Neil F. Comins • William J. Kaufmann III

1. Neil F. Comins • William J. Kaufmann III Discovering the Universe Tenth Edition CHAPTER 19 Astrobiology

2. In this chapter, you will discover… • what qualities scientists believe a world must have in order to support life • why many scientists are open to the possibility that primitive life exists elsewhere in the solar system • how scientists estimate the number of planets orbiting other stars that could support complex life • how scientists search for life beyond our own solar system—and the results of those searches • how we are trying to communicate with advanced extraterrestrial civilizations

3. Viking Mars Lander Astronomer and renowned science popularizer Carl Sagan poses by a model of the Viking lander. This image was taken in Death Valley, California, where the background creates the feel of a Martian landscape. Sagan was instrumental in choosing some of the experiments flown on the Viking spacecraft.

4. Chemical Building Blocks • The chemical building blocks of life exist throughout the Milky Way Galaxy. • Organic molecules and water have been discovered in interstellar clouds, in some meteorites, in comets, and in newly forming star and planet systems. • Five elements can bond covalently (that is, by sharing electrons) with three or more elements: boron (B), carbon (C), nitrogen (N), silicon (Si), and phosphorus (P). • Bonding by donating or receiving electrons (called ionic bonding) creates bonds that are unsuitable for the formation of the complex, flexible, rapidly changing molecules that life requires. • Carbon is unique among the five elements that can make at least three covalent bonds in that its bonds are flexible yet strong. • As counter-examples, silicon-silicon bonds come apart under the slightest disturbance; silicon-oxygen bonds create gels and liquids that are very hard to alter.

5. Creating Complex Molecules (a) Atoms (denoted by capital letters) that can bond strongly to only two other atoms can make linear chains, as depicted here. However, when such atoms bond to atoms that can only make one bond, denoted here as Y and Z,the chain stops. In no case can atoms X, Y, and Z combine to create nonlinear chains other than loops. (b) When an atom, like carbon (C), can share electrons with more than two other atoms (in carbon’s case, with four atoms), then the complex, nonlinear chains essential for life can form. Chains of carbon atoms form the backbone of organic molecules. For example, glucose, with carbon, oxygen (O), and hydrogen (H), is a nonlinear molecule that serves as a nutrient for many life-forms; it is a sugar. The lines indicate bonds between atoms.

6. Non-Carbon Organic Molecules? When any element other than carbon that can make three or more covalent bonds combines, it makes compounds that are either too soft, too hard, too reactive, or too inert to be useful in supporting life. Consider silicon. (a) The silicon-oxygen pair that creates the backbone of silicone is too inert to allow such molecules to react rapidly and thereby serve as organic molecules. Furthermore, these bonds produce gel or liquid compounds, as shown. (b) When the backbone is silicon-oxygen-oxygen, the bonds are rigid, as in this quartz rock.

7. Carbonaceous Chondrite Carbonaceous chondrites are meteorites that date back to the formation of the solar system. This sample is a piece of the Allende meteorite, a large carbonaceous chondrite that fell in Mexico in 1969.

8. Miller-Urey Experiment Updated Modern versions of this classic experiment prove that numerous organic compounds important to life can be synthesized from gases that were present in Earth’s primordial atmosphere. This experiment supports the hypothesis that life on Earth arose as a result of ordinary chemical reactions.

9. Miller-Urey Experiment Updated This photograph shows Harold Urey observing his experiment as it was underway.

10. Zones for Habitable Planets • If a planet’s environment is otherwise hostile—such as having too much radiation, being too windy, or being too seismically active—life may either not be able to spread or may quickly become extinct. • If the star is too massive, it will explode before advanced life on any of its planets has a chance to evolve very far. If the star is too small, the planet would have to be very close to it to be warm enough for life, but then tidal forces from the star would lock the planet in synchronous rotation, making most of its surface either too hot (daytime side) or too cold (nighttime side) for life to flourish. • Even a world in synchronous rotation with a suitable temperature on the star-facing side is very unlikely to support life because water in its atmosphere will drift to the night side, permanently freeze, and the star-lit side will eventually become arid.

