Transcript Document 7461540

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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.
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
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)
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 Earth’s
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?
READING ASSIGNMENTS
Goldsmith & Owen
The Search for Life in the Universe
1st WEEK:
2nd WEEK:
3rd WEEK:
4th WEEK:
5th WEEK:
Ch. 1, 2 & 7 (p. 163-166)
Ch. 6 & 3
Ch. 4 & 5
Ch. 11 & 17
Ch. 7 (pp. 167-186) & Ch. 8
LAST WEEK:
Ch. 10 & Ch. 16
THIS WEEK:
Ch. 13 & Ch. 14
AST 251
Life on Other Worlds
Lecture 8
The Notion of Habitability
Origin of Life (wrap up)
Rise of Oxygen in the Earth’s atmosphere
Why Water? Why Carbon?
Traditional view of Habitability
Expanded view: the Extremophiles
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 it’s 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 they’re 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
doesn’t have this property…
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 C’s) than ribose or deoxyribose (5 C’s).
• p-RNA = another, similar variant whose strands don’t 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…
Cyanobacteria: aquatic, photosynthetic bacteria (make O2)
Chloroplasts of plants are almost identical to cyanobacteria!
(eukaryotes evolved via symbiosis of less complex organisms)
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.
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
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
Burgess Shale, British Columbia
Very well-preserved site of Cambrian fossils
Some fossils from
~540 Myrs ago
Hallucigenia
Life’s raw materials: Water as solvent
H-bonding
Hydrophilic/
Hydrophobic
Behaviour
surface tension
High heat of fusion
(evaporative cooling)
+ High specific heat
(for thermal stability)
Life’s 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
Life’s raw materials: Water as solvent
218
atm
374 C
747 K
1 atm
1/167
atm
0C
273 K
100 C
373 K
Cosmic Abundances
Protons
Element
Abundance
(number)
Comment
1
H
100,000
Life, stars
2
He
10,000
Inert
6
C
3
Life
7
N
1
Life
8
O
7
Life, Earth’s Crust
10
Ne
1
Inert
11
Na
0.2
12
Mg
4
14
Si
4
16
S
2
20
Ca
0.2
26
Fe
0.3
Earth’s Crust
Earth’s Core
Life’s 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
Si=Si and triple Si≡Si 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
• Si’s 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)
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?
Habitable zones around stars: Basic view
The “Habitable Zone” has classically
been defined as that region of planetary
orbits that allow liquid water on their
surfaces (~1 atm pressure).
Star’s mass in solar masses
D
1. Star emits luminosity L*
2. 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 Earth’s 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
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?
Extremophiles:
Thermophiles to Psychrophiles
Newly discovered archaea
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 > 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 don’t 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
Other extremophiles
Oxyphiles – organisms that love oxygen. That’s 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]
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?)
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
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 crust…so 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!
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.