Lecture 20

Tuesday, July 2nd 2002. Reading: Cosmic Perspective Chapters 13 and S5

Astrobiology

What is Life?

something that ingests, metabolizes, and excretes: Cars, Candle Flames

Departure from Thermodynamic Equilibrium: Lightening, Ozone Layer

Biochemical definitions -- nucleic acids, proteins, etc.: chauvinistic

Carl Sagan's Favorite: "Life is a system capable of reproduction, mutation, and reproduction of its mutations": Impractical to apply when searching other worlds.

Life on Earth

• primordial disk of dusty gas surrounding the protosun.
• clumps of matter stick together, gain more mass and gravitationally attract more matter: planetesimals
• Earth slowly builds up from planetesimals; molten, differentiates
• Primordial Earth collides with Mars-sized planetesimal, creates the Moon
• Earth's surface bombarded by impact from debris, surface is molten. lasts until about 3.9 billion years ago.
• Surface cools, atmosphere and oceans build up.

3.8 billion years ago Oldest Carbon evidence for life

3.5 billion years ago Oldest fossils of blue-green algae and stromatelite bacteria

Biology

All life on Earth shares the same biochemistry. Carbon compounds (Organic): long chains of Carbon with Hydrogen, Oxygen, and Nitrogen attached. Same substances that Earth's surface and atmosphere are mostly made of. Primary structural unit of chain molecules are amino acids. H,C,N,O,S all arranged into specific patterns. Amino acids form more complex chains of molecules called Proteins. Proteins give structure to living cells and provide energy to the cell. DNA Deoxyribose Nucleic Acid. A very, very long molecule shaped like a ladder with rungs twisted into a double helix. The "rungs" are called base pairs and are a kind of molecular code. The code contains information on the construction of amino acids and proteins. The molecule can split in half and make a copy of itself. The molecule is "alive"! That all living things on Earth share the same biochemistry (all organisms that use DNA use the same code book of base pairs) speaks to a common origin of life on Earth. A singular spark of life ~ 3.8 billion years ago leading to an unbroken chain of life. An enormous diversity of lifeforms evolve from that single spark of life! Evolution of life is a process that leads to the diversification of life. It is not a drive toward greater complexity, it is only a drive toward greater diversity.

Lifeforms change with time due to random mutations that arise from a variety of sources: High Energy E&M radiation, Particle Radioactivity, Cosmic Rays, Chemical Reactions, and random errors in the copying of DNA (caused by the Uncertainty principle). Most mutations are lethal to the organism (i.e. cancer), but some are beneficial. Over long periods of time species can change; those that are best suited for survival in their current environment thrive over others. In times of changing environments those species capable of rapid adaptation survive over others. This process is called Natural Selection. Despite the increase in complexity of some organisms over time, the overwhelming majority of life on Earth is still in the form of simple one-celled organisms: bacteria. Nature likes to stick with what works.

Origin of Life

Miller-Urey Experiment: place compounds common in the early Earth's atmosphere in a container (nitrogen, water, carbon dioxide, etc. Note: no oxygen) and put energy into it with electrical sparks (to simulate lightening). Forms an organic sludge containing amino acids!

NOTE: Life is found practically everywhere on Earth (deep sea ocean vents, dry desserts of Antarctica, etc.). Commonality: Require liquid water. Liquid water is the best solution for organic chemistry. Life is not found to exist on Earth where there is no liquid water present.

Life on other worlds in our Solar System

Venus: Once had liquid oceans (probably water) but runaway greenhouse effect evaporated all water and cooked the planet. Definitely no life there now. fossil records from earlier epochs before the runaway greenhouse?

Mars: Liquid water on surface in past, thicker atmosphere, warmer temperatures. A world very much like Earth. Could life have started there and been extinguished? Could there still be some life hanging on? Fossils of past life?

Europa: Liquid oceans of water beneath a frozen ice crust. Heat from interior geologic activity (tidal friction). Life in those oceans?

Titan: Organic Molecules in atmosphere. Liquid Methane on surface (similar solvent properties as water). Heat from Saturn and a greenhouse effect. Life with different biochemistry?

Panspermia

Or what if life originated on some distant world in another planetary system? Simple bacteria spores might survive the eons in the vacuum and then settle down on Earth and take root. Might we be aliens on our own world?

Other Planetary Systems

Planetary scientists fully expect that in the next several decades planets like Earth will be found around many stars.

Not all stars are expected to have planets. The most massive stars (O and B stars) live so fast and die so young that planets barely have a chance to form around them before the stars blow up as supernovas.

Double stars systems also would tend to not have planets as the gravitational effects of the 2 stars would tend to eject planets from the system.

That still leaves ~200 billion stars in the Milky Way galaxy alone that could have planets.

Some of those planets must lie in a habitablity zone and have liquid water present. Given that it seems so easy to make the building blocks of life, how easy is it for life to start going and survive? We really have no clue.

Intelligent Life in the Universe

UFOs, Alien Abductions, and Ancient Astronauts

They would have become technologically advanced and likely become space-faring. They could build space ships that held whole generations of space explorers, and travel at relativistic speeds to make the journies between stars not so long for them (although those of us at lower relative speeds observe their journies to take aeons).

If that were the case, then where are they? Should they not have come this way already?

• Perhaps we are the first in our galaxy to reach this level. It's highly unlikely, but someone has to be first.
• Our Sun is an unremarkable star in a galaxy of 100s of billions of stars just like it. It's possible that space explorers just haven't made it out this way yet.
• Or maybe they have...

Some suggest that the phenomenon of UFOs may be extraterrestrial visitors coming to our world. Or that strange historical accounts or grand technological accomplishments of humans in the past are evidence of visits from extraterrestrials. There are those claiming to have been abducted by aliens and have terrible medical experiments performed on them. This all falls into the realm of pseudoscience. There has never been any concrete evidence of extraterrestrials having anything to do with UFOs. UFOs could be so very many things. Why assume automatically that there is an otherworldly explanation? But those who want to believe will do so even despite evidence to the contrary. "Extraordinary Claims Require Extraordinary Evidence" - Carl Sagan. To this date there has never been any such evidence of an extraterrestrial connection to UFOs. The fact the the majority of the human population lives in cities now and has very little familiarity with the sky goes a long way to explaining most UFO sightings. How many of you have ever seen ball lightening, a stealth fighter or bomber in flight, swamp gas, Insect swarms flying through electric fields? All of these things are real and in the sky. Most people have no idea what they are looking at when they see them. Alien abduction stories bare a resemblance to post traumatic stress disorder. People experience something horrible in their lives and their brains suppress the memories. The memories come back in dream-like states where the mind is highly susceptible to confabulation and fantasy. There is such a large main-stream consciousness of the typical alien abduction story (thanks to TV and Movies) that it's no wonder that "abductees" tell stories that are remarkably similar to one another. Their stories often involve elements that sound like a condition called sleep paralysis. When you are in REM sleep and are dreaming your body paralyzes the major voluntary muscles so that you cannot injure yourself while you dream. Sometimes we can fall into dream states before we have become completely unconscious. Our bodies become paralyzed and we can even dream in the state, but yet we are also partially awake. It's called waking dreams it's a real and studied phenomenon. Its happened to me on more than one occasion. Pretty spooky.

SETI

All of this skepticism over UFOs and alien abductions does not hinder astronomers from thinking that intelligent life could exist in the Universe. In fact most think that it can. A project now known as the Search for Extraterrestrial Intelligence (SETI) was begun back in the 1960s to search the heavens for radio signals that reach us from some other technological civilization in the Cosmos. NASA funded the program in the 1980s but Congress pulled the plug on the project in the early 1990s. Seems the Congress felt that tax payer money shouldn't be spent on a project that they thought was more like science fiction than science. They sought private funding for their research and have now established the SETI institute. The idea is that radio communication is simple, inexpensive, and the Universe is relatively quiet at most radio frequencies. So the first choice for an alien civilization to attempt to make contact would be via radio communication. So the SETI project scans billions of radio frequencies everyday from across the sky searching for some signal that would stand out as being unequivocally unnatural. They do this with supercomputers and a network of radio telescopes around the world. To date there have been no detections. But that doesn't mean we will never find anyone out there.

Radio waves travel at the speed of light (they are light afterall). Earth has been producing radio waves for our own communication purposes for about 100 years now. Those signals escape from Earth and spread away from Earth in a sphere. So the Earth is surrounded by a sphere of about 100 light-years in radius of radio waves. Any aliens farther away than 100 light-years could not possibly know about us yet. Likewise if they have only started producing radio signals in the last 100 years we will not know about them for some time. If a signal were ever detected it would not likely be easy to decode, unless it were specifically designed to be decoded by any species with sufficient understanding of simple mathematics. Plus 2-way communication would be excruciatingly slow. We could exchange information over periods of decades and centuries. But it would not be like a real conversation.

The Drake Equation

Frank Drake of UC Santa Cruz developed a simple equation which is just the multiplication of all the probabilities involved to make an estimate of this very thing.

N_c is the number of civilizations in our Galaxy with whom we can communicate.

R_s: Rate of star formation in the Galaxy
     ~ 40 stars/year

f_p: Fraction of Stars that have planets
      ~ 0.5

n : Average number of planets that are habitable in a planetary system
      1 - 5

f_l: Fraction of habitable planets on which life arises
      0.1 - 1.0

f_i: Fraction of planets with life on which intelligence arises
      0.01 - 1.0

f_c: Fraction of intelligent civilizations that communicate via radio waves
      0.1 - 1.0

L : Average lifetime of a civilization
      100 - 10⁹ years

• If you make optimistic estimates you find that the Universe is teaming with life. Billions of civilization in each galaxy.
• If you make pessimistic estimates you find that we are alone in the Universe.

Both ideas are huge!!

The Universe did at least make us. So the probability of intelligent life in the Universe is 100%.

"We are starstuff contemplating the stars" - Carl Sagan

Evaluations...

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