Questions

Review Questions

  1. Why are extreme life-forms on Earth, such as those shown in the photograph that opens this chapter, of interest to astrobiologists?

  2. What is meant by “life as we know it”? Why do astrobiologists suspect that extraterrestrial life is likely to be of this form?

  3. How have astronomers discovered organic molecules in interstellar space? Does this discovery mean that life of some sort exists in the space between the stars?

  4. Mercury, Venus, and the Moon are all considered unlikely places to find life. Suggest why this should be.

  5. Why do astronomers think Enceladus could support life? What would be the easiest way to search for this life?

  6. Why are scientists interested in searching for life in Lake Vostok, Antarctica?

  7. Many science-fiction stories and movies—including The War of the Worlds, Invaders from Mars, Mars Attacks!, and Martians, Go Home—involve invasions of Earth by intelligent beings from Mars. Why Mars rather than any of the other planets?

  8. Summarize the three tests performed by the Viking Landers to search for Martian microorganisms.

  9. What arguments can you give against the idea that the “face” on Mars (Figure 27-9) is of intelligent origin? What arguments can you give in favor of this idea?

  10. Suppose the Curiosity rover measured an enhanced abundance of the carbon isotope 12C in an ancient riverbed. What might this tell us?

  11. Suppose someone brought you a rock that he claimed was a Martian meteorite. What scientific tests would you recommend be done to test this claim?

  12. The Martian meteorite ALH 84001 contains magnetite, which can be produced by microorganisms on Earth. Why isn’t this good evidence for Martian life?

  13. Why are astronomers interested in the microtunnels within several Martian meteorites? Why don’t these prove the existence of Martian life?

  14. Why are most searches for extraterrestrial intelligence made using radio telescopes? Why are most of these carried out at frequencies between 103 MHz and 104 MHz?

  15. Explain why telescopes that would look for the infrared spectra of other planets need to be placed in space (Hint: See spectrum in Figure 27-13).

Advanced Questions

Problem-solving tips and tools

The small-angle formula, discussed in Box 1-1, will be useful. Section 5-2 gives the relationship between wavelength and frequency, while Section 5-9 and Box 5-6 discuss the Doppler effect. Section 6-3 gives the relationship between the angular resolution of a telescope, the telescope diameter, and the wavelength used. You will find useful data about the planets in Appendix 1.

  1. In 1802, when it seemed likely to many scholars that there was life on Mars, the German mathematician Karl Friedrich Gauss proposed that we signal the Martian inhabitants by drawing huge geometric patterns in the snows of Siberia. His plan was never carried out. (a) Suppose patterns had been drawn that were 1000 km across. What minimum diameter would the objective of a Martian telescope need to have to be able to resolve these patterns? Assume that the observations are made at a wavelength of 550 nm, and assume that Earth and Mars are at their minimum separation. (b) Ideally, the patterns used would be ones that could not be mistaken for natural formations. They should also indicate that they were created by an advanced civilization. What sort of patterns would you have chosen?

  2. Assume that all the terms in the Drake equation have the values given in the text, except for N and L. (a) If there are 1000 civilizations in the Galaxy today, what must be the average lifetime of a technological civilization? (b) What if there are a million such civilizations?

  3. (a) Of the visually brightest stars in the sky listed in Appendix 5, which might be candidates for having Earthlike planets on which intelligent civilizations have evolved? Explain your selection criteria. (b) Repeat part (a) for the nearest stars, listed in Appendix 4.

  4. It has been suggested that extraterrestrial civilizations would choose to communicate at a wavelength of 21 cm. Hydrogen atoms in interstellar space naturally emit at this wavelength, so astronomers studying the distribution of hydrogen around the Galaxy would already have their radio telescopes tuned to receive extraterrestrial signals. (a) Calculate the frequency of this radiation in megahertz. Is this inside or outside the water hole? (b) Discuss the merits of this suggestion.

