Upcoming Dates

1
Upcoming Dates
Paper due in class Monday March 14
4pm WILL NOT ACCEPT E-MAILED PAPERS
PENALTY FOR LATE PAPERS Final exam
schedule at http//www.artsandscience.utoronto.c
a/current/exams (also linked from course web
site)
2
Paper writing advice and guidance
Strongly recommend the Writing Centres web
site http//www.utoronto.ca/writing/index.html
Especially the guidelines on Writing about
Physics http//www.utoronto.ca/writing/physguide.
html (linked from Supplementary Resources on
the course web site) Also feel free to consult
with your TA as you make progress --by email or
in person by appointment.
3
A large crater on Titan?
Radar map from Cassini (crater diameter 440
km) Could have formed when a comet or asteroid
tens of kilometers in size slammed into Titan.
This is the first impact feature identified in
radar images of Titan.
4
Channels formed by liquid methane on Titan?
Radar map from Cassini (longest channel 200
km) Could be channels in which liquid methane
flowed from the slopes of the crater, carrying
debris, towards the bright area in the upper right
5
Ozone destruction due to solar flares?
Ozone levels declined by up to 60 in the
stratosphere above the Arctic in Spring 2004.
Nitrogen oxides in the upper atmosphere climbed
to their highest levels in two decades. This
could be related to high levels of solar flare
activity in Fall 2003.
Charged particles in the solar wind break up N2
and help form nitrogen oxides This process
destroys ozone (O3)
6
What triggered Snowball Earth events? A new
suggestion Solar system passed through a dense
cloud in space a rare occurrence A dust
layer built up in the Earths upper atmosphere
--it absorbed and scattered sunlight, but allowed
heat to escape from Earth The result was runaway
ice buildup and global glaciations What else
could lead to increased dust levels in the
atmosphere?
7
READING ASSIGNMENTS
Goldsmith Owen The Search for Life in the
Universe
1st WEEK Ch. 1, 2 7 (p. 163-166) 2nd WEEK
Ch. 6 3 3rd WEEK Ch. 4 5 4th WEEK
Ch. 11 17 5th WEEK Ch. 7 (pp. 167-186)
Ch. 8 LAST WEEK Ch. 10 Ch. 16 THIS
WEEK Ch. 13 Ch. 14
8
AST 251 Life on Other Worlds Lecture 8
The Notion of Habitability Origin of Life (wrap
up) Rise of Oxygen in the Earths atmosphere Why
Water? Why Carbon? Traditional view of
Habitability Expanded view the Extremophiles
9
First living thing How to get there?
How to get from oligonucleotides (short strands)
to the first self-replicating molecule (e.g., RNA
replicase, if its going to be RNA)?

• Issues
• Shorter strands are easier to replicate and
  easier to imagine assembling.
• But, being a good copy-machine requires some
  technology
• Experiments to identify RNA molecules that can
  copy others have found
• molecules as short as about 120 nucleotides, but
  not yet fully self-replicating
• This can be partially solved by copying smaller
  strands that assemble into
• the replicase molecule.
• Being a good copy-machine requires a stable, 3D
  structure (folding), whereas
• being copied requires a relaxed (unfolded)
  configuration.
• This can also be helped by copying shorter
  molecules that self-assemble.
• Complementary RNA strands form a double helix
  much like DNA. This helix must
• be ripped apart or prevented from forming when
  theyre copied.
• This can be done, at the expense of adding some
  more machinery to the replicase (e.g., something
  to peel off new strand, or displace one strand
  relative to the other)
• Alternatively, there might have been a previous
  molecule besides RNA that doesnt have this
  property

10
Other interesting possibilities

• Maybe another molecule was the first nucleic
  acid?
• Possibilities
• TNA nucleic acid with a backbone of threose
  sugars that are simpler
• (4 Cs) than ribose or deoxyribose (5 Cs).
• p-RNA another, similar variant whose strands
  dont twist better for
• being copied worse for catalytic activity.
• PNA peptide nucleic acid backbone made of
  amino acids
• (peptides), rather than nucleic acids.
  Therefore they are closer
• to proteins.
• All of these show Watson-Crick pairing each
  nucleotide has a partner with whom it fits
  together. This vastly improves their stability,
  rate of formation, and ease of copying.
• Maybe there is a mineral catalyst that helps in
  assembling the molecules,
• vesicles and protocells?
• J. Ferris, RPI A kind of clay (montmorillonite)
  can catalyze the formation of oligonucleotides
  and also the creation of vesicles from lipids
  (100x speedup).
• The handedness of molecules could derive from
  the structure of a crystal that organized them
  together

