Reminder:
Homework #4 due Tuesday, 4:00 pm
EVOLUTION OF ATMOSPHERES
The dominant gasses arising from outgassing were
carbon dioxide and water vapor, with minor
amounts of nitrogen, sulfur, argon, …
Each terrestrial planet’s outgassed atmosphere
was roughly the same at the beginning.
Why do they differ now?
Mercury is
too small and
too hot to hold
onto an
atmosphere.
Mars lost much of its
atmosphere because of its
small size & lack of a
magnetosphere.
Current atmosphere resembles
its original atmosphere in
composition (essentially CO2).
The fate of its water is still a
matter of debate. There
appears to be substantial
amounts of subsurface frozen
water.
Venus and the Earth:
started with more or less identical atmospheres.
Their atmospheres have subsequently followed very
different paths.
WHY?
Slightly higher temperatures at Venus’ distance from the Sun
made it difficult for water to stay in liquid state.
 Liquid water exists in abundance on the Earth
 Carbon dioxide dissolves in oceans
 Photosynthetic life creates oxygen (oxygen has a short
lifetime in the atmosphere - must be constantly replenished).
Evolution of
Atmospheres:
Earth vs. Venus
because of the Earth’s temperature:
 On Earth there are oceans
Original CO2 has dissolved into oceans
and is tied up in carbonate rocks,
rocks (carbonates) keep levels of
CO2 just balanced in atmosphere
 keeps planet WARM but not HOT
if planet were hotter, CO2, H2O would
be boiled out of oceans and baked
out of rocks
 more CO2, H2O enter Atmosphere
Liquid water may have existed early in
Evolution of
Atmospheres: Venus’ history – but most vaporized into
Earth vs. Venus atmosphere: T was hotter on Venus
H2O vapor is a greenhouse gas - trapped
energy thus making planet hotter; eventually
T so high that water boils
‘runaway’ because more H20 goes into the
atmosphere as it evaporates; no water left
on planet to dissolve CO2 – out of balance!
eventually stabilized when H20 broken down
by UV sunlight (H escaped to space, O
reacted with minerals) and there was no
further CO2 to bake out of the Venus surface
●
●
●
This is called the runaway greenhouse
effect
It happened on Venus because Venus is
closer to the Sun. We do not think it can
happen on the Earth.
So - Earth has less atmosphere
because most of our CO2 is
frozen in rocks
(e.g., limestone)
Which of the following worlds has the
most substantial atmosphere?
red)
Mars
blue)
Earth
yellow) Venus
green) Mercury
Which of the following worlds has the
most substantial atmosphere?
red)
Mars
blue)
Earth
yellow) Venus
green) Mercury
The greenhouse effect keeps the temperature so high that
essentially all of the CO2 remains in gaseous form
Earth's stratosphere is heated primarily by which
process?
red) Ozone absorbs ultraviolet radiation.
blue) Atoms and molecules absorb infrared sunlight.
green) Greenhouse gases absorb infrared radiation.
yellow) Ozone absorbs visible sunlight.
red) Ozone is broken apart
by ultraviolet radiation.
What Determines a Planet’s Surface Temperature?
In the absence of the Greenhouse Effect:
 the planet's distance from the Sun
 the planet’s overall reflectivity
•
the higher the albedo (reflectivity), the less light
absorbed  planet cooler
What Determines a Planet’s Surface
Temperature?
●
With a greenhouse effect.
 Greenhouse effect increases the
energy (heat) in lower atmosphere,
keeping the surface warmer
 It works like a blanket
Greenhouse Effect on the Planets
●
Greenhouse Effect warms Venus, Earth, & Mars
 on Venus: it is very strong
 on Earth: it is moderate
 on Mars: it is weak
 avg. temp. on Venus & Earth would be freezing
without it
To Life!
“Life”
 How is life defined?
 What is needed for life?
 How hard it is for life to
form?
 What environments are
suitable for life?
How is “LIFE” defined?
This is extremely difficult. We can look at
commonalities of what we have defined as living…
 Order - life has structure
How is “LIFE” defined?
This is extremely difficult. We can look at
commonalities of what we have defined as living…
 Order - life has structure
 Reproduction
 Growth & development
 Energy utilization
 Senses & reacts to environment
 Evolutionary adaptation
All six properties of life are important,
but biologists consider evolutionary
adaptation to be the most important.
Evolution: “change with time”
Organisms need to be able to
encode their structural
information in order to
reproduce.
In Earth-based life, this
encoding is accomplished
through DNA.
DNA Replication
–
–
–
–
–
–
–
Complete double helix
Strands separate into 2 helices
Two identical copies of the DNA in the cell
Cell division: one copy to each daughter cell
Heredity: ensured by exact copying, but
Errors: occur occasionally -> evolution
Origin of Life: need simpler mechanism (RNA?)
Will Life Elsewhere Use DNA?
 Heredity and evolution are essential
 DNA does the job on Earth today, but fairly complex
 RNA may have been the first mechanism - simpler
 No inherent reason the same complex mechanism is
universal

Some type of molecule has to
provide the mechanism for heredity
and evolution
ERRORS ARE IMPORTANT!
Changes (mutations) in this encoding
will lead to changes in the organism.
Mutations and Evolution
 Causes of mutations (errors in hereditary coding):
–
–
–
Ultraviolet (UV) light
Chemical agents (carcinogens)
Nuclear radiation (mostly natural cosmic rays)
 Effect of mutations:
–
–
–
Harmless – no positive or negative consequences
Fatal
Evolution – survival & reproductive advantage
If the change produces an organism
better suited to its environment, it
is more likely to be passed on, i.e.,
the organism changes (evolves).
Natural selection
Artificial selection
What are the necessities of life?
 Nutrient source(s) – building blocks of organism
 Energy (sunlight, chemical reactions, internal
heat)
 Liquid water
(or possibly
Hardest
to findsome
on other liquid)
other planets
Common Characteristics of Life?
 Carbon based
 Have a protective membrane
 Need liquid water
 Use energy to maintain internal state
 Can get energy from environment
 Conduct metabolic processes (use stuff, make waste)
 Responds to stimuli
 Grow, reproduce (replicate)
 Evolve and adapt to the environment as a population
Obtaining Energy
Living organisms can obtain energy through
 “eating”, energy & nutrients from other organisms
 extraction from chemical reactions in the
environment (black smokers - ocean vents)
 extraction from radiative energy (e.g.,
photosynthesis)
Metabolism
Metabolism: chemical reactions within living
organisms. It takes place within cells.
 Why in cells? Chemical reactions much
faster than in the open
 Collects the raw materials for the
chemical reactions
 Provides the energy for the
reactions
 Provides enzymes to catalyze the
reactions
 Instructions for enzymes encoded in
DNA
Enzyme
 A specialized substance that acts as a
catalyst to regulate the speed of the many
chemical reactions involved in the
metabolism of living organisms.
 Without enzymes, life as we know it
would not exist.
Metabolism and Cells
 Metabolism:
–
–
Four forms of metabolism defined by:
●
Sources of carbon (direct or indirect)
●
Sources of energy (light or chemical)
The four forms of metabolism are quite general and should apply
to life anywhere
 Cells:
–
Needed environment for metabolism at acceptable rate
 Origin of Life (on Earth and elsewhere):
–
Look for cells as sites of metabolism
Carbon and Energy Sources
●
Carbon:
–
–
●
Heterotroph: eat other organisms
Autotroph: self-feeding by converting atmospheric CO2
Energy:
–
Photoautotrophs (plants): photosynthesis: CO2 + H2O +
sunlight
sugar
–
Photoheterotrophs (rare prokaryotes): carbon from food
but make ATP using sunlight
–
Chemoheterotrophs (animals): energy from food
–
Chemoautotrophs (extreme prokaryotes): energy from
chemicals and not sunlight
Why Carbon based?
 Can bond to as many as 4 atoms at a time.
 Can form skeleton of long chains of atoms
(polymers).
 The complexity of life requires complex
molecules.
Silicon can also form 4 bonds and is
relatively abundant, however…
 Bonds are weaker than those of carbon (fragile:
complex Si-based molecules don’t last long in
water)
 Does not normally form double-bonds like
Carbon; this limits the range of chemical
reactions and molecular structures.
 Carbon is more mobile in the environment - it
can travel in gaseous form, e.g., CO2
Environmental limits to life
(as we know it) ?
 Is the planet of interest missing any of the key ingredients? (water,
energy, nutrients)
 Are temperatures below –15 or beyond +115 C?
 Is it really cushy? – does it have an atmosphere
Importance of liquid water
●
●
Importance:
– Contact: organic chemicals float in the cell and find
each other
– Transportation: bring chemicals in and out of cells
– Participant in reactions, e.g.,:ATP, photosynthesis
Necessity:
– Life on Earth: all use water
– Dormant without water: for a limited time only
– Elsewhere: need a liquid (are there alternatives?)
Water
Liquid water plays a fundamental role in life:
 Make chemicals available (dissolved)
 Transports chemicals
 Plays a role in many metabolic reactions
Cells
 All life on Earth is made of cells - microscopic units in
which living matter is separated from the outside world
by a membrane.
 All cells on Earth share common characteristics (e.g.,
use of ATP, DNA, …), leading to conclusion that they
share a common ancestor
 All cellular life is carbon based (organic molecules)
Components of Cells

Carbohydrates: energy needs and structures
 Lipids: Source of energy & major component of
membranes. Lipids can spontaneously form membranes
in water.
 Proteins: participate in a vast array of functions;
structural, enzymes, catalysts. Built from long chains of
amino acids.
 Nucleic acids: instructions for reproduction
 70 amino acids known to
exist; only 22 are found in
life on Earth.
 Only left handed versions
are found in living
organisms
 Both of these traits
suggest a common
ancestor for life on Earth.
Based upon the cellular structure of an
organism, living cells come in two types:
Prokaryotes
Eukaryotes
The prokaryotes
 simplest type of cell
 lack a cell nucleus
 Most are unicellular
 two domains: bacteria &
archaea
 asexual reproduction
 many do not require free
oxygen
Eukaryotes
 cells are organized into complex
structures enclosed within membranes.
Have a nucleus.
 typically much larger than prokaryotes
 May be unicellular, as in amoebae, or
multi-cellular, as in plants and humans.
 both sexual and asexual reproduction