Key Ideas and Terms

8-1 Planets and the chemical building blocks of life are found throughout space

• Astrobiology is the study of how life originates and evolves throughout 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 or meteorites.
• The Miller-Urey experiment demonstrated the possible environmental conditions of an early Earth.

8-2 Europa and Mars are promising places for life to have evolved

• Besides Earth, the planet Mars and Jupiter’s moon Europa are the most likely candidates to have had the right conditions for the origin of life.
• 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. Three decades later, NASA successfully landed two robotic rovers named Spirit and Opportunity at two very different sites on opposite sides of Mars. More recently, the Phoenix Lander and the LCROSS NASA missions found solid evidence of water on Mars and our Moon.
• Europa appears to have extensive liquid water beneath its icy surface. Future missions may search for the presence of life there.

8-3 Meteorites from Mars have been scrutinized for life-forms

• SNC meteorites come from Mars and are identified by the chemical composition of trace amounts of Martian atmospheric gas trapped within them.
• ALH 84001 is the only known specimen of a rock that was on Mars during the era when liquid water most likely existed on the planet’s surface and provides only circumstantial evidence for Martian life.

8-4 The Drake equation helps scientists estimate how many civilizations inhabit our Galaxy

• The fact that life exists on Earth means that extraterrestrial life, including intelligent species, might evolve on planets around distant stars, given sufficient time and hospitable conditions.
• The collective effort of scientists looking for intelligent life beyond Earth is known as the Search for Extraterrestrial Intelligence, or SETI.
• The Drake equation is a tool for estimating the number of intelligent, communicative civilizations in our Galaxy.
• As radio waves travel at the speed of light, radio communication is probably more plausible than face-to-face visits with extraterrestrials.

8-5 Searches with space-based infrared telescopes and Earth-based radio telescopes for Earthlike planets and alien civilizations are under way

• Astronomers have discovered many planets by searching for the slight dimming of starlight as a planet “transits” its host star.
• Radio searches focus on a frequency that is fairly free of interference in the neighborhood of the microwave emission lines of hydrogen (H) and hydroxide (OH). This region of the microwave spectrum is called the water hole, because the letters H and OH together make H₂O, the chemical symbol for water.
• 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 using increasingly sophisticated techniques.
• A new generation of orbiting telescopes may be able to detect terrestrial planets around nearby stars. If such planets are found, their infrared spectra may reveal the presence or absence of life.