Life in the Universe. 2© Sierra College Astronomy Department Life in the Universe Life on Earth Is there life beyond Earth? Great for science fiction

Life in the Universe

2© Sierra College Astronomy Department


Life on Earth Is there life beyond Earth?

Great for science fiction plots, but at present there is no undeniable evidence that aliens have been here.

Many astronomers of the past have suggested that life existed elsewhere. Kepler – thought there were inhabitants on the Moon. Herschel – claimed life existed on nearly all the

planets. Lowell – thought he saw canals on Mars.

Before searching for life elsewhere in the universe we must first look at how life arose on the Earth.



Life on Earth

Three recent developments have led to the idea that life may not be so uncommon beyond Earth. Life arose quite early in Earth’s history. Laboratory experiments show that the chemical make-

up of the young Earth could readily combine to form organic molecules, suggesting that life formed from naturally occurring chemistry.

We have discovered microorganisms which live in extreme Earth conditions which may be similar to those found on other worlds



Life on Earth

When did life arise on the Earth? After the Earth formed 4.5 billion years ago, life

had little chance to survive. First several hundred million years was era of heavy

bombardment. These bombardments could vaporize oceans killing

any life that might have formed. Studies of the craters on the Moon suggest that the

heavy bombardment ended 4.2 to 3.9 billion years ago.

Remarkably life may have been thriving 3.85 billion years ago – consequently it appears lifer arose in a geological blink-of-an-eye once conditions became hospitable.



Life on Earth

Fossils and Geologic Time Scale The key to understanding ancient life is to look for fossils,

relics of organisms which lived and died long ago. These fossils can be found under layers of sediments

which are carried by rivers or lie under ocean floors. Grand Canyon shows billions of years of Earth’s history. Relative ages of rocks and fossils are easy to determine: each

deeper layer formed earlier. Radiometric dating confirms this and estimates the absolute ages

of the material. Based of the layering of the rocks and fossils Earth’s

history can be divided into several distinct intervals or geological time scales.



Life on Earth

Fossil Evidence of Early Life Since the Earth is so geologically active, it is difficult

to find rocks which are over 3 billion years old. The evidence of the existence of life 3.5 billion years

ago is suggestive and subtle since microscopic fossils are difficult to detect. What we see today is what was left behind by ancient

bacteria in rocks called stromatolites. Also, the ratio of carbon-13 to carbon-12 in rocks

containing fossils has been found to be lower relative to rocks that do not contain fossils.

Some rocks dating back to 3.85 billion years ago have the lower ratio and this is suggestive that ancient life formed at this distant time in the past.



Life on Earth

How did life progress on Earth? Theory of Evolution

In the 19th century, most biologists agreed that species change through time.

In 1859, Charles Darwin explained how life might undergo those changes based on two observations:

Overproduction and struggle for survival. Individual variation.

Conclusion: unequal reproductive success what is often termed natural selection or survival of the fittest.



Life on Earth

The Mechanism of Life DNA (deoxyribonucleic acid) is the genetic

material of all life on Earth. It consists of 4 chemical bases: adenine (A), thymine

(T), guanine (G), cytosine (C) which pair up in two long strands and wind together in a double helix.

DNA is self-replicating, which is the key to heredity. Any change (small or large) to this sequence from

imperfect duplication is called a mutation. Most mutations are lethal and kill the cell with the mutation. But some are beneficial and these can be passed to offspring.



Life on EarthThe First Living Organism DNA has the same basic chemical nature in all life. Also, all organisms build proteins from the same set of

amino acids. This suggests that all life had a common ancestor which

arose some 3.85 billion years ago. The present day black smokers which thrive deep underwater at

high temperatures may resemble this early form of life. High temperatures may promote faster and more diverse

chemistry which allows life to form quickly. Looking at DNA from all life, biologists have composed a

“tree of life”, which suggests how the DNA changed to other types of life. Consists of 3 main domains: bacteria, archaea, and eukarya.



Life on Earth

The Origin of The First Living Organism Where did this first life come from? Some experiments done in the 1950s (and since) showed

a mixture of early-Earth organic molecules plus lightning can produce all the major molecules of life including amino acids and DNA bases.

