Conditions for life

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Conditions for life
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Our solar system
http//photojournal.jpl.nasa.gov
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Earth, our home
http//visibleearth.nasa.gov
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Earth Goldilocks Zone

• Earths position (third rock from the Sun) is
  in the Goldilocks Zone (0.9 1.4 AU)
• that is, in a position that is not too hot and
  not too cold (just right)
• Venus is too hot, Mars is too cold, Earth is just
  right
• note 1 Astronomical Unit (AU) 149,598,000 km

http//www.dailymail.co.uk
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Venus Earth Mars
Distance from Sun 0.72 A.U. 1 A.U. 1.52 A.U.
Mass 4.87 x 1024 kg 5.98 x 1024 kg 6.42 x 1023 kg
Density 5.25 g cm-3 5.52 g cm-3 3.94 g cm-3
Gravity 0.88 Earth gravity 1 Earth gravity 0.38 Earth gravity
Radius 6052 km 6378 km 3397 km
Atmospheric pressure 90.9 atm 1 atm 0.069 atm
Surface temperature 460 ?C 15 ?C -59 ?C
Atmospheric CO2 96 0.0389 95
Atmospheric N2 3.5 77 2.7
Water vapour 0.01 1 0.03
Oxygen 0 21 0.13
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Venus
http//nssdc.gsfc.nasa.gov/photo_gallery/photogall
ery-venus.html
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moon
Venus!
http//blog.thomaslaupstad.com/2007/04/12/moon-ven
us-and-earthshine/
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Mars
http//nssdc.gsfc.nasa.gov/photo_gallery/photogall
ery-mars.html
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Earth, Venus, and Mars

• Earth actually has similar composition with Venus
  and Mars but water is not stable or not present
  in Venus or Mars
• Venus is very hot (460 ?C) hot enough to melt
  lead
• has a dense atmosphere, mainly composed of CO2
• atmosphere is shrouded with clouds of sulfuric
  acid and water droplets
• because of thick cloud cover, Venus surface
  receives only 44 of solar radiation that Earth
  does
• but heat from surface is nearly completely
  absorbed by clouds and atmosphere
• runaway greenhouse warming

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Runaway greenhouse warming in Venus

• Venus has no way of removing CO2
• Perhaps early in Venus history, Venus had plate
  tectonics and oceans to control climate
• But as the Sun became hotter, ocean boiled away
• Nearer Sun, hotter. H2O readily evaporates from
  surface
• Because of its proximity to the Sun, there is no
  cold trap (for water to condense into ice)
• water vapor rises to high altitudes, where it is
  more easily destroyed by solar UV
• H2O dissociates to H and O.
• H, being light, escapes, but O reacts with other
  molecules to form other molecules
• means increasingly more water is being lost

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• Absence of cleansing action by H2O precipitation
  permits CO2 atmosphere to grow
• Absence of water means rock weathering also
  ceases to capture CO2 from the atmosphere and
  lock it to form carbonate rocks
• CO2 traps infrared radiation from surface gt
  temperature rises gt more liquid water evaporates
  gt enhances Greenhouse further
• this positive (amplifying) feedback produces a
  runaway greenhouse effect
• All water is eventually removed from the
  atmosphere
• yields a massive, very hot, and dry CO2 atmosphere

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• Continuous volcanic out gassing of materials like
  sulfur dioxide without water cleansing produces
  sulfuric acid clouds
• without its abundant water, Earth would probably
  be like Venus
• Venus is hotter than Earth because of its
  greenhouse gases rich atmosphere
• without these gases, Venus would actually be -20
  ?C and any water would be frozen
• hotter than Earth not so much because Venus is
  closer to the Sun

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and the problem with Mars

• Mars is very cold (-59 ?C) and has a very thin
  CO2 atmosphere
• Mars may have once had water in a very large
  northern basin, Oceanus Borealis
• Mars is 90 lighter than Earth
• lower gravity on Mars
• means Mars cannot hold onto its early atmosphere
• Mars is also too small
• to have sustained plate tectonics
• rocks cannot return CO2 into the atmosphere
• geochemical cycle has stopped
• so Mars loses heat easily

