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.
| ![](images/energy_3_small.gif) 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. |