1. EVOLUTION OF ATMOSPHERES

2. The dominant gasses arising from outgassing were carbon dioxide and water vapor, with minor amounts of nitrogen, sulfer, argon, …
Each terrestrial planet’s outgassed atmosphere was roughly the same at the beginning.
Why do they differ now?

3. Mercury is too small and too hot to hold onto an atmosphere.

4. Mars lost much of its atmosphere because of its small size & lack of a magnetosphere.
The atmosphere it retains 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.

5. started with more or less identical atmospheres.
Venus and the Earth:
started with more or less identical atmospheres.
Their atmospheres have subsequently followed very different paths.
WHY?

6. 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).

7. 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

8. 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

9. 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)

10. 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

11. What Determines a Planet’s Surface Temperature?
With a greenhouse effect.
it increases the energy (heat) in lower atmosphere, keeping the surface warmer
It works like a blanket

12. 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

13. Before proceeding to life… A brief visit to a missed topic…
The Origin of the Earth’s Moon

14. The Earth-Moon
double planet does not fit well into the nebular theory
planetesimal accretion predicts both should have the same chemical composition.
They don’t - there are subtle but significant differences
Moon is composed of less dense material than Earth

15. The general view is that the Earth’s moon was created as a result of the impact of a large object, perhaps as large as Mars, with the Earth very early in its existence.
The moon was formed from the debris of this collision, which included lower density “mantle” material from the Earth.

16. The Moon stabilizes the tilt of Earth's spin axis, leading to stable seasonal changes. This is not the case for the other terrestrial planets.

17. Time to switch gears

18. “Life” How is life defined? What is needed for life?
How hard it is for life to form?
What environments are suitable for life?

19. How is “LIFE” defined? Order - life has structure
This is extremely difficult. We can look at commonalities of what we have defined as living…
Order - life has structure

20. How is “LIFE” defined? Order - life has structure Reproduction
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

21. Evolution: “change with time”
All six properties of life are important, but biologists consider evolutionary adaptation to be the most important.
Evolution: “change with time”

22. Organisms need to be able to encode their structural information in order to reproduce. In Earth-based life, this encoding is accomplished through DNA.

23. 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?)

24. 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

25. ERRORS ARE IMPORTANT! Changes (mutations) in this encoding will lead to changes in the organism.

26. 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

27. 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).

28. Natural selection
Artificial selection

29. 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)
Hardest to find on other planets
Applies to “life as we know it.”

30. 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
Fire – actually has several of these characteristics; viruses have few
All living entities have the ability to reproduce and perpetuate genetic variation

31. 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)

32. Metabolism

33. Metabolism: chemical reactions within living organisms
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

34. Synthesis reactions = “building up” reactions
Example: Making starch from glucose
- requires energy
Decomposition reactions = “breaking down” reactions
Example: Breaking glucose into CO2 & H2O
- releases chemical energy

35. Metabolism and Cells Metabolism: Cells:
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

36. 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.
Fire – actually has several of these characteristics; viruses have few
All living entities have the ability to reproduce and perpetuate genetic variation

37. 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
Fire – actually has several of these characteristics; viruses have few
All living entities have the ability to reproduce and perpetuate genetic variation

38. Environmental limits to life (as we know it) ?
Is the planet of interesting 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
>125 protein and carbohydrates and genetic materials break down
Below freezing chemical reactions are too slow to support life
Water can be intermittent – just has to be available as a liquid
Atmosphere – thick enough to block radiation and incoming meteorites; moderate temperatures; gravity issue – planet or moon has to be large enough to hold an atmosphere
Energy – light or chemical
Nutrients – all planets and moons in the solar system have the same general chemical composition. Must have a tectonic or volcanic or water or atmospheric process to make these available.

39. Importance of Water Importance: Necessity:
Contact: organic chemicals float in the cell and find each other
Transportation: bring chemicals in and out of cells
Participant in reactions:
ATP
Photosynthesis
Necessity:
Life on Earth: all use water
Dormant without water: for a limited time only
Elsewhere: need a liquid
What are the alternatives?

40. Water Liquid water plays a fundamental role in life:
Make chemicals available (dissolved)
Transports chemicals
Plays a role in many metabolic reactions