Physics 55: Two Classes About Astrobiology Professor Henry Greenside April 9, 2012

Great Scientific Question: Does Life Exist Outside of Earth? 1.Why life is likely elsewhere in the universe 2.How to search for life 3.Where are the aliens? 4.Travel to the stars? Not likely, not even by probes

What is Life? Start with discussion: How is fire like or not like a living creature? Duke-Maryland game, 2002

Brownian Motion? Belousov-Zhabotinsky Reaction?

Is a star living or not?

Characteristics of Life Requires nutrients (atoms, molecules) Requires energy (chemicals, sunlight) Requires water (???) Produces waste products Can reproduce Can respond to environmental changes Can evolve. Biologists and computer scientists have especially made progress with artificial life: evolving reproducing mutating computer codes “living” in memory of computer, viruses and ecologies spontaneously appear.

Key Slide: Key Slide: Why life is likely elsewhere: Same ingredients (atoms) everywhere Life started rapidly on Earth Laboratory experiments suggest spontaneous formation of biological molecule, comets support this. Earth life can survive extreme environments Earth does not seem special stars in a galaxy, galaxies Universe is ancient, lots of time for life to have started elsewhere

Relative Abundance of the Elements: Why H-C-N-O Based Life is Likely

Relative Abundance of the Elements On Earth's Surface

What Can You Make With H, C, O, N, P? Polysaccharides (sugars) Nucleic acids, DNA, RNA Amino acids and proteins

“Inner Life of a Cell” Movie

Why is Life Likely: Life Appeared on Earth as Soon as Life Was Possible! Earth “born” 4.5 billion years ago Heavy bombardment sterilized Earth 4 billion years ago, life appeared shortly after, 3.85 billion years ago!

Life Appeared on Earth as Soon as Life Was Possible Australian stramatolites Some are 3.5 billion years old! Evidence of age: - Radiodating of fossils - Increased ratio of carbon-12 isotope to carbon-13 isotope in rocks indicates presence of life.

Two Possibilities About Rapid Appearance of Life Life can arise easily and spontaneously. Panspermia: life arrived on Earth from some other location, perhaps Mars or comets or from outside solar system, but that just shifts question of origin to another location (and Mars was also bombarded).

Possible Example of Panspermia: Rocks from Mars on Earth Martian meteorite that crashed in Tissint, Morocco, July 2011

Why Is Life Likely: Suggestive Chemistry Experiments Miller-Urey Experiment, 1953 Amino acids, nucleic acids found

Why Life Might Be Likely: Extreme Forms of Life on Earth Bacterial mats Yellowstone Extremophiles: acidophile, anaerobe, lithoautotroph, oligotroph, cryptoendolith, xerophile, piezophile, radioresistant, etc black smoker Pitch Lake, Trinidad

But “Advanced” Life Forms Might be Rare: Complexity Transitions Are Difficult Single cell creatures persisted in ocean for first billion years, then multi-cell creatures appeared. Dinosaurs existed for about 130,000,000 years but, to our knowledge, never developed technological civilization. Humans around 3-4 millions years, but technology arose only in last 100,000 years!

Looking For Life in the Solar System Key assumption: liquid water is essential!

Stellar Habitable Zone: Where Liquid Water Might Occur

Habitable Zone = Goldilocks Zone

Only One Other Planetary Candidate : Mars Mars Traces of water found No signs of life

Water on Mars

Gravitational Tides Might Allow Life Outside the Goldilock's Zone This complicates search for life around other stars, remote moons hard to study!

Life in Europa's Icy Ocean? 4 th largest moon, about same size as Earth's Moon Gravitational tides warm ice, create salty ocean possibly ~60 miles deep, versus ~7 miles for Earth.

Life Around Other Stars Galactic Habitable (“Goldilocks”) Zone

Many Exoplanets Now Known: ~760 Planets In 600 Planetary Systems!

Not Know Yet if Earth-Like Planets Are Rare

Searching For Exolife Terrestrial Planet Finder (TPF) Block light from star, observe planets directly!

Look Spectroscopically for Biosignatures Water vapor Excess oxygen Excess methane (found on Mars!) Complex molecules like chlorophyll

Infrared Spectroscopic Signature of Life Based onSolar Sysem

SETI: The Search for Extraterrestrial Intelligence

Drake Equation: How To Estimate Number Of Intelligent Civilizations in Galaxy

How To Search For Intelligent Life Arecibo Radio Telescope in Puerto Rico

How to Communicate?

Interstellar Travel: Implications for SETI

How to Communicate: Lincos? ?

How to Communicate?

Where is Everybody? Galaxy has radius of 50,000 light-years so time to colonize less than ten million years with v ~ 0.1c, ok grant 100,000,000 years, still much less than 1/10 age of universe!

Travel to the Stars by Fusion

Implications We are the only technological civilization in the Milky Way. Civilizations are common but it's hard to colonize space (interstellar travel is hard! There is a galactic civilization but it is hiding from us.

Why is the search for life worthwhile?

Class Discussion: are snowflakes alive? Prof Kenneth Libbrecht. Caltech