Habitability - the ability of worlds to sustain life - is a unifying theme in NASA's current exploration efforts. The concept goes back to the most basic questions: "Are we alone? Is there life elsewhere in the cosmos?"
In our solar system, the formative and evolutionary processes that acted on the planets made at least one of them a platform for the development of life - Earth
The search for life beyond Earth raises many questions.
- How have specific planetary environments evolved with time, when and in what way were they habitable and does life exist there now?
- What roles do the positions and masses of giant planets play in the formation of habitable planets and moons and the delivery to them of the chemical ingredients of life?
- What are the hazards that threaten the long-term survival of humankind on our planet, and what threats do we face as individuals and as a species as we move off the Earth into the rest of the Solar system?
Conditions for habitability remain poorly understood. The narrowest definition - that a habitable environment requires liquid water in order to sustain life as we know it - makes Jupiter's moon Europa (which has subsurface liquid water) and Venus (which may have lost oceans of water sometime in its history) intriguing targets for exploration. The Cassini mission's discovery that Saturn's Enceladus might have liquid water near its surface had increased interest in further study of this intriguing moon.
But life might occur where other liquids substitute for water, and in planetary environments where organic molecules are briefly exposed to liquid water and then preserved. This possibility makes Saturn's moon Titan a high-priority for more detailed exploration.
NASA Solar System Exploration Planetary Database
NASA Astrobiology Homepage
Planet Quest: Alien Safari
NASA Astrobiology Roadmap