Life on Other Worlds

Guidepost This chapter is either unnecessary or critical, depending on our point of view. If we believe that astronomy is the study of the physical universe above the clouds, then this chapter does not belong here. But if we believe that astronomy is the study of our position in the universe, not only our physical position but also our role as living beings in the origin and evolution of the universe, then everything else in this book is just preparation for this chapter. Astronomy is the only science that truly acts as a mirror. In studying the universe up there, we learn what we are down here. Astronomy is not really about stars, galaxies, and planets; it is about us.

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

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. This complex mammal contains about 30 AU of DNA. The Tobacco Mosaic Virus contains a single strand of RNA, about 0.1 mm long

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.

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.

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.

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.

The Origin of Life on Earth (2) ~ 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.

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

The Origins of Life on Earth (3) 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.

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.

Formation of Cells Cell membranes First cell membranes may have formed before the beginning of life: Single amino acids can be assembled into long protein-like molecules, which form microspheres when they cool in water. Cell membranes

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

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.

Earth Calendar (SLIDESHOW MODE ONLY)

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

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.

Life in Our Solar System Other planets or their moons are unlikely to have ever provided suitable conditions for life. Most promising candidate: Mars. Claimed traces of microscopic fossils may well be regular mineral formations in the rock. Meteorite ALH84001,0 probably originated on Mars. Possibly some evidence of past life on Mars, but questionable.

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

Interstellar Communications (SLIDESHOW MODE ONLY)

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 certain length that is a product of two prime numbers  Only two ways to arrange them in a rectangle.

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

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 Only certain wavelength ranges are suitable for this search. SETI program is highly controversial because of the uncertain prospects of positive results.

The Drake Equation Factors to consider when calculating the number of technologically advanced civilizations per galaxy: Nc = N* · fp · nLZ · fL · fl · FS 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.

Drake Equation (SLIDESHOW MODE ONLY)

New Terms DNA (deoxyribonucleic acid) amino acid protein enzyme RNA (ribonucleic acid) chromosome gene natural selection mutant Cambrian period Miller experiment primordial soup chemical evolution stromatolite life zone water hole SETI Drake equation

Discussion Questions 1. What would you change in the Arecibo message if humanity lived on Mars instead of Earth? 2. What do you think it would mean if decades of careful searches for radio signals for extraterrestrial intelligence turned up nothing?

Quiz Questions 1. Which element is the physical basis for life chemistry on Earth? a. Hydrogen. b. Helium. c. Oxygen. d. Carbon. e. Silicon.

Quiz Questions 2. On Earth why is carbon-based life more successful than silicon-based life? a. Silicon molecules from rings rather than chains. b. Carbon molecule chains are stronger and more stable. c. Most of the silicon on Earth is locked up in silicate rocks. d. Carbon molecule chains can store, extract, and utilize energy. e. Silicon molecule chains require a higher temperature to be stable.

Quiz Questions 3. How are the instructions for life stored in DNA molecules? a. As a sequence of protons and electrons. b. As Digital Numeric Analogs (DNA). c. As a sequence of base pairs. d. In base ten code. e. In binary code.

Quiz Questions 4. How many different bases (A, C, etc.) make up the base pairs of DNA molecules? a. 2. b. 4. c. 23. d. 26. e. 52.

Quiz Questions 5. When a cell divides, how does a DNA molecule duplicate its stored information for the two new cells? a. The D, N, and A are highlighted, copied, and then pasted into the new molecule. b. The original DNA molecule lines up with the new blank cell nucleus and burns a copy of itself into the new cell. c. The DNA molecule divides along the long dimension of the ladder, splitting the base pairs. Corresponding new bases attach to each half to form two new identical DNA molecules. d. The DNA molecule places its information in temporary storage, in RNA molecules that later reinstall the information into the two new cell nuclei. e. A tightly wound 1.5 meter segment of the DNA molecule containing about 4.5 billion base pairs breaks off and goes into the new cell nucleus, as a duplicate copy of the entire sequence of base pairs.

Quiz Questions 6. How can the instructions stored in DNA molecules be changed? a. By chance mismatching of base pairs during cell division. b. By exposure of reproductive cells to radioactive isotopes. c. By exposure of reproductive cells to cosmic rays. d. By cutting the DNA and adding base pairs. e. All of the above.

Quiz Questions 7. Why must the DNA instructions change for a species to survive? a. Without changes, the DNA instructions will degrade. b. Without variety some species will die of boredom. c. To produce mutants for experimentation. d. To adapt to changes in the environment. e. To create larger and stronger life forms.

