Chapter 18 Life in the Universe

Galaxyrise Over Alien Planet by D. Berry

Cosmic evolution - phases in the history of the universe Particulate Galactic Stellar Planetary Chemical Biological Cultural

Figure 18.1 Arrow of Time

Living organisms React to environment and can often heal themselves when damaged Grow Reproduce Genetic change and evolution to adapt to a changing environment

Assumption of mediocrity Life on earth depends on a few basic molecules Elements in these molecules are common to all stars Laws of science same everywhere Life must have originated elsewhere than on earth

Chemicals on earth before life formed Outgassing in early earth produced hydrogen, nitrogen and carbon compounds Ammonia, methane, carbon dioxide and water formed

Energy input Energy from radioactivity, volcanism, lightning, UV radiation and meteoritic impacts modified chemicals Formed amino acids and nucleotide bases Organic (carbon-based) molecules that are the basis of life

Amino acids and nucleotides Amino acids build proteins, which control metabolism Sequences of nucleotide bases form genes, which are part of DNA molecules DNA controls synthesis of proteins and determines characteristics of living organisms Genes carry hereditary information to the next generation

Figure 18.2 DNA Molecule

Miller-Urey experiment took primordial chemicals, input energy, and generated amino acids A later experiment generated nucleotide bases Did not produce life, but did synthesize biological molecules Demonstrated chemical evolution

Figure 18.3 Miller-Urey Experiment

Figure 18.4 Chemical Evolution

Interstellar origin? Some scientists claim organic material came from interstellar space Experiment exposed icy mixture of water, methanol, ammonia and carbon monoxide to UV radiation Complex organic molecules formed

Figure 18.5 Interstellar Globules

Diversity and culture Simple one-celled life appeared on earth about 3.5 billion years ago More complex one-celled life appeared about 2 billion years ago Multicelluar organisms appeared about 1 billion years ago Insects, reptiles, mammals, etc. Biological evolution led to the strongly favored trait of intelligence Cultural evolution followed

Figure 18.6L Life on Earth

Figure 18.6R Life on Earth

Life as we know it Means carbon-based life originating in liquid water environment Did it happen elsewhere in the solar system? No on Moon or Mercury - no protective atmosphere Not on Venus - atmosphere too dry and hot Jovian planets have no solid surface, Pluto too cold Europa and Titan possibilities Mars a possibility

Figure 18.7 Murchison Meteorite

Alternative biochemistries Carbon is basis of life forms on earth Maybe silicon could be basis of life forms Ammonia could be an alternative instead of water

Intelligent life in our Galaxy Distances in Galaxy too great to detect life with current technology Instead rely on estimating likelihood of intelligent life in Galaxy

Drake Equation Named after Frank Drake, who pioneered a probability type calculation

Number of technological civilizations in galaxy is Rate of star formation X Fraction of stars having planetary systems X Average number of habitable planets in those planetary systems X Fraction of those planets on which life arises X Fraction of those planets on which intelligent life evolves X Fraction of those planets developing technology X Average lifetime of a technological civilization

Figure 18.9 Drake Equation

Rate of star formation Roughy 100 billion stars in Galaxy Galaxy has been around 10 billion years 10 stars per year forming

Fraction of stars having planetary systems Informed guess - fraction of 1 or so

Number of habitable planets per planetary system Determined by distance to star (too hot or cold) Determined by spectral class of star Affected by orbit of star and if star in orbit in a binary star system Depends on position in Galaxy - supernovae damage and gravitational effects of close- encounter stars Estimate 1/10 (1 habitable planet per 10 planetary systems)

Figure Stellar Habitable Zones

Figure Galactic Habitable Zone

Figure Binary-Star Planets

Fraction of habitable planets on which life arises Can make guesstimates from chemical formation Optimistically assign this 1

Fraction of life-bearing planets on which intelligence arises Any hint of intelligence is highly favored by evolution Guess this fraction to be 1

Fraction of planets on which intelligence develops technology Technological societies developed independently at several locations on earth Give this factor a 1

Average lifetime of a technological civilization Modern civilization for only 100 years or so Maybe 1000 years? Multiply all of the factors together and end up with there should be 1000 technological civilizations scattered throughout our Galaxy It is unlikely there is time enough for these civilizations to communicate

Search for Extraterrestrial Intelligence If technological civilizations last 1 million years, there are 1 million in existence Average about 100 light-years apart 200 years for two-way communication In current fastest space ship, trip there and back would take 1 million years

Figure Pioneer-10 Plaque

Radio search Radio waves best for communication - not scattered by dusty interstellar space We would not broadcast to other stars We would passively “listen” toward F, G and K stars near the sun Earth is currently stronger (man-made) radio emitter than the sun

Figure Earth’s Radio Leakage

Most probable radio wavelengths Near 20 cm - H emission is so common H radiates at 21 cm OH radiates near 18 cm They together make water, which our life form is based on Wavelengths between 18 and 21 cm are called a “water hole” Galactic background minimized

Figure Water Hole

SETI Search for Extraterrestrial Intelligence Project Phoenix searched in 1-3 GHz range Nothing resembling intelligence yet detected

Figure Project Phoenix

Consequences of two civilizations meeting Consider the history of earth What happened whenever a more “advanced” civilization met a less “advanced” civilization?