If life is widespread on a planet, its atmosphere should show signs of its presence. The air we exhale has more carbon dioxide and less oxygen than the air we inhale, and in the same way, the combined ?breathing? of all the life on a planet will change the chemistry of its atmosphere. If life exists on the planet, these changes may be large enough for us to notice.

We need to know which gases should we search for. We know the answers for our own world, but predicting how an alien biology might interact with its atmosphere isn?t easy. ?As astrobiologists we?ve got to be sure that we?re not too Earth-centric,? says Michael Meyer, senior scientist for astrobiology at NASA Headquarters in Washington, D.C.

?We have to be very careful about how foreign biology might be different from our own, especially when you get to the bigger molecules,? says David Des Marais, principal investigator for the Ames Research Center team of NASA?s Astrobiology Institute. ?Large molecules like DNA and chlorophyll represent later, highly significant innovations of life on Earth, but also the ones that may have differed elsewhere.?

Scientists have speculated that silicon, a primary component of sand and a close cousin to carbon, could form the basis of an extraterrestrial biology. Alien life might forgo sunlight and depend instead on the geothermal energy in hydrogen and sulfur compounds emitted from the planet?s interior, much like the deep-sea vent ecosystems here on Earth.

?Suppose,? says Meyer, ?that there is silicon-based life. [It might be] photosynthetic, and you would still end up with oxygen in the atmosphere. You could go there and the life could be completely different, but some of the chemistry could still be the same [as on Earth].?

A new space telescope called the Terrestrial Planet Finder (TPF) will focus on simple gases such as oxygen, ozone, carbon dioxide, methane, and water vapor. Oxygen is a good indicator of life. But scientists caution that oxygen has been abundant on Earth only since the advent of photosynthesis — probably a billion years or more after life began. A small amount of Oxygen can be created without life by ultraviolet radiation that splits water vapor into hydrogen and oxygen. But that oxygen would be readily consumed by rocks and minerals on the planet?s surface in the same kind of reactions that produce rust. Volcanic gases also react with oxygen and remove it from the atmosphere. Geological processes alone usually work against the accumulation of oxygen. An oxygen-rich atmosphere is, therefore, out of chemical equilibrium, suggesting that some active agent, such as photosynthetic life (green plants), is constantly replenishing the supply.

To have the greatest chance of spotting extraterrestrial life, astrobiologists must consider how life looked on the young Earth, when the atmosphere was different. Scientists suspect that for roughly the first billion years of its history, life on Earth had not yet evolved oxygen-producing photosynthesis. Instead, the microorganisms that then dominated the planet tapped the energy in gases leaking out of the Earth?s interior, with some microbes creating methane as a byproduct. On a planet with a similar geology to Earth, methane levels greater than about 100 parts per million would suggest the presence of life. But methane would be a more ambiguous discovery than oxygen, because planets of a different geological make-up might produce abundant methane without life.

In 1990, the Galileo spacecraft made a flyby of our planet on its journey to Jupiter. As it passed, Galileo?s sensors detected high levels of oxygen and methane in Earth?s atmosphere and signs of chlorophyll on the ground. Chlorophyll, which absorbs light at the far-red end of the visible spectrum, is a ?red flag? for life. The TPF won?t be sensitive to chlorophyll on a planet?s surface because atmospheric water vapor, which is opaque in the mid-infrared frequency range that TPF will observe, will hide the surface below. Even without chlorophyll, signs of oxygen and methane — which TPF can detect — would make a persuasive case for life.

If the TPF finds a habitable planet with lots of oxygen and some methane in its atmosphere, it would be a momentous discovery. But would such data really prove life is there? Astrobiologists say such evidence would be ?very compelling.? The next step would be an even larger space telescope that could analyze surface features such as continents and observe the planet?s changing seasons. Perhaps it would be possible to send a probe across interstellar space to take a close-up look, which would provide incontrovertible evidence. However, even using advanced propulsion technologies, a probe might take decades to reach a neighboring star.

Today, we know of more than 80 planets outside our solar system that might possibly harbor life, and more are being found all the time.

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