  5. Imagine that a civilization in another planetary system is sending a radio signal toward Earth. As our planet moves in its orbit around the Sun, the wavelength of the signal we receive will change due to the Doppler effect. This gives SETI scientists a way to distinguish stray signals of terrestrial origin (which will not show this kind of wavelength change) from interstellar signals. (a) Use the data in Appendix 1 to calculate the speed of Earth in its orbit. For simplicity, assume the orbit is circular. (b) If the alien civilization is transmitting at a frequency of 3000 MHz, what wavelength (in meters) would we receive if Earth were moving neither toward nor away from their planet? (c) The maximum Doppler shift occurs if Earth’s orbital motion takes it directly toward or directly away from the alien planet. How large is that maximum wavelength shift? Express your answer both in meters and as a percentage of the unshifted wavelength you found in (b). (d) Discuss why it is important that SETI radio receivers be able to measure frequency and wavelength to very high precision.

  6. Astronomers have proposed using interferometry to make an extremely high-resolution telescope. This proposal involves placing a number of infrared telescopes in space, separating them by thousands of kilometers, and combining the light from the individual telescopes. One design of this kind has an effective diameter of 6000 km and uses infrared radiation with a wavelength of 10 μm (micrometers). If it is used to observe an Earthlike planet orbiting the star Epsilon Eridani, 3.22 parsecs (10.5 light-years) from Earth, what is the size of the smallest detail that this system will be able to resolve on the face of that planet? Give your answer in kilometers.

Discussion Questions

  1. Suppose someone told you that the Viking Landers failed to detect life on Mars simply because the tests were designed to detect terrestrial life-forms, not Martian life-forms. How would you respond?

  2. Science-fiction television shows and movies often depict aliens as looking very much like humans. Discuss the likelihood that intelligent creatures from another world would have (a) a biochemistry similar to our own, (b) two legs and two arms, and (c) about the same dimensions as a human.

  3. The late, great science-fiction editor John W. Campbell exhorted his authors to write stories about organisms that think as well as humans but not like humans. Discuss the possibility that an intelligent being from another world might be so alien in its thought processes that we could not communicate with it.

  4. If a planet always kept the same face toward its star, just as the Moon always keeps the same face toward Earth, most of the planet’s surface would be uninhabitable. Discuss why.

  5. How do you think our society would respond to the discovery of intelligent messages coming from a civilization on a planet orbiting another star? Explain your reasoning.

  6. What do you think will set the limit on the lifetime of our technological civilization? This factor is L in the Drake equation. Explain your reasoning.

  7. The first of all Earth spacecraft to venture into interstellar space were Pioneer 10 and Pioneer 11, which were launched in 1972 and 1973, respectively. Their missions took them past Jupiter and Saturn and eventually beyond the solar system. Both spacecraft carry a metal plaque with artwork (reproduced below) that shows where the spacecraft is from and what sort of creatures designed it. If an alien civilization were someday to find one of these spacecraft, which of the features on the plaque do you think would be easily understandable to them? Explain your reasoning.

801

Web/eBook Questions

  1. Any living creatures in the subsurface ocean of Europa would have to survive without sunlight. Instead, they might obtain energy from Europa’s inner heat. Search the World Wide Web for information about “black smokers,” which are associated with high-temperature vents at the bottom of Earth’s oceans. What kind of life is found around black smokers? How do these life-forms differ from the more familiar organisms found in the upper levels of the ocean?

  2. Search the World Wide Web for information about the Mars Express orbiter and the Spirit and Opportunity rovers. What discoveries have these missions made about water on Mars? Have they found any evidence that liquid water has existed on Mars in the recent past? Describe the evidence, if any.

  3. Like other popular media, the World Wide Web is full of claims of the existence of “extraterrestrial intelligence,” namely, UFO sightings and alien abductions. (a) Choose a Web site of this kind and analyze its content using the idea of Occam’s razor, the principle that if there is more than one viable explanation for a phenomenon, one should choose the simplest explanation that fits all the observed facts. (b) Read what a skeptical Web site has to say about UFO sightings. A good example is the Web site of the Committee for Skeptical Inquiry, or CSI. After considering what you have read on both sides of the UFO debate, discuss your opinions about whether intelligent aliens really have landed on Earth.