1. Is there other life in our solar system? E X P L O R I N G A S T R O B I O L O G Y: Alien life in our solar system isn’t a new idea. Astronomer Percival Lowell was convinced that an advanced civilization built canals on Mars, and Venus was once thought to have oceans and rainforests. Public imagination and popular culture have reinforced such ideas. Canals On Mars, Oceans On Venus

2. Is there other life in our solar system? E X P L O R I N G A S T R O B I O L O G Y: Mars and the rest of our solar system turn out to be far less hospitable than Percival Lowell and others thought—but finding microbial or extremophile life is still a possibility. Changing Expectations Olympus Mons, Mars, 2003 Mars Express mission. Image: ESA Europa’s icy surface

3. Is there other life in our solar system? E X P L O R I N G A S T R O B I O L O G Y: Astrobiologists have located several places in our solar system where conditions could support life as we understand it. Jupiter’s moon Europa, the planet Mars, and perhaps even Venus could conceivably be home to living organisms. Saturn’s moon Titan could hold clues to our own planet’s evolution. Jupiter’s moon Europa Mars Venus Saturn’s moon Titan Best Bets

4. Is there other life in our solar system? E X P L O R I N G A S T R O B I O L O G Y: An ocean of liquid water probably exists under Europa’s cracked, icy surface, kept warm by tidal forces. Evidence supports this idea, and Europa is considered a likely location for finding life in our solar system today. Life On Europa?

5. Is there other life in our solar system? E X P L O R I N G A S T R O B I O L O G Y: Life On Mars? Mars, once considered our best candidate for finding life beyond Earth, is now known to have a surface that is savagely cold, dry, and barren. But new indications of warmth and liquid surface water in Mars’ past are rekindling hopes that life was there in the past—and perhaps still is.

6. Is there other life in our solar system? E X P L O R I N G A S T R O B I O L O G Y: Life On Mars Now? If Mars was warmer and wetter in the past, it may have supported life on the surface—but even the toughest organisms would be unlikely to survive the planet’s current surface conditions. So, if there’s any life on Mars, it would be beneath the surface in ice deposits or aquifers.

7. Is there other life in our solar system? E X P L O R I N G A S T R O B I O L O G Y: Venus is a world where daily temperatures at the surface can be as high as 870°F. Sulfuric acid clouds and a thick carbon-dioxide atmosphere trap the Sun’s heat in a runaway greenhouse effect. Life On Venus? Venus surface image, taken by the Russian lander Venera, showing part of the lander’s structure at the lower right

8. Is there other life in our solar system? E X P L O R I N G A S T R O B I O L O G Y: Venusian Clouds Far above the surface of Venus, the temperature and pressure are Earth-like, and photosynthesis is possible. On Earth, some bacteria live in the droplets of water in clouds, and many others are able to tolerate strong acid. Could the acidic clouds of scorching Venus be a home for life?

9. Is there other life in our solar system? E X P L O R I N G A S T R O B I O L O G Y: Life On A Saturn Moon? Saturn’s “titanic” moon, one of the biggest in our solar system, probably doesn’t harbor life— but it does have abundant organic substances, including methane and hydrocarbons. Titan’s environment may resemble that of the early Earth, and could increase our understanding of where we came from and how our solar system formed. Titan’s surface (above, in false-color) and hazy clouds (right)

10. Is there other life in our solar system? E X P L O R I N G A S T R O B I O L O G Y: The Cassini-Huygens Mission The Cassini-Huygens mission, which reached Saturn in 2004, is also investigating Titan. Cassini includes instruments to map the surface with radar and take pictures. The Huygens probe was designed to make direct observations of Titan’s environment as it descends by parachute through the moon’s thick atmosphere. Artist’s Conception