11
Cyanobacteria aquatic, photosynthetic bacteria
(make O2)
Chloroplasts of plants are almost identical to
cyanobacteria! (eukaryotes evolved via symbiosis
of less complex organisms)
12
The rise of Oxygen in the Atmosphere
Stromatolites
The Age of Banded-Iron Formations About 2-3
billion years ago Photosynthesis produces
oxygen In a reducing environment, oxygen gets
bound up in minerals like iron oxide. These
precipitate, creating banded-iron formations of
Fe2O3 (hematite) but only until the
environment becomes oxidizing.
13
Banded Iron Formations sedimentary rock
deposited while O2 was building up in the
atmosphere (2-3 billion years old)
Rise of oxygen was not uniform - it happened in
fits and starts
14
Early eukaryotes and the Cambrian explosion
Oxygen is a very efficient chemical with which 
to extract energy from environment Anaerobic
organisms are simple. The rise of oxygen allowed
the rise of complex animals Cambrian Explosion
proliferation of shelly animals (which make
good fossils, not like jellyfish!) occurred
about 540 million years ago
15
Burgess Shale, British Columbia
Very well-preserved site of Cambrian fossils
16
Some fossils from 540 Myrs ago
Hallucigenia
17
Lifes raw materials Water as solvent
H-bonding
Hydrophilic/ Hydrophobic Behaviour
High heat of fusion (evaporative cooling)
High specific heat (for thermal stability)
surface tension
18
Lifes raw materials Why water?

• Thermal properties
• Liquid over large range in T (273 K 373 K
  31)
• Expands on freezing protects liquid under ice
  layer
• High heat content good thermal insulator
• High heat of fusion good for evaporative
  cooling
• Other physical properties
• High surface tension
• Shields UV radiation partially
• Excellent solvent
• Chemical properties
• Relatively non-reactive with organic compounds
• Strongly polar supports membrane formation,
  protein folding
• Participates in weak Hydrogen Bonds
• Dissolves many gases and useful nutrients

19
Lifes raw materials Water as solvent
218 atm
374 C 747 K
1 atm
1/167 atm
0 C 273 K
100 C 373 K
20
Cosmic Abundances

21
Lifes raw materials Carbon or Silicon?

• Both C and Si have 4 valence electrons, and can
  form single, double, and triple bonds. So, are
  they equally good at making biological polymers,
  enzymes, and informational molecules? Probably
  not.
• Chemical properties
• Si atoms are larger than C atoms, which reduces
  the flexibility of their
• electronic orbitals in forming bonds. This
  makes it more difficult to form double
• SiSi and triple SiSi bonds.
• Si-Si bonds are weaker than C-C bonds by a
  factor of 2
• Si-H and Si-O bonds are stronger than Si-Si
  bonds, whereas C-H, C-O, C-C are
• similar. This means Si-backbone polymers have
  very different chemistry.
• Silicones (Si-O chains) are very stable (inert),
  most suitable for lubricants.
• Si does not create aromatic ring compounds
  like C does (e.g., benzyne).
• Physical properties
• Sis compounds are heavier more refractory in
  general it forms few liquid or
• gaseous compounds, and is difficult to cycle in
  the environment (e.g., SiO2
• glass sand)

22
Martians and Humans
Should we put humans on Mars? What are the
benefits? What are the drawbacks? What
would be the implications (ethical social) of
Terraforming Mars -- making it Earthlike?
What steps would be required to accomplish it?
23
Habitable zones around stars Basic view

