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Home , Astrobiology, Drake equation

Lab #7: and Drake’s Equation

This week, we’ll be talking about astrobiology, or the study of outside of . You will discuss and answer the following “thought” questions in smaller groups, and share your thoughts and ideas as a class. Most of these questions don’t have one correct answer, so it is important to validate and explain your thinking.

1. Preliminary Questions

Discuss the following questions in groups of 3 or 4, and write down responses and thoughts in your notebook. At the end of each set of questions, we’ll discuss them as a class.

1.1. Definitions

1. What defines life? What criteria should be used to distinguish “living things” from “non-living” things? Explain and discuss your reasoning.

2. What defines “intelligent” life? Are we intelligent? Are chimpanzees? What would we need from alien lifeforms/ if we are to detect their existence or to communicate with them?

1.2.

3. Name the major chemical elements that constitute life as we know it on earth. What are possible sources for these elements (i.e. where did they come from)? Comment on the ultimate source of , and other organic elements.

4. What is so significant about the element (Si), in relation to the element Carbon (C)? (Hint: periodic table.) Why is Si more special than Germanium (Ge) or Tin (Sn)?

5. One of the most oft-mentioned topics in “life outside of Earth” discussions is the existence of liquid . (Bonus: why is water so important?) Comment on how the dis- –2– tance from the host affects the possibility for liquid water on a . Are there ways to cheat this “habitability zone”, i.e. can a planet be very far or close to a star and still have water?

1.3. Our Planet

6. Explain how the orbit of Earth affects the seasons. What would happen if the Earth’s orbit were more eccentric? What would the seasons be like on Halley? Comment on how the eccentricity of a planet’s orbit affects its habitability.

7. The Earth’s is threaded by a weak magnetic field. A magnetic field strongly deflects the path of charged . Why is this significant for ? Com- ment on the effects of the Earth’s magnetic field on its habitability.

8. How might the have affected early development of life on Earth? Comment on possible roles played by the moon in early evolution on Earth.

1.4. More Questions!

9. Our understanding of life is somewhat (severely?) limited in that we only know of one instance of evolved life: one common ancestor on one planet, etc. Might life exist in conditions very different from what we discussed above?

10. What are some of the ways in which a species can become extinct? Are some of these fates preventable by sufficiently “advanced” civilizations? Unique to them? Can you make educated guesses for a typical lifetime of an intelligent ? Some values for perspective: Age of the : ∼13,700,000,000 years Age of the Earth: 4,600,000,000 years Earliest evidence of fossil : 3,500,000,000 years ago First multicellular : 1,500,000,000 years ago Earliest invertebrates: 800,000,000 years ago Fish+amphibian domination: 590,000,000 - 248,000,000 years ago –3–

Mammals dominant: since 65,000,000 years ago Homo sapiens originated: 250,000 years ago Human civilization: ∼ 10,000 years old. Radio communication: ∼< 100 years old! 11. The human population on Earth doubles every 30-40 years. Taking the of the Earth to be 6.7 billion people at present day, and assuming a steady growth rate, estimate a rough value for the population in the year 2106. In the year 2206? Comment on the effects of inhabiting (or some of the Jovian moons) in solving the overpopulation problem. The nearest star to the is Proxima Centauri, 4.2 light years away. Comment on the feasibility of colonization of other in solving the overpopulation problem.

2. Drake’s Equation:

The following equation was created by Prof. (UCSC).

N = R × fp × ne × fL × fI × fC × L

R is the rate of in our .

fp is the fraction of stars that have habitable . ne is the average number of planets that in the habitable zone.

fL is the fraction of the star systems from fp that actually develop life. fI is the fraction of the above with intelligent life. fC is the fraction of the above that are willing and able to communicate. L is the expected lifetime of such a civilization.

1. What does N represent? Show that it has proper units. Make sure you understand how it works!

2. R is about 10 per year. With your partner(s), make educated guesses on the values of the other parameters. Explain the reasoning and thoughts behind your values.

3. What do you get for N? How does your answer compare to the observed value of N?

4. Comment on the validity, uncertainties, and the applicability of the . Is it scientific? Is it useful? What does it tell us about life outside our own ?