Late in the twentieth century, a small unmanned spacecraft flew past a rocky planet. Onboard sensors scanned the planet and found some unusual patterns. The atmosphere was rich in oxygen and methane, two highly reactive gases. Green light from the planet's star reflected into space off a substance covering most of the land. Organized radio signals streamed from the planet--transmissions that natural processes can't produce. The scientists who operated the spacecraft regarded all of these patterns as unmistakable evidence of life.

The planet, of course, was Earth. The spacecraft was the planetary probe Galileo, which spent the last half of the 1990s studying Jupiter and its dramatic moons. Mission scientists used Earth and Venus as gravitational slingshots to boost Galileo on its journey to Jupiter. During one of the Earth flybys, a team led by the astronomer Carl Sagan conducted a fascinating test: Could a spacecraft detect life on a planet without landing on it?

In Earth's case the answer was reassuring. The atmosphere provided two strong clues. Plants have altered the ingredients of Earth's air, making our atmosphere unique in the solar system. The most notable ingredient is oxygen, which composes 21 percent of the air we breathe. Free oxygen molecules react with rocks, soils, metals, and chemical compounds in the air. Rusty old cans in your garage are evidence of that process, called oxidation. Such reactions quickly remove most oxygen from the atmospheres of other planets. On Earth, however, another constant process churns the gas into the air just as quickly: photosynthesis. Earth's greenery has released enough oxygen to maintain a concentration of that vital gas in the atmosphere for the past 2 billion years.

Another unusual gas, methane, exists at much lower levels in the air--about 1 part per million. Methane doesn't last long in the presence of oxygen because oxygen molecules combine readily with methane to make water and carbon dioxide. Therefore, something on the ground must continually replenish the methane, just as plants do for oxygen. Volcanic eruptions and other natural events produce some methane but not enough to account for the levels seen. To fully explain the surplus, we must turn to biological sources: bacterial life in bogs (hence the name "marsh gas") and in the guts of termites, cattle, and other animals.

Plant life on Earth absorbs mainly red and blue wavelengths of light from the Sun to drive the chemical reactions of photosynthesis. The remaining wavelengths, primarily green, reflect into space--giving our continents their distinctive hue and providing one strong clue about the presence of life on our planet.

The light reflected from Earth's surface was a third clue. Large green patches detected on the continents by Galileo meant that some substance trapped other colors--primarily red and blue light--from the Sun. No known soils or minerals act in that way. The only logical explanation was widespread plant life, using chlorophyll to harness the Sun's energy. Pigments within algae, grasses, and leaves absorb mostly red and blue light to drive the chemical reactions of photosynthesis. The plants then reflect the remaining wavelengths of light, in the green part of the visible spectrum. That gives our planet its distinctive tinge.

A spacecraft flying past Earth 2 billion years ago would have seen these clues that life had taken root on the planet, Sagan's team noted. A fourth sign arose during the twentieth century, and it was the dead giveaway. An instrument on Galileo registered strong signals within narrow radio bands. The signals were too orderly to flow from the turbulent magnetosphere around Earth or from lightning or other natural bursts of energy. The radio waves also seemed to carry information: They pulsed and varied with distinct patterns. Those modulations, the researchers concluded, were hallmarks of the web of communication woven by an intelligent society.

We can use the observations of Galileo and our knowledge about the many forms of life on Earth to guide our quest for life on other worlds. If technological civilizations exist on planets around other stars, we may have a chance to detect their communications with radio telescopes on Earth. However, finding simpler forms of life elsewhere poses a challenge. We don't yet have the technology to measure the atmospheres of planets beyond our solar system, nor can we see the colors of light reflected from such planets. Within our solar system, no mission to other planets or moons has spotted any of the obvious signs of life that Galileo detected on Earth. But we know that life here has many other guises. Some organisms live far underground, drawing chemical energy from minerals and warm fluids. Others eke out bare existences within rocks in frigid Antarctica. Bacteria grow in the scalding chemical stews of hot springs in Yellowstone National Park and other geothermal sites. Most intriguing of all, ecosystems thrive on fiery ridges thousands of feet deep in the ocean, where new chunks of seafloor ooze (continued)