11. Zone for Habitable Planets This figure summarizes the locations in the Galaxy and in orbit around stars where habitable planets might be found. Earth, of course, is in such a location.

12. Hyperthermophiles These microscopic thermophiles (heat-loving organisms) live in water that is between 80°C and 100°C (85°F–140°F).

13. Hyperthermophiles • Geologists and biologists have discovered life on Earth in some incredibly challenging environments, such as on the ocean floor, in a lake far under the Antarctic ice pack, deep inside our planet’s crust, and even in hot geothermal vents. • It therefore seems reasonable to believe that life could have originated off Earth under similarly challenging conditions. • Scientists consider at least four places in our solar system as possible habitats for life past or present. They are Jupiter’s moons Europa, Ganymede, and Callisto, and the planet Mars.

14. Hyperthermophiles Tube worms (light-green tubes) with hemoglobin-rich red plumes. They reside around black smokers—vents in the ocean bottom that are in the same temperature range as the hot springs shown in the previous figure. These vents are over 3 km (2 mi) under water.

15. SETI • Astronomers are using radio telescopes to search for signals from other self-aware life in the Galaxy. This effort is called the search for extraterrestrial intelligence, or SETI. • SETI is primarily done at frequencies where radio waves pass most easily through the interstellar medium. So far, these searches have not detected any life outside Earth.

16. Water Hole The so-called water hole is a range of radio wavelengths from about 3 to 30 centimeters that happens to have relatively little cosmic noise. Some scientists suggest that this noise-free region would be well-suited for interstellar communication.

17. Radio Telescope Used for SETI The Arecibo observatory’s radio telescope, with a diameter of 305 m (1000 ft) is the largest single-aperture telescope in the world. It is located in Arecibo, Puerto Rico. In the past decade, it was used in an all-sky survey, along with an antenna located in the Mojave Desert in California to search for extraterrestrial intelligence. In 1996, an antenna in Canberra, Australia, joined the network.

18. Drake Equation: Used to calculate civilizations which are likely to exist in the Milky Way N = R* fp ne f l fi fc L R* = rate at which Sunlike stars form in the Galaxy fp = fraction of Sunlike stars that have planets ne = number of planets per solar-type star system suitable for life fl = fraction of those habitable planets on which life actually arises fi = fraction of those life-forms that evolve into intelligent species fc = fraction of those species that develop adequate technology and then choose to send messages out into space L = lifetime of that technologically advanced civilization

19. Human Memorabilia in Space Humans have beamed radio signals into space, hoping that the message will someday be intercepted by an alien civilization. This is a visual version of the signal sent in 1974 from the Arecibo radio telescope toward the globular cluster M13. The Pioneer and Voyager spacecraft, now in interstellar space, also carry messages from Earth.

20. Human Memorabilia in Space The plaques on Pioneer 10 and Pioneer 11 provide information about where we are, what we look like, and some of the science we know.

21. Human Memorabilia in Space Images and sounds sent on Voyager 1 and Voyager 2 were stored on phonographic records, long before DVDs were even a twinkle in an engineer’s eye. There are also instructions for playing the record, which contains information about our biology, our technology, and our knowledge base. Each record also contains the sounds of children’s voices. It is remotely possible that another race might someday discover the spacecraft.

22. Summary of Key Ideas

23. Astrobiology and SETI • The chemical building blocks of life exist throughout the Milky Way Galaxy. • Organic molecules and water have been discovered in interstellar clouds, in some meteorites, in comets, and in newly forming star and planet systems. • Astronomers are using radio telescopes to search for signals from other self-aware life in the Galaxy. This effort is called the search for extraterrestrial intelligence, or SETI. SETI is primarily done at frequencies where radio waves pass most easily through the interstellar medium. So far, these searches have not detected any life outside Earth.

24. Drake Equation and Space Memorabilia • The Drake equation is used to estimate the number of technologically advanced civilizations in the Galaxy whose radio transmissions we might discover. Estimates of this number vary from 1 to millions. • Everyday radio and television transmissions from Earth, along with intentional broadcasts into space, may be detected by other life-forms.

25. Key Terms astrobiology Drake equation habitable zone organic molecule SETI water hole