CAPE CANAVERAL, Fla. (Reuters) - The U.S. space agency launched on Sunday the second of two unmanned space missions to search the planet Mars for signs of water, a key ingredient of life. NASA‘s Mars Polar Lander probe lifted off from Cape Canaveral under gray skies precisely on time at 3:21 p.m. EST (2021 GMT). "We have liftoff of the Delta 2 rocket carrying the Mars Polar lander, NASA‘s first visit to the Red Planet‘s southern pole," said launch commentator Lisa Malone as the rocket soared skyward. The landing craft, shrouded in a protective heat shield, separated from the rocket‘s upper stage about 42 minutes later, high above the Indian Ocean, to begin its 757 million km interplanetary voyage. The probe‘s sistership, the Mars Climate Orbiter, was launched three weeks ago to observe the planet from an orbit 422 km high. Together with European nations, NASA hopes to launch unmanned craft to Mars every two years. NASA officials put the cost of the two latest probes at around $328 million. If all goes according to plan, the Mars Polar Lander will touch down on the edge of the Martian south pole, a cap of frozen carbon dioxide, on Dec. 3. The craft will slow its descent with a parachute and rocket engines and land on three tripod-like legs. The main goal of the mission is to detect traces of water, which would boost the theory that life could exist on the planet. A 2-metre robot arm on the lander will be used to scoop up samples of the Martian soil for analysis in a mini-laboratory where they will be heated and electronically sniffed to detect water. The mission will also give earthlings their first chance to eavesdrop on another world. The lander will record the sounds of Mars with a microphone donated to NASA by the Planetary Society, a space advocacy group once-led by the late Carl Sagan.