Life in the Universe - sceweb.uhcl.edusceweb.uhcl.edu/blanford/23Life.pdf · Cosmic Evolution If we...

Life in the Universe

Life and Its Origins

The Drake Equation

SETI

Cosmic Evolution

If we are going to be looking for life

elsewhere in the universe, we need to

define what we mean by “life.”

It turns out not to be so easy, particularly if

we want to allow for types of life that do

not appear on Earth!

Cosmic Evolution

These are some generally agreed-upon

characteristics that any life-form should

have:

Ability to react to the environment

Ability to grow by taking in nourishment

and processing it into energy

Ability to reproduce, with offspring having

some characteristics of the parent

Ability to evolve

Cosmic Evolution

on Earth

We have very little information about the

first billion years of Earth’s existence; Earth

was simply too active at that time.

It is believed that there were many

volcanoes, and an atmosphere of hydrogen,

nitrogen, and carbon compounds.

As Earth cooled, methane, ammonia, carbon

dioxide, and water formed.

Cosmic Evolution

on Earth

Earth was subject to volcanoes, lightning,

radioactivity, ultraviolet radiation, and

meteoroid impacts.

Over a billion years or so, amino acids and

nucleotide bases formed. The process by

which this could have happened has been

re-created in the laboratory.

Cosmic Evolution on Earth

This is a schematic

of the Urey–Miller

experiment, first

done in the 1930s,

that demonstrated

the formation of

amino acids from the

gases present in the

early Earth’s

atmosphere, excited

by lightning.

Cosmic Evolution on Earth The Urey-Miller experiment shows that the basic building blocks of

life can form naturally. Amino acids and other organic compounds

naturally tend to link up to form more complex structures.

Early oceans on Earth were probably filled with a rich

mixture of organic compounds: the primordial soup

Chemical evolution leads to the formation and survival of

the most stable of the more complex compounds.


This image shows protein-like

droplets created from clusters of

billions of amino acid molecules.

These droplets can grow, and can

split into smaller droplets.

On the left are fossilized remains

of single-celled creatures found in

2-billion-year-old sediments.

On the right are living algae. Both

resemble the drops in the top

image.


The Murchison

meteorite

contains 12

different amino

acids found in

Earthly life,

although some of

them are slightly

different in form.

It is also possible that the source of complex organic

molecules could be from outside Earth carried here on

meteorites or comets.

The Physical Basis of Life All life forms on Earth, from viruses to complex mammals

(including humans) are based on carbon chemistry.

Carbon-based DNA and RNA molecule strands are the basic

carriers of genetic information in all life forms on Earth.

The

tobacco

mosaic

virus

contains a

single

strand of

RNA, about

0.1 mm long

This

complex

mammal

contains

about

30 AU of

DNA.

Information Storage

and Duplication

All information guiding all

processes of life are stored in

long spiral molecules of DNA (=

deoxyribonucleic acid)

Basic building blocks are four

amino acids: adenine (A), cytosine

(C), guanine (G), and thymine (T)

Information is encoded in the

order in which those amino acids

are integrated in the DNA

molecule.

N.B. A links only with T and G links

only with C

Processes of Life in the Cell

Information stored

in the DNA in the

nucleus is copied

over to RNA

(ribonucleic acid)

strands, which act

as messengers to

govern the

chemical processes

in the cell.

Duplication and Division

In the course of cell division, the

DNA strands in the nucleus

(chromosomes) are duplicated by

splitting the double-helix strand up

the middle and replacing the open

bonds with the corresponding

amino acids.

The process must be sufficiently

accurate, but also capable of

occasional minor mistakes to

allow for evolution.


Simple one-celled creatures, such as algae,

appeared on Earth about 3.5 billion years

ago

More complex one-celled creatures, such

as the amoeba, appeared about 2 billion

years ago

Multi-cellular organisms began to appear

about 1 billion years ago

The entirety of human civilization has been

created in the last 10,000 years

Three Questions About the

Evolution of Life

1) Could life originate on another world if conditions

were suitable?

Urey-Miller experiment et al. indicate: probably

yes.

2) Will life always evolve toward intelligence?

If intelligence favors one species over another:

probably yes.

3) How common are suitable conditions for the

beginning of life?

Investigate conditions on other planets and

statistics of stars in our Milky Way

Intelligent Life in the Galaxy

The Drake

equation,

illustrated here

in a cartoon, is a

series of

estimates of

factors that must

be present for a

long-lasting

technological

civilization to

arise.

The Drake Equation (one form)

Nic = R*PpPeNePlPiLic

Nic Number of intelligent civilizations

R* Rate of star formation

Pp Probability that the star has planets

Pe Probability that the star will last long enough to form life

Ne Number of planets of suitable temperature

Pl Probability that a planet will develop life

Pi Probability that intelligent life will develop

Lic Lifetime of the intelligent civilization

The Drake Equation (another form)

Factors to consider when calculating the number of technologically

advanced civilizations per galaxy:

Most of the factors are highly uncertain.

Possible results range from 1 communicative civilization within a few

dozen light years to us being the only communicative civilization in the

Milky Way.

Nc = N* · fp · nLZ · fL · fl · FS


N*: We have a very good estimate of the

number of stars in the Milky Way: 2 x 1011

fP: Fraction of stars having planetary systems:

quite a few; planetary systems like our own

have not been detected yet, but we would

not expect to be able to detect them using

current methods: 0.01 - 0.5


Number of habitable planets per planetary system:

probably significant only around A, F, G, and K type stars.

Smaller stars have too-small habitable zones, and larger

stars have too-short lifetimes.


In addition, there

are galactic

habitable zones

– there must not

be too much

radiation, or too

few heavy

elements.


Finally, it is very

unlikely that a planet in

a binary system would

have a stable orbit

unless it is extremely

close to one star, or

very far away from

both.

Considering stellar

type, galactic habitable

zones and binary stars,

we can estimate nLZ = 0.01 - 1.


fL: Fraction of habitable planets on which

life actually arises:

Experiments suggest that this may be

quite likely; on the other hand, it might

be extremely improbable!

We can estimate fL = 0.01 - 1.


fI: Fraction of life-bearing planets where

intelligence (abstract reasoning) arises:

Here we have essentially no facts, just

speculation and opinion.

We can estimate fI = 0.01 - 1.

I personally believe that even the lower

estimate is too optimistic (on Earth there is

only 1 out of millions of different life forms).


FS: Fraction of planet’s lifetime during which

where life develops and uses technology:

Again, we have no facts, but it does seem

reasonable to assume that intelligent life

will develop technology sooner or later.

We can estimate FS = 10-8 – 10-4.


Nc = N* · fp · nLZ · fL · fl · FS

Pessimistic: Nc = 2 x 1011 · 0.01 · 0.01 · 0.01 · 0.01 · 10-8

= 2 x 10-5

The number of communicative civilizations in

a galaxy like the Milky Way

Optimistic: Nc = 2 x 1011 · 0.5 · 1 · 1 · 1 · 10-4

= 10 x 106

The Search for Extraterrestrial

Intelligence (SETI)

If the average lifetime of a technological

civilization is one million years, optimistically

there should be a million such civilizations in

our Galaxy, spaced about 30 pc, or 100 ly, apart

on average.

This means that any two-way communication

will take about 200 years (if there is in fact a

technological civilization 100 light-years away

from us).


Intelligence (SETI) We are communicating – although not deliberately –

through radio waves emitted by broadcast stations.

These have a

24-hour

pattern, as

different

broadcast

areas rotate

into view.


Intelligence (SETI) If we were to deliberately broadcast signals that we wished

to be found, what would be a good frequency?

There is a feature

called the “water

hole” around the

radio frequencies of

hydrogen and the

hydroxyl molecule.

The background is

minimal there, and

it is where some

have been focusing

many of their

searches.

The Arecibo Message

At the rededication of

Arecibo Radio

Observatory, blocks

of 1679 pulses were

emitted, which can be

arranged in only two

ways: 23 rows of 73 or

73 rows of 23.

Resulting 23x73 grid

contained basic

information about our

human society.


Intelligence (SETI) In addition to sending messages to possible extraterrestrial

civilizations, there are also programs to listen for intelligent

messages from space: SETI.

Only certain

wavelength ranges

are suitable for this

search

SETI program is

highly controversial

because of the

uncertain prospects

of positive results.

Signals would be

overwhelmed by

background noise


Intelligence (SETI) We have already launched interstellar probes;

this is a plaque on the Pioneer 10 spacecraft.

Life in the Universe - sceweb.uhcl.edusceweb.uhcl.edu/blanford/23Life.pdf · Cosmic Evolution If we...

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