Key Ideas

Organic Molecules in the Universe: All life on Earth, and presumably on other worlds, depends on organic (carbon-based) molecules. These molecules occur naturally throughout interstellar space.

• The organic molecules needed for life to originate were probably brought to the young Earth by comets, meteorites, and interplanetary dust particles. Another likely source for organic molecules is chemical reactions in Earth’s primitive atmosphere. Similar processes may occur on other worlds.

Life in the Solar System: Besides Earth, other worlds in our solar system—the planet Mars, Jupiter’s satellite Europa, and Saturn’s satellite Enceladus—may have had the right conditions for the origin of life.

• Europa appears to have extensive liquid water beneath its icy surface. Future missions may search for the presence of life there. Enceladus also appears to contain a large subsurface ocean, and some of this material is ejected through ice volcanoes for easier sampling.
• Mars once had liquid water on its surface, though it has none today. Life may have originated on Mars during the liquid water era.
• The Viking Lander spacecraft searched for microorganisms on the Martian surface, but found no conclusive sign of their presence. The Mars Science Laboratory is designed to assess the past and present suitability of Mars for microbial life.
• An ancient Martian rock that came to Earth as a meteorite shows features that have been suggested as remnants of Martian life. Most of these features can be explained by geologic processes.

Radio Searches for Extraterrestrial Intelligence: Astronomers have carried out a number of searches for radio signals from other stars. No signs of intelligent life have yet been detected, but searches are continuing and use increasingly sophisticated techniques.

• The Drake equation is a tool for estimating the number of intelligent, communicative civilizations in our Galaxy.

Telescope Searches for Earthlike Planets: A future generation of orbiting telescopes may be able to detect numerous terrestrial planets around nearby stars. If such planets are found, their infrared spectra may reveal the presence or absence of life.