in our solar system have been less fortunate. Closer to the Sun than Earth is, Venus has suffered a "runaway" greenhouse effect, as shown on the opposite page.

Fractured plains and the remnants of vast lava flows surround Maat Mons, a 5-mile-high extinct volcano on Venus. Vertical scale is exaggerated 22.5 times in this 3-D map, created from radar data collected in 1990 by NASA's Magellan orbiter. The simulated orange hues are based on color images from two Soviet missions to Venus in the 1980s.

The Sun shines most brightly in visible light . Biologists believe this may be why our eyes evolved to perceive only that range of electromagnetic energy. The Sun is a garden-variety star, so the energy from many other stars also peaks in or near visible light . For centuries visible light was all astronomers studied. They didn't have the technology to gather other forms of light. visible light provides a reasonably accurate approximation of the universe because stars provide the bulk of its illumination. If we can see stars in visible light , we can see most of what's bright out there. We also see a little bit of just about everything else, since nearly all objects that glow emit some of their energy at visible wavelengths of light.

Like UV light, infrared light also just eludes our vision. We commonly think of infrared light as heat and sense it with our skin as warmth. Consider the heating element on your stove. Even after you turn it off and the red glow fades away, you can still feel its heat and you know not to touch it. Every warm object in the universe, including your body at this moment, emits infrared light. Special night-vision goggles can reveal this eerily glowing world to us.

All stars produce infrared light as well as visible light . But some relatively "cool" objects shine very faintly in visible light , so infrared offers the best way to detect them. These include the tiniest stars , which are the most numerous in the universe. They

Cooked by the Sun, crustal rocks on Venus released quantities of greenhouse gases into the atmosphere. Fully 96 percent of the Venusian atmosphere is a choking shroud of carbon dioxide . As shown at left, infrared radiation (red) rising from the superhot surface of the planet is largely absorbed and reemitted back toward the surface in an infernal feedback loop. This "runaway" greenhouse effect finally stopped when the surface temperature reached about 900 degrees Fahrenheit, hot enough to vaporize lead or to bake a pizza in under 10 seconds.

will live for tens of billions of years after the Sun has used up its fuel. Still smaller objects, called brown dwarfs , are failed stars that never quite ignited nuclear fusion in their cores. They glow faintly with energy left over from their gravitational collapse. Planets also emit infrared light from their own internal heat. We may spot planets directly in other solar systems with infrared telescopes of the future.

Infrared light also figures in a notorious phenomenon of the day: the greenhouse effect . visible light from the Sun bathes Earth and warms the surface. The land and sea then reradiate much of this energy into the atmosphere as infrared light. Certain gases--most notably water vapor , carbon dioxide , and methane --act like the glass panes in a greenhouse and prevent the heat from escaping into space. This effect is natural and not necessarily bad. Without it our planet might have been too cold for life to arise. However, during the twentieth century the exhaust from our factories and cars has stoked carbon dioxide in the atmosphere to ever-increasing levels. By trapping more heat, this extra gas may make the globe's average temperature rise by several degrees in the near future.

The effects of such rapid changes are uncertain. But we know that once a delicate thermal balance is perturbed, it may never recover. Our neighbor Venus is a case in point (above). That planet never shed the carbon dioxide atmosphere it had at birth. (continued)