NASA is concerned about aliens. No, not undocumented immigrants or the scary space invaders loved by Hollywood. The real science questions are related to the search for microbial life in our solar system: on Mars, Europa, Titan, or Enceladus. Each of these has a possibility of supporting life: Mars has a geological and climate history most like Earth; Europa and Enceladus both have extensive oceans of warm liquid water under an ice crust; and Titan has a complex organic chemistry but is too cold for liquid water.
If life exists on any of these worlds, it is likely to have begun there, providing evidence of a second (or third or fourth) genesis within our solar system. Finding any lifeforms, or even life precursors or fossils, would be tremendously exciting to scientists and the public alike.
Mars’s habitability is a central focus of today’s missions to the Red Planet, and the results encourage us to move on to a search for life itself. Robotic landers planned for the 2020s may carry the first life-detection experiments since the Viking mission in 1976, or alternatively include the return of samples from Mars to be analyzed in terrestrial laboratories. The discipline called “planetary protection” establishes guidelines to minimize biological cross-contamination between worlds. We think we know how to handle these issues, although assuring the public that our protocols are safe may be a challenge. But now there is also serious discussion of landing humans on Mars. This would introduce a new set of concerns, since surely any humans who land on Mars would want to return eventually to Earth, and there is no way to sterilize humans or their hardware. By landing there, the astronauts might compromise our hope of ever finding indigenous martian life.