How to Detect a Bio Signature

Send probes to Mars to seek out the water, one of the essential requirements for life. Powerful cameras aboard orbiting spacecraft have detected evidence of past water flow within Martian craters. In 2008, the Phoenix Mars lander detected present-day water ice near the planet's north pole. In 2004, the Spirit rover discovered evidence of Mars' watery history in a rock called "Humphrey."

Place a narrowly focused camera on a satellite with an ultra-high grade digital camera, and observe a small patch of sky over several years to detect earth-sized planets with orbits similar to Earth's in other solar systems. The Kepler space telescope, launched in 2009, is doing precisely that, seeking the tell-tale changes in luminosity that reveal the presence of planets. Over three years of study, astronomers expect the mission to identify terrestrial planets within the so-called "habitable zone" of stars. Analyze this publicly available data, looking at the light signatures from candidate planets for signs of water, methane and other indicators that life may be present.

Sniff out Martian methane, using high-dispersion infrared spectrometers attached to powerful Earth-based telescopes. Although it doesn't constitute an open-and-shut case, the presence of methane strongly suggests that biological activity may be happening. The discovery of methane in Mars' atmosphere startled scientists in 2003. However, researchers later determined that the gas exists only in specific regions, suggesting that geologic activity may account for it.

Analyze the light spectra from worlds circling distant stars. NASA's plans include spacecraft that will be capable of separating the light of orbiting planets from that of the much brighter stars around which they orbit. Researchers at Oxford University's Environmental Change Institute have developed mathematical techniques for discerning the presence of multicellular life from the spectra of light from worlds orbiting other stars. They claim that they would be able to tell, for instance, whether the planet has forests.

Send probes that pierce the icy surfaces of Europa and Ganymede, which are moons of Jupiter, and Enceladus, which orbits Saturn. Data from the Galileo and Cassini spacecraft suggests that watery oceans lie underneath the icy surfaces of each moon. NASA's long-term plans include probes that will sample the water that lies underneath the ice, potentially detecting signs of multicellular life.

Place a Martian meteorite under a microscope, and look for signs of ancient life. Such meteorites fall to Earth more often than you might think. In 1996, scientists studying the martian meteorite ALH84001 claimed to have detected evidence for past bacterial life within the rock. Other experts have since disputed the claim, but evidence derived from ALH84001 remains the closest that scientists have yet come to detecting a genuine bio signature from another world.