Quiz Questions 8. How do the instructions stored in a DNA molecule at the nucleus of a cell get out to where they are needed to conduct the business of life? a. DNA molecules transport the information throughout the cell. b. The instructions are copied and transported by RNA molecules. c. The information is not needed at any location other than the cell nucleus. d. Short strands of DNA sequences break loose and follow outward-flowing currents. e. None of the above.

Quiz Questions 9. What evidence do we have that life on Earth began in the sea? a. The base pairs of DNA molecules’ sequences always begin with base C. b. The oldest fossils are of the remains of ocean creatures. c. About 75% of Earth's surface is covered with water. d. Fish are more primitive than mammals. e. Water is the best common solvent.

Quiz Questions 10. What was the significance of the Miller-Urey experiment? a. It was the first time that humans created life. b. It verified the suspected conditions of primeval Earth. c. It formed new amino acids that had not been found before. d. It formed proteins in simulated conditions of primeval Earth. e. It shows that complex organic molecules can form naturally.

Quiz Questions 11. What is chemical evolution? a. The process of binding molecules together to form progressively more complex molecules that cannot reproduce themselves. b. The process of forming progressively more complex life forms that derive their energy from sources other than sunlight. c. The evolution of life that has occurred since the first cell divided and reproduced itself. d. The development of the periodic table. e. The process that forms stromatolites.

Quiz Questions 12. How may intelligence have begun? a. By species developing multiple escape strategies. b. By developing an external skeleton for protection. c. By developing lungs for respiration. d. By developing the ability to make and use tools. e. By collecting the seeds of wild plants and planting them for later harvest.

Quiz Questions 13. Why do we think that liquid water is necessary for the origin of life? a. Water is the best common solvent. b. Amino acids can easily form in water. c. Amino acids need a medium in which to link together to form proteins. d. Organisms need a medium to transport nutrients and waste. e. All of the above.

Quiz Questions 14. Other than Earth, where in the solar system is the most likely place to find life? a. Earth's moon. b. Mercury. c. Mars. d. Europa. e. Titan.

Quiz Questions 15. If a planet is to remain in the life zone of a G or K main sequence star for 4 to 5 billion years, it must form near _____ of the life zone. a. the outer edge b. the middle c. the inner edge d. Either a or b above. e. Either b or c above.

Quiz Questions 16. Why are upper main sequence stars unlikely sites for intelligent civilizations? a. The short lifetime of such stars allows little time for intelligent life to evolve. b. These stars are too hot for life to evolve on a planet that is 1 AU away. c. Upper main sequence stars are very rare. d. Both a and b above. e. All of the above.

Quiz Questions 17. Why do we suspect that travel between stars is nearly impossible for intelligent life forms? a. The distance between stars in the disk of our galaxy is typically several light years. b. The maximum speed that can ever be approached is the speed of light. c. To accelerate to and from high speeds requires a tremendous amount of fuel. d. Both a and b above. e. All of the above.

Quiz Questions 18. Why do SETI programs only observe at wavelengths between 1 and 30 centimeters? a. The first commercial radio stations transmitted in this range of wavelengths, and we expect that any communicating extraterrestrial that has picked up our early signals will attempt to contact us at the same wavelength. b. Funding for SETI projects is so low that military surplus radar antennae that operate at this range of wavelengths are all that can be afforded. c. We expect that intelligent beings must have feet the size of ours, and thus would communicate at wavelengths around this natural unit of length. d. At wavelengths longer than 30 cm our galaxy emits a lot of interference, and at wavelengths less than 1 cm our atmosphere is opaque. e. The FCC reserves this range of wavelengths for SETI programs.

Quiz Questions 19. Where is the “water hole” that we suspect is a good place to find extraterrestrials? a. Devils Tower. b. Roswell New Mexico. d. At the poles of Mercury. e. Beneath the frozen icy crust of Europa. c. Between 18-cm and 21-cm wavelengths.

Quiz Questions 20. Which variable in the modified Drake equation would be likely to give us the greatest difficulty in estimating an accurate and precise value? a. The number of planets per star that lie in the life zone for longer than 4 billion years. b. The fraction of a star's life during which a technological society survives. c. The fraction of life forms that evolve to intelligence. d. The fraction of suitable planets on which life begins. e. The fraction of stars with planets.

Answers 1. d 2. b 3. c 4. b 5. c 6. e 7. d 8. c 9. b 10. e 11. a 12. a 13. e 14. c 15. a 16. a 17. e 18. d 19. e 20. b