How deep to dig for life on Mars?
A team of NASA scientists now believe they know why organic molecules vanish from surface of Mars. Their findings may help determine how deep probes will have to dig to look for evidence of life on Mars. In 1976, NASA’s Viking Lander tested the Martian soil for the presence of organic molecules — possible evidence of past or present life. It found none at all. For a time, the case for life on Mars seemed closed. Yet Viking didn’t detect even the minute amounts of organics that should have been left behind by meteorite impacts. Since then, scientists have come to believe that ultraviolet light wipes out any organic molecules on the surface of Mars. (Unlike Earth, Mars lacks a thick atmosphere with an ozone layer to screen out the Sun’s intense ultraviolet rays.) Now a team of NASA scientists has analyzed this process in detail. They have found that ultraviolet light creates extremely reactive oxygen molecules  known as “superoxide ions” — in the Martian soil. Any organic molecules pr
