1. Astrobiology: Life on Other Worlds Chapter 26

2. This chapter is either unnecessary or vital. If you believe that astronomy is the study of the physical universe above the clouds, then you are done; the previous 25 chapters completed your study of astronomy. But, if you believe that astronomy is the study not only of the physical universe, but also of your role as a living being in the evolution of the universe, then everything you have done so far from this book has been preparation for this chapter. As you read this chapter, you will ask four important, related questions: What is life? How did life originate on Earth? Could life begin on other worlds? Could humans on Earth communicate with civilizations on other worlds? Guidepost

3. You won’t get more than the beginnings of answers to those questions here, but often in science asking a question is more important than getting an immediate answer. You have explored the universe from the phases of the moon to the big bang, from the origin of Earth to the death of the sun. Astronomy is meaningful, not just because it is about the universe but because it is also about you. Now that you know astronomy, you see yourself and your world in a different way. Astronomy has changed you. Guidepost

4. I. The Nature of Life A. The Physical Basis of Life B. Information Storage and Duplication C. Modifying the Information II. Life in the Universe A. The Origin of Life on Earth B. Geologic Time and the Evolution of Life C. Extremophiles D. Life in Our Solar System E. Life in Other Planetary Systems III. Intelligent Life in the Universe A. Travel Between the Stars B. Radio Communication C. How Many Inhabited Worlds? Outline

5. The Physical Basis of Life All life forms on Earth, from viruses to complex mammals (including humans) are based on carbon chemistry. Carbon-based DNA and RNA molecule strands are the basic carriers of genetic information in all life forms on Earth. The Tobacco Mosaic Virus contains a single strand of RNA, about 0.1 mm long. This complex mammal contains about 30 AU of DNA.

6. Information Storage and Duplication All information guiding all processes of life are stored in long spiral molecules of DNA (Deoxyribonucleic Acid). Basic building blocks are four Amino acids: Adenine, Cytosine, Guanine, and Thymine Information is encoded in the order in which those amino acids are integrated in the DNA molecule.

7. Processes of Life in the Cell Information stored in the DNA in the nucleus is copied over to RNA (ribonucleic acid) strands, which acts as a messenger to govern the chemical processes in the cell.

8. Duplication and Division In the course of cell division, the DNA strands in the nucleus (chromosomes) are duplicated by splitting the double-helix strand up and replacing the open bonds with the corresponding amino acids. Process must be sufficiently accurate, but also capable of occasional minor mistakes to allow for evolution

9. The Origin of Life on Earth Life develops into more complex forms through gradual evolution, spanning many thousands of generations. Life began in the sea as single- celled creatures. Those, as well as early multi-celled creatures, had no hard parts to leave fossils. Earliest, microscopic fossils date back ~ 4 billion years

10. The Earliest Fossils Oldest fossils known: stromatolites Built up layer by layer from single-celled creatures, similar to bacteria, ~ 3.5 billion years ago

11. The Miller Experiment Miller Experiment in 1952: Simulating conditions on Earth when life began ~ 4 billion years ago: Water (oceans), primitive atmosphere gases (hydrogen, ammonia, methane), and energy from electric discharges (lightning) Experiment produced some of the fundamental building blocks of life: amino acids, fatty acids, and urea

12. The Origins of Life on Earth (2) Miller experiment shows that basic building blocks of life form naturally. Amino acids and other organic compounds naturally tend to link up to form more complex structures. Early oceans on Earth were probably filled with a rich mixture of organic compounds: the “Primordial Soup”. Chemical evolution leads to the formation and survival of the most stable of the more complex compounds.

13. Extraterrestrial Origin of Life on Earth Alternative theory: Most primitive living entities transported to Earth in meteorites or comets. Some meteorites do show traces of amino acids. Theory of extraterrestrial origin of life is currently untestable.

14. Geologic Time In geologic terms, higher life forms, in particular mammals and humans, have evolved only very recently. Humans have existed for only ~ 3 million years.

15. The Cambrian Explosion ~ 1/2 billion years ago, in the Cambrian Period, the diversity and complexity of life on Earth dramatically increased  “Cambrian Explosion ” Best-known fossils from the Cambrian period: Trilobites All known fossils from the Cambrian period are from sea creatures. No traces of life on land until ~ 400 million years ago

16. Extremophiles Some life forms can withstand extreme conditions, hostile to most other life on Earth. These are called Extremophiles. Withstanding extreme conditions may have been necessary if life already began during the era of heavy bombardment. Weiku bugs survive at an altitude of 13,800 feet on Mauna Kea (Hawaii). Colonies of thermophilic (heat- loving) organisms thrive in temperatures up to 70 o C (13 o F).

17. Life in Our Solar System Other planets or their moons are unlikely to have ever provided suitable conditions for life. There is evidence for past water on Mars, but no evidence for life now or any time in the past. Meteorite ALH84001,0 probably originated on Mars. Claimed traces of microscopic fossils may well be regular mineral formations in the rock. High Methane concentrations apart from volcanic regions suggests that Methane may be produced biologically.

18. Three Questions About the Evolution of Life 1) Could life originate on another world if conditions were suitable? 2) Will life always evolve toward intelligence? 3) How common are suitable conditions for the beginning of life? Miller experiment etc. indicate: probably yes If intelligence favors one species over another: probably yes  Investigate conditions on other planets and statistics of stars in our Milky way

19. Some Requirements of Life Liquid water (for chemical reactions and as transport medium) Atmosphere (to avoid rapid vaporization of water; gasses needed for organic compounds) Moderate temperatures (keep water liquid; avoid disintegration of organic compounds; activate complex chemical reactions) Time for life to evolve from simple organic compounds into higher life forms: several billion years

20. Requirements for Life in Other Planetary Systems Planetary systems are probably common. Stable orbit around the star  consider only single stars Time for evolution  consider only F5 or less massive stars Moderate temperatures  Life zone around the star

21. Communication with Distant Civilizations Direct space travel to other stars not feasible due to large distances (long travel times) Viable alternative: Radio communication Even for radio communication: Long answer times due to light-travel time Messages can be arranged in blocks of a certain length that is a product of two prime numbers  Only two ways to arrange them in a rectangle.

22. The Arecibo Message At dedication of Arecibo Radio Observatory, blocks of 1679 pulses were emitted, which can be arranged in only two ways: Resulting 23 x 73 grid contained basic information about our human society 23 rows of 73 or 73 rows of 23

23. The Search for Extraterrestrial Intelligence (SETI) In addition to sending messages to possible extraterrestrial civilizations, there are also programs to listen for intelligent messages from space: SETI. Signals would be overwhelmed by background noise SETI program is highly controversial because of the uncertain prospects of positive results. Only certain wavelength ranges are suitable for this search.

24. The Drake Equation Factors to consider when calculating the number of technologically advanced civilizations per galaxy: N c = N * · f p · n LZ · f L · f l · F S Most of the factors are highly uncertain. Possible results range from 1 communicative civilization within a few dozen light years to us being the only communicative civilization in the Milky Way