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Earths shields

• Earth supports life because it has abundance of
  water and water exists primarily as liquid (not
  as vapour or ice)
• Earth is also shielded from UV by the ozone layer
  in the stratosphere
• Earths magnetic fields shield us from
• the Suns solar wind (flux of electrons, protons,
  and charged helium nuclei) that travel several
  hundreds of kilometers per second
• can kill a human
• cosmic rays (protons and heavier nuclei
  particles) travelling at near light speeds
• rays come from extra solar activities such as
  supernova explosions

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Water
The water molecule
E.A. Mathez, 2009, Climate Change The Science of
Global Warming and Our Energy Future, Columbia
University Press.
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Water

• 2 atoms of hydrogen and 1 atom of oxygen - H2O
• One of the most unique and most important
  molecule on Earth
• Ice (solid water) has a lower density than liquid
  water, so ice floats
• other molecules solid phase will sink (higher
  density)

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• Ice forms at the surface of water
• ice is now a heat insulator for the waters below
• below ice is liquid water (crucial if life is to
  continue in cold weather)
• Bipolar charge because of atom arrangement
• -ve on oxygen side ve on hydrogen side
• bipolarity charge makes water stable and solvent
  for many substances
• many chemical reactions can take place in water
• Bipolarity makes water stable means
• large amount of energy needed to evaporate water
• large amount of energy has to be removed for
  freezing water

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• Specific heat for water is among the highest
• amount of energy to raise 1 gram of substance by
  1 ?C
• to heat 1 g water by 1 ?C requires 1 calorie or
  4.186 J
• compare that to 1 g dry air (udara) which
  requires 1.006 J (about 1/4 less of that for
  water)
• this means water can absorb and release
  relatively large amounts of heat with very little
  change in its temperature
• high specific heat of water is one reason why
  oceans are much slower to respond to the heating
  or cooling of atmosphere
• also why seasonal change in temperature of oceans
  are much less that that of the atmosphere

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• Hottest and coldest temperature ever recorded
• on land 58 ?C (Libya desert) and -88 ?C
  (Antarctica)
• range 146 ?C
• on ocean 36 ?C (Persian Gulf) and -2 ?C (near
  poles)
• range 38 ?C only (much less than that for land)
• Ocean is a natural thermostat
• annual sea surface temperature variation
• 2 ?C in tropics, 8 ?C in middle latitudes, 4 ?C
  in polar regions
• global average ocean temperature 17 ?C
• releases and absorbs heat over decades to
  centuries, whereas the atmosphere does the same
  but in days to weeks

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Why is water still on Earth?

• Earths atmosphere is layered
• troposphere (8-15 km) then stratosphere
• Upper troposphere is very cold
• liquid water condenses into ice before it can
  reach stratosphere, making the stratosphere very
  dry
• if water escapes into stratosphere and higher, UV
  rays would dissociate water molecule into H and O
• H, being light, would not be held down by gravity
  and would escape into space
• eventually all water would be lost from Earth
• this cold trap is essential to trap water on
  Earth

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Hydrological cycle
(T)
(E)
Capillary rise
Water entry into soil is called inflitration.
Runoff is water flowing on the soil surface,
unable to enter into soil (unable to inflitrate
into soil)
Environmental soil physics by Daniel Hillel,
1998, Academic Press
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Water balance within the root zone
Run in
RI R I CR ?? RO ET P
Balance looks deceptively simple, but some
parameters are difficult to measure in practice,
such as RI, RO, CR, P, and ET (especially the T
component)
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• Water balance can be simplified by using some
  assumptions
• no irrigation, so I 0
• flat land or incoming water same as outgoing
  water by runoff, so RI RO
• deep water table (i.e., gt 2 m), so CR 0
• balance over long term (i.e., a year), so no
  change is soil moisture between the period, so ??
  0
• Simplified equation
• R ET P
• this equation, though much simpler, has to be
  used with care because it uses a lot of
  assumptions which may not be appropriate in some
  conditions

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Carbon cycle

• Carbