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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).
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
Evolution of Atmospheres:
Earth vs. Venus
Evolution of Atmospheres:
Earth vs. Venus
Liquid water may have existed early in Venus’ history – but most vaporized into 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?
Order - life has structure
This is extremely difficult. We can look at commonalities of what we have defined as living…
How is “LIFE” defined?
Order - life has structure Reproduction Growth & development Energy utilization Senses & reacts to environment Evolutionary adaptation
This is extremely difficult. We can look at commonalities of what we have defined as living…
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 some other liquid)
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
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
Metabolism: chemical reactions within living organisms. It takes place within cells.
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:– 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
Carbon and Energy Sources
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