• Star emits luminosity L
• This luminosity is distributed in a sphere of
  size 4pD2,
• so the flux hitting the planet is F L
  /4pD2
• 3. If the planet has an albedo a, a fraction 1-a
  of this sticks
• 4. The planet must re-radiate this so (1-a)F s
  T4
• 5. For Earths albedo, this gives about
• T 270 (L/Lsolar)1/4 (AU/D)1/2 K
• 6. But planets are warmer than this if their
  atmospheres
• have greenhouse gases

stellar type
The Habitable Zone has classically been
defined as that region of planetary orbits that
allow liquid water on their surfaces (1 atm
pressure).
24
Continuously habitable zones
Stars get brighter as they age on the main
sequence so their habitable zones move out with
time. The CHZ is the region in which liquid
water can exist the whole time - remember
Faint Young Sun paradox
The Sun has increased in luminosity by 30,
causing the HZ to move out by 14.
How correct is it to associate liquid surface
water with life?
25
Extremophiles Thermophiles to Psychrophiles
Newly discovered archaea
26
Life as an extremophile
Life as a hyperthermophile (high
temperature) Problem At high T, membranes become
too fluid and permeable. Solution change the
lipids to be more waxy Problem at T gt 70 C, DNA
RNA starts to degrade Solution increase the
salt solution within the cell to protect them.
Solution rely more on the stabler G-C base pair
rather than T-A or T-U (seen in
RNA, not DNA) Problem Proteins dont fold as
well at high T Solution Evolve more
stably-folding proteins (e.g., tighter
hydrophobic cores) Life as a psychrophile (low
temperature) Problem At low T, membranes become
too stiff. Solution change the lipids to be
more greasy. Problem Water freezes, and ice
crystals break cells Solution use antifreeze
molecules to inhibit crystal growth Problem Not
enough energy to overcome chemical
barriers Solution Evolve more active enzymes
27
Other extremophiles
Oxyphiles organisms that love oxygen. Thats
us! Problem Oxygen reactions produce reactive
species like oxygen free radicals,
which damage DNA responsible for much of aging
and some cancers. Inevitable side product of
respiration, photosynthesis and UV absorption
used as a tool to kill some pathogens.
Solution Develop anti-oxidants (e.g., some
vitamins and flavinoids) Halophiles organisms
that live in high-salt environments. Problem
Reverse osmotic pressure dessicates cells
Solution Produce something inside cell (usu.
glycene, sometimes potassium) whose
osmotic pressure balances that of salt outside
cell. Xerophiles organisms that live in
extremely dry environments. Problem water
evaporates. Solution Protect surface (desert
varnish) Solution Increase interior osmotic
pressure, or let cell dry out Problem Oxygen
free radicals accumulate as cell dries DNA
breaks Solution Fix it! Side benefit extreme
radiation resistance D. Radiodurans incredible
resistance
28
Xerophiles

• Desert Varnish exists in the driest places on
  Earth
• Varnish includes bacteria that
• arrange clay and manganese above them to shield
  them
• from the elements oxidize Mn to produce ATP
• are great for showing where pollutants in water
  exist or
• where off-road vehicles stir up alkaline dust.

• Lichens a symbiosis of fungi and algae
• Margulis 1964
• dry out completely and photosynthesize
• only when wet
• The first step in creating soil out of rock
• (e.g., Sierra Nevada polished by glaciers 12 kyr
  ago, heavily wooded now.)
• Edible! (Manna?)

29
Even more extremophiles
Acidophiles/Alkalophiles organisms that love
acidic/basic conditions Problem Proteins can
be degraded by changes in pH (e.g.,
ceviche) Solution Use molecular pumps to keep
the interior pH close to neutral.
Fresh water
acidic basic
30
Even more extremophiles
Piezopiles organisms that live at high
pressure Pressure increases by 1 atm ( 15 pounds
per square inch) every 10 meters in water, or
every 5 meters in rock. Benefit Water is
liquid for a higher range of temperatures as the
pressure goes up this allows
liquid water to tens of kilometers depth
T goes up 25 C per km in crustso 121 C
about 4 km Problem Pressure changes the
packing of DNA and membrane lipids Problem
Pressure inhibits reactions that lower the
density (more products than reactants) Solution
? Life in Vacuum 1964 Surveyor 3 camera in
space for 2.6 years, unprotected.
On returning from the Moon, viable
streptococcus bacteria are cultured from it!
31
More amazing life
Longevity Viable microbes from ice cores (Lake
Vostok) up to 20 Myr From bee abdomens in
amber 25 Myr From salt in salt mines many Myr
(controversial) Multicellular extremophiles?
Tartigrades (water bears) in a dry (tun) state,
can withstand temperatures up to 151 C, X-rays,
vacuum, and pressures of 6000 atmospheres.
Life without light? Autolithotrophic
communities (SLiMe) Basalt rock water has
C,N,O,H, S just need energy Energy from
oxidation of S H and reduction of S and
nitrates. Note life had to be like this before
photosynthesis was invented.