NASA's Curiosity rover has discovered more building blocks of life on Mars  after carrying out a chemistry experiment never before conducted on another planet, scientists said Tuesday. The organic molecules are not definitive evidence of past life, the NASA-led team emphasized, because they could also have formed on the red planet or crash-landed on meteorites. But it proves that these important clues to Martian history have been preserved on the surface for more than 3 billion years, they