Strands of RNA (ribonucleic acid) which resemble single strands of DNA have been reproduced in the laboratory. Many biologists presume that RNA came first followed by DNA. Microscopic enclosed membranes can form which may have

surrounded self-replicating RNA on the early Earth.



Life on Earth

Could life have migrated from elsewhere? The idea that life started elsewhere and

then came to the Earth (via meteor impacts) is called panspermia.

While the prospect of life traveling and surviving through space seems difficult, we have evidence of organic molecules in meteorites and tests that show microbes can survive space for years.



Life on Earth

A Brief History of Life on Earth As Earth formed, chemical reactions form the

first organic molecules. After the heavy bombardment ended, the

common ancestor of life formed. Life rapidly grew and diversified, but remained

single cell organisms for 1 billion years. The land was still inhospitable until the ozone

layer formed – this required atmospheric oxygen



Life on Earth Nearly all of the original oxygen was released via

photosynthesis from single-celled cyanobacteria some 3.5 billion years ago.

For more than 1 billion years, this oxygen reacted with surface rocks and little stayed in the atmosphere.

Eventually, some 2 billion years ago, the oxygen began to accumulate, but would not be “breathable” until just a few hundred million years ago. This allowed new species (e.g. plants and animals) to form. Many other species did not need oxygen or even found oxygen to

be toxic.



Life on Earth About 540 million years ago, tiny plants and

animal organisms changed dramatically in about 40 million years and formed into all the basic plants (phyla) that we find on Earth today.

The dramatic change in the diversity of life is called the Cambrian explosion.

Early dinosaurs and mammals arose at the same time (225 to 250 million years ago). Dinosaurs died off 65 million years ago (by a meteor

impact?) and the mammals then rose to dominance. The earliest humans formed only a few million

years ago (after 99.9% of Earth’s history).



Life on EarthWhat is necessary for Life to exist? While animals need moderate temperatures and

abundant oxygen, simpler life can live under much more extreme conditions and locations (extremophiles). Underground, high and low temperatures.

There seem to be 3 basic requirements. A source of nutrients Energy to fuel the activities of life Liquid water (the biggest constraint)



Life in the Solar System To look for life elsewhere, we need to

search for places where the basic necessities of life exist – the habitable worlds. This eliminates most of worlds in our solar

system. Moon and Mercury are barren and dry. Venus too hot for liquid water. Jovian planets are gaseous. This leaves Mars and a few of the moons

orbiting the Jovian planets, notably Europa.



Life in the Solar SystemMars Percival Lowell thought he saw canals on

Mars, but we are quite confident now that there are no civilizations on Mars.

Nevertheless, we have good evidence the liquid water once flowed on the Martian surface.

Today it contains subsurface ice which could be heated to form areas of liquid water underground.



Life in the Solar SystemMissions to Mars, looking for life The Viking missions took soil samples and

looked for chemical changes that could be attributed to biological processes. 3 experiments suggested that life may be

present, but also ordinary chemical reactions could have caused the same results.

A fourth experiment found little organic material, the opposite of what one would expect if life were present.



Life in the Solar System The Mars Phoenix mission detected water under

the surface, though soil was basic and may have trouble harboring life (perchrolates)

Pathfinder, Spirit and Opportunity studied the Martian conditions to see if life might have existed.

The Mars Express orbiter detected methane gas. Methane should disappear within a few centuries due

to chemical reactions. So, something is supplying Mars with methane. It could come from comet impacts, volcanoes, or life.

Volcanism seems to be the most likely candidate.



Life in the Solar SystemMartian Meteorites One meteorite which landed in Antarctica 13,000

years ago and found in 1984 was clearly of Martian origin.

Inside the meteorite were complex organic materials and structures which looked like nanobacteria , very small bacteria which have been discovered on Earth.

These structures can also be made by chemical and geological means.

Contamination from being on the Earth may also explain the presence of organic materials.



