Discovering the Universe Ninth Edition

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

2. WHAT DO YOU THINK? • Why is water so important to the formation of life? • What element is uniquely suited to be the foundation of life as we know it, and why? • How do astronomers search for extraterrestrial intelligence? • Have astronomers located any extraterrestrial civilizations? • If advanced alien civilizations exist, is there any way they might know of our existence?

3. 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 extraterrestrial life

4. 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.

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 (a) 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. (b) Chemical analysis of newly fallen carbonaceous chondrites discloses that they are rich in organic compounds.

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 a slime of organic material created in the original Miller-Urey experiment.

10. 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.

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

12. 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.

13. 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.

14. 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.

15. Region of the Search for Extraterrestrial Intelligence The red circle, centered on the solar system, shows the region of the Milky Way in which SETI searches can reasonably expect to detect radio emissions from alien civilizations. This reasoning assumes that the signals are similar in nature to the kinds of radio emissions we generate on Earth.

16. 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.

17. 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.

18. 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.

19. Summary of Key Ideas • 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.

20. Summary of Key Ideas • 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.

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

22. WHAT DID YOU THINK? • Why is water so important to the formation of life? • Water allows many interactions between atoms and molecules dissolved in it.

23. WHAT DID YOU THINK? • What element is uniquely suited to be the foundation of life as we know it, and why? • Carbon is the only atom that can bond flexibly (but not too flexibly) with three or more other atoms.

24. WHAT DID YOU THINK? • How do astronomers search for extraterrestrial intelligence? • They search for radio signals from other advanced civilizations.

25. WHAT DID YOU THINK? • Have astronomers located any extraterrestrial civilizations? • No. Extraterrestrial civilizations have not yet been discovered.

26. WHAT DID YOU THINK? • If advanced alien civilizations exist, is there any way they might know of our existence? • Yes. They might detect our intentional radio broadcasts into space, our everyday radio and television broadcasts, or, possibly, one of our spacecraft traveling in interstellar space.