AST 251 Life on Other Worlds Lecture 7

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AST 251 Life on Other Worlds Lecture 7
Origins and Evolution of Life Carbon
cycle Lifes beginnings Molecular basis of life
as we know it
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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 THIS WEEK Ch. 10 Ch. 16
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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.
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Upcoming Dates
Pick up TA comments on paper outlines
TODAY Final paper due in class Monday
March 14 4pm WILL NOT ACCEPT E-MAILED
PAPERS PENALTY FOR LATE PAPERS
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Carbon on Earth Carbon is either in the form of
atmospheric gases (CO2 and CH4), both of which
are greenhouse gases, or in solid materials
(living things, fossil fuels, rocks) Processes
that take carbon out of solids and put it into
the atmosphere include -volcanism -loss of
vegetation (deforestation) -burning of fossil
fuels What is fossil fuel? Coal, oil, natural
gas are the remains of plants that lived
millions of years ago. Complex hydrocarbons that
can be burned, which releases CO2 CH4  2 O2 --gt
CO2  2 H2O
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Normally, CO2 cycles through the atmosphere,
crust, and ocean. This cycle is a thermostat that
regulates our climate.
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Snowball Earth
Earth experienced several episodes of GLOBAL
glaciation 750-580 million years ago and 2-3
billion years ago. Geological evidence for
glaciers at sea level, at the equator!
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POSITIVE FEEDBACK Increasing glaciation caused
even more glaciation As Earth got whiter, it got
colder (reflecting sunlight away) How did the
Earth ever snap out of it? Strange finding
evidence for a very WARM climate directly on top
of the snowball earth sediments!!
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X
X
During the Snowball Earth episodes, the normal
carbon cycle stopped. CO2 built up in the
atmosphere. Catastrophic warming followed
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The Earth emerged from the Snowball episodes as
CO2 levels built up in the atmosphere The
response of climate to increasing CO2 levels is
not immediate. It takes about 400,000 years to
warm up. Geological evidence shows that the
Earth went into an extreme warming trend after
the Snowball episodes. How does CO2 warm the
Earth? It traps heat, like a greenhouse.
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Probing the Origins of Life on Earth Life may
have originated before the Moon forming impact -
but would have had to restart What was early
life like? Earths oldest rocks hold
clues Western Australia Northwestern
Canada Greenland
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Earliest evidence of liquid water on Earth
Zircon crystals (ZrSiO4 Zirconium Silicate) 50 mm
diameter
crystallization age 4.4 Gyrs ago A high oxygen
(18O) isotope ratio indicates that the crystals
experienced low temperature interaction with
liquid water in a near surface environment
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In other words The Earth had oceans 4.4 BILLION
years ago!!! This is about 200 million years
after the Moon-forming impact Life requires
liquid water could life have existed 4.4 billion
years ago? Earths oldest sedimentary rocks are
about 3.8 billion years old
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Worlds oldest sedimentary rock 3.8 billion
year old sedimentary rocks in Greenland
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Carbon isotopes and the earliest evidence for
life on Earth (controversial!) Photosynthetic
organisms take CO2 from the air and fix it into
organic matter Isotopes are chemically almost
identical, but living organisms distinguish
between them All biochemistry is picky about
which carbon isotope it uses Carbon-12 is
preferred over Carbon-13 (biochemical reactions
using lighter isotopes go faster)
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Carbon isotopes and the earliest evidence for
life on Earth (controversial!) Thus, organic
matter is richer in Carbon-12 than atmospheric
CO2 This imprint can be left in rocks
isotopic fossils The 3.8-Gyr-old sediments in
Greenland have an enhanced ratio of
Carbon-12/Carbon-13 Does this mean there was
life 3.8 billion years ago?
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Oldest morphological fossils (or just plain
rocks???)
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Microfossil from Gunflint Chert, Western Ontario
(north shore of Lake Superior)
1.9 Gyrs old
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850-Myr-old INCONTROVERTIBLE microfossil (Bitter
Springs Chert, Australia)
Chemical fossils contain molecules of biological
origin
Cyanobacteria aquatic, photosynthetic bacteria
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Where did life begin?
Darwin in a warm, shallow pond or tide pool
no longer considered consistent with the Hadean
Earth Miller Urey Oro in reducing conditions
in surface water, with a lot of energetic input
(e.g., lightning) some scientists now
think the atmosphere was oxidizing
rather than reducing Baross Hoffman 1985 In
or near hydrothermal vents
(first discovered 1979) pro
shielded from surface rich in organics and
energy early oceans would have had a
lot of them. Dont need reducing
atmosphere (in fact, use
reactions for energy in mixing region). con
temperatures may get high enough to degrade
important molecules (so mixing with cold water
required?) Arrhenius, Hoyle, Wickramsinghe On
another planet.
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The Miller-Urey Experiment (1953)
After 1 week 10-15 of carbon goes into
organic compounds, incl. amino acids
REDUCING
CH4, NH4, H2
Water
Juan Oros experiment (1961)
HCN NH4 energy gives adenine,
an important
component of
RNA, DNA,

ATP
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Was the early atmosphere oxidizing or reducing?
Oxidizing More than 1 O per 2 Hs
Reducing Fewer than 1 O per 2 Hs
Fe2O2, CO2, etc Bad for C-C bonds!
H2O
CH4, NH4, etc Good for C-C bonds!
H2O
Does it matter? Maybe the chemistry for the
origin of life didnt
depend on the atmosphere.
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Where did life begin?
Darwin in a warm, shallow pond or tide pool
no longer considered consistent with the Hadean
Earth Miller Urey Oro in reducing conditions
in surface water, with a lot of energetic input
(e.g., lightning) some people now think
the atmosphere was oxidizing rather than
reducing Baross Hoffman 1985 In or near
hydrothermal vents
(first discovered 1979) pro
Shielded from surface rich in organics and
energy early oceans would have had a
lot of them. Dont need reducing
atmosphere con temperatures may get high enough
to degrade important molecules (so mixing
with cold water required?) Arrhenius, Hoyle,
Wickramsinghe On another planet.
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Did the organic compounds have to come from Earth?
28 September, 1969 (2 months after Apollo moon
landing) A large meteorite fell near
Murchison, Australia analysis shows it is rich
in organic compounds, including amino acids.
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Question How would you define life? Come up
with definitions appropriate for the following
contexts 1. Artificial life in a computer
2. The first living thing on Earth 3. Life
anywhere in the Universe Also ponder 4. What
is the relation between life and energy?
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Be sure to look at Part II of Lecture 7.