Transcript Searching for Life in Our Solar System: Chapter 6

Searching for Life in Our Solar
System: Chapter 6
• Introduction
• Environmental requirements of life
– Elements of the periodic table
– Energy for metabolism
– Liquid solution for living systems
• Concept of “habitability zones”
• Passing the baton to Professor Geller
Searching for Life in Our Solar
System
• Where would you go to search for life in our solar
system?
• Systematically, how would you prioritize the
criteria to use in the process of searching?
• For each criteria, for what would you specifically
look?
• What methodologies would you use?
• How confident/uncertain would you be relative to
proving or disproving “life signs”?
Elements of the Periodic Table
C
Relative Abundance of Elements in the Sun
12
Log of Relative
Abundance (Si)
H
10
He
8
O
C
Ne
6
Mg
Si
Ar
N
4
Na
2
Li
0
Ca
Al
P
Cl
K
Fe
B
Be
0
Fe
S
5
10
15
20
25
Atomic Number
30
35
40
45
Elements of the Solar System
• Source and abundance of H (hydrogen) and He
(helium) due to Big Bang (conversion of energy
into mass)
– 98% of solar mass
• Source of all “other” elements was the interior of
stars (chemical reactions among elements and
subatomic particles)
– Rare relative to H and He
• Planetesimals everywhere should contain elements
needed for life
Elements of the Solar System:
Role in Living Systems
• 25 of 92 natural elements are known to be essential
to life on Earth
• 4 of the above make up 96%
– Carbon (C), Oxygen (O), Hydrogen (H) and Nitrogen
(N)
• Remainder (4%) are mostly Phosphorus (P), Sulfur
(S), calcium (Ca) and Potassium (K)
• Compare to solar abundance and discuss relative to
evolution of biochemistry in origin of life
Energy for Metabolism
• Introduction
• Sunlight and photochemical energy
– Energy decreases with square of distance from
source (e.g., Sun)
– Example: leaf on Earth versus leaf twice as far
out from Earth (1/4 as much energy)
– Example: 10 times further out, energy would be
1/102 or 0.01times as much
Energy for Metabolism
• Chemical energy: ongoing chemical reactions
that can supply energy for metabolism
Example: FeS + H2S
FeS2 + H2 + free energy
• Requirements
– Atmosphere with mixing
– Liquid medium with mixing
– Solid-liquid reactor/mixing
Energy for Metabolism
• Internal sources of heat for mixing
–
–
–
–
Moon and Mercury (?)
Earth, Mars and Venus
Jovian planets (surface lacks solid surface)
Jovian moons (Io, Europa)
• Atmospheric mixing
– Jovian planets (chemical and electrical energy)
• Conclusions
– Our solar system (internal geology and liquid medium)
– Other systems?
Liquid Solution for Living
Systems
• Introduction
– Life on Earth in water….~4 BYA
– First 3 BY of life in water alone
– All life tied to watery medium (plants, animals and
microbes)
– “Habitability” of Earth f [water]
• Simplicity and complexity of the nature of the
water molecule
– Deceptively simple in structure
– Exquisite in function
Water and Its Properties:
Elemental Composition
• Elemental composition of H2O
– Hydrogen is the most abundant element in
universe
– Oxygen is the 5th most abundant element
• Conclusion: H2O is probably common
throughout the Universe
Water and Its Properties: Polarity
• Composition and structure: a polar molecule
H+
O
-
H+
• Features
– Attraction is electrical
– Hydrogen bonding among two molecules of H2O
• Exquisite properties of H2O arise from chemical
attractions because it is a polar molecule: emergent
properties
Water and Its Properties: Liquid
State
• Water is liquid over broad range of temperatures
0oC
to
100oC
• Other
Compound
Chemical Formula
Freeze Vapor
(oC)
(oC)
Range
(oC)
Water
Ammonia
Methane
H2O
NH3
CH4
0
-78
-182
100
-33
-164
100
45
18
Ethane
C2H6
-183
-89
94
Water and Its Properties: Liquid
State
• Significance of broad range for water to exist
as a liquid
– Temperatures on many planetary bodies must
have been or are within range so water exists as a
liquid
– Water in a liquid state is not too cold to slow
reactions and bonding of carbon and not too hot
to break bonds
Water and Its Properties:
Cohesion of H2O Molecules
•
•
•
•
Cohesion: H2O molecules are “sticky”
Liquid: fragile hydrogen bonds (5% of covalent)
Each bond last 1x10-12 seconds
Constant formation and breakage of bonds: any
moment a substantial number of H2O molecules are
bonded to neighbors
• Consequence: more structure than other liquids
Water and Its Properties: Water
Moderates Temperatures on Earth
• Water stabilizes temperatures by absorbing heat
from air
• Mechanism
– Specific heat (SH): usually high SH for H2O
– Change temperature less when absorbing or losing heat
(e.g., metal pot with water)
– Due to hydrogen bonding
• Relevance
– Absorption of heat in the summer by ocean and re-release
of heat (i.e., evening, winter)
– Evaporative cooling
• Define: vaporization and loss of heat
• Evaporative cooling of tropical systems
• Evaporative cooling of your body
Water and Its Properties: Freezing of
Oceans and Lakes
• Water is unusual in that H2O is less dense as a
solid than a liquid (it floats)
• Mechanism
– H2O expands when it solidifies
– Due to hydrogen bonding
• Consequence
– Ponds, lakes and ocean freeze from the top down
Water and Its Properties: Solvent of Life
• Water is versatile solvent, due to its polarity and
dissociation
H2O
H+
+ OH– In pure water, 1 molecule in every 554 million is dissociated
(10-7 molar concentration)…pH scale
– Statistically very rare BUT exceedingly important in
chemistry of life (e.g., proteins, macromolecules)
• H+ and OH- are very reactive and affect much of the
chemistry of living systems on Earth (e.g.,
membranes)