Above left: If we find evidence of life beyond Earth, it will probably be very tiny. Did you know that microbes are the largest population of living things on Earth? Above right: Many strange creatures live in extreme conditions right here on Earth, such as hydrothermal vents deep in the oceans. So it wouldn’t be too surprising to find life in seemingly hostile environments on other planets.
	A scientist’s explanation of what some features mean to the search for evidence of liquid water on Mars.

Is there life—or has there ever been life—beyond Earth? Scientists are trying to find out. One thing’s for sure, if we find life elsewhere, it will probably be in conditions where people couldn’t survive: Too hot or too cold, the wrong kind of air, too much solar radiation, or a host of other things that are deadly to humans. But life is robust, and on Earth it finds a way to thrive in the unlikeliest of conditions. The only absolute is that life—at least on Earth—needs liquid water.

According to Gene McDonald at the Astrobiology Research Element at NASA’s Jet Propulsion Laboratory (JPL), water has unique properties that make it the best solvent for the kind of chemistry that you need to start and sustain life. Neither ice nor water vapor will do. Biochemistry requires molecules to move around, which they can’t do nearly as well in a solid. In vapor, it’s hard to keep parts together on a molecular level.

Why search for evidence of water on other planets?
Where there’s water, there might be—or might have been—life. Given what we know about Mars and about the conditions necessary for life, Mars is a likely candidate, and a relatively close place to look. Scientists don’t expect to find liquid water currently on the surface of Mars, though there appears to be frozen water in the form of polar caps and ground ice. But liquid water may have existed in Mars’s distant past—which means there may be evidence of ancient life on Mars as well.

What about life in the universe beyond our solar system?
Check out Drake’s equation for a fun mathematical way to calculate the likelihood of life elsewhere.

Scientists at the SETI (Search for Extraterrestrial Intelligence) Institute, once directed by Carl Sagan, search for radio signals and other evidence that would indicate intelligent life beyond Earth. Find out more at SETI@home.