Lecture 22: Astrobiology

Reading Assignment: Arny: Essay 3, Cosmos: Chapters 2, 11, 12, and 13

What is Life?

"I'll know it when I see it" <--- Not good enough

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.

History of the Universe

t = 0: Big Bang

t = 10-35 seconds: Inflation

t = 10-6 seconds: protons and antiprotons form

t = 10 seconds: electrons and positrons "freeze out" and annihilate

t = 10 minutes: Nucleosynthesis of Helium and trace light elements

t = 1 million years: First neutral atoms form and Universe becomes transparent to radiation (origin of the CMB).

t = 1 billion years: protogalaxies form; first generation of stars produce heavy elements.

t = 2 - 10 billion years quasars; galaxies mergers; clusters of galaxies

t = 10 billion years: Solar System forms from cloud of dust and gas

Life on Earth

Early History of Earth:

3.8 billion years ago Oldest Carbon evidence for life

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

***Geological Timeline*****

Origin of Life

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!

Organic molecules are very common in the Cosmos. Find them all the time in interstellar clouds. Must have been common in the early Earth atmosphere.

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

We've mentioned that there are other worlds in the Solar System that either at one time or another have had (or still have) liquid water on their surfaces. Given the abundance of organic matter on these worlds might life have once existed on these worlds? Might it still exist?

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?


Idea that life could have originated on either Mars or Earth, and meteor impacts sends rocks containing life into space that later crash land on the other planet. Cross contamination? Could we be Martians? Or could there be life on Mars that originated on Earth?

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

There are now upwards of 40 other known planets orbiting other stars. These planets are all being found indirectly by their gravitational influences on their parent star. Since the search for such planets is very young only the most massive planets will be found first (they have the strongest gravitational effect on their parent star). Also the first systems to be found are likely to have the big planets closer to their star than the Jovian planets are to our own Sun (because they will then have shorter orbital periods and stronger gravitational tugs on their parent stars).

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

Life has the possibility to be abundant in the Universe. In all of those living worlds, how often does intelligence arise?

UFOs, Alien Abductions, and Ancient Astronauts

If it turns out to be very possible for intelligent life to exist then, certainly there must have been many intelligent civilizations that would have evolved billions of years before us.

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 no 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?

Carl Sagan once suggested that perhaps we are the first in our galaxy to reach this level. It's highly unlikely, but someone has to be first. He also pointed out that 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.

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? The planet Venus is mistaken for a UFO all the time because it is very bright, is often viewed low to the horizon and therefore experiences atmospheric scintillation which makes its color change rapidly. Have you ever seen a stealth fighter or bomber in flight? What about swamp gas? Insect swarms flying through electric fields? All of these things are in the sky and 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. What's more 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.


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.

That didn't stop the true believers. 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

Can we estimate, based on the things we have learned in this class, the number of intelligent civilizations that we could contact via radio communication?

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.

Nc = Rs · fp · n · fl · fi · fc · L

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

Rs: Rate of star formation in the Galaxy

fp: Fraction of Stars that have planets

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

fl: Fraction of habitable planets on which life arises

fi: Fraction of planets with life on which intelligence arises

fc: Fraction of intelligent civilizations that communicate via radio waves

L : Average lifetime of a civilization

Both ideas are huge!!

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

"These are some of the things Hydrogen atoms do, given 15 billion years of evolution" - Carl Sagan

"We are, in the most profound sense, children of the Cosmos" - Carl Sagan

"We are starstuff contemplating the stars" - Carl Sagan

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