Life in the Solar SystemLife on Europa Europa has enough tidal heating to possibly form

a subsurface ocean underneath its icy crust. Life there could form like the “black smokers” on

Earth. Larger life forms could exist in the vast oceans,

but energy sources are limited and this would tend to limit the size of any life there.



Life in the Solar SystemLife on Ganymede, Callisto and Titan Ganymede and Callisto might have

subsurface oceans, but their internal heat is small and liquid water would not be terribly abundant.

Titan has no native liquid water, but an abundance of organic materials. Could life evolve from the lakes of methane? Water might be brought in from comets, but

this would eventually freeze.



Life Around Other StarsBeyond the Solar System Where in the Galaxy might we find life? Since technology might allow us to obtain

surface pictures or spectra, we restrict ourselves to considering extrasolar planets with habitable surfaces.

So far all detected extrasolar planets (except maybe one or two) are gaseous giants and are unlikely to have surface life. However, they may be surrounded by moons

which may support life.



Life Around Other StarsConstraints on Star Systems A star must be stable and live long enough

to allow a planet to develop life. Stars greater than a few solar masses are

ruled out (but this is only about 1% of all stars).

A star must allow stable planet orbits. Binary and multiple star systems are much

less likely to have this – about 50% of all star systems.



Life Around Other Stars A third constraint is a planet must form in

the habitable zone. This is a region where a terrestrial type planet

would have the right surface temperature for liquid water to exist.

Stars less massive than the Sun have smaller zones.

A star like the Sun (or more massive) would have the largest zone.

Even if we restricted our search to Sun-like stars, we would still have to consider billions of stars in our Galaxy.



Life Around Other StarsFinding Habitable Planets Two upcoming missions may be able spot Earth

sized planets. Kepler will look for transits of planets across other

stars. The Space Interferometer Mission (SIM) may be

able to detect Earth sized planets. A decade or so from now, the Terrestrial Planet

Finder (TPF) or something like it may be able to image extrasolar planets. Infrared spectra from future telescopes can look for

signatures of life



Life Around Other StarsRare Earth? Some feel that an Earth type planet (with its

complex type of life) is rare: Galactic constraints

Too close to the galaxy’s center and the rate of supernovae are too great.

Too far from the center and “metal” content is too low. This leaves about 10% of the galaxy’s disk that might be

habitable. A stellar system needs a Jupiter-like planet to sweep-

out and deflect meteors that might wipe out life on Earth.

Climate stability Plate tectonics and the carbon dioxide cycle. Earth’s large Moon keeps axial tilt relatively stable.



Life Around Other StarsRare Earth? Counterarguments to the Rare Earth

Hypothesis The above conditions may not affect the

creation and advancement of complex life as much as we think.

There may be other overlooked conditions and processes that could assist the creation and advancement of complex life.



The Search for Extraterrestrial Intelligence

What About “Intelligent” Life beyond the Solar System? SETI (Search for Extraterrestrial Intelligence) is trying to

find signs of alien communication How many civilizations out there?

The (modified) Drake Equation suggests the number of civilizations we might be able to contact:

# of civilizations = NHP × flife × fciv × fnow

where NHP is the number of habitable worlds

flife is the fraction of these world which actually have life

fciv is the fraction of these worlds which have interstellar communications

fnow is the fraction of these worlds which have a civilization at the present time

It is hard to know exactly what any of these numbers are at the present time.



Interstellar Travel If one is restricted to going no faster than

the speed of light, then interstellar travel will be difficult.

In any event, vast new energy sources must be used to propel a ship. Hydrogen scoopers. Nuclear bombs or nuclear power Matter-antimater.



Interstellar Travel If there is advanced alien life out there, why

haven’t we seen them (Fermi Paradox)? We are alone and there is no other advanced life out

there. Civilizations are common, but no one has colonized

the galaxy because Technology prevents a widespread travel. The desire to explore is unusual. Civilizations destroy themselves before they can colonize the

stars. There is a galactic civilization, but it has not revealed

itself!!!

THE END

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Life in the Universe. 2© Sierra College Astronomy Department Life in the Universe Life on Earth Is there life beyond Earth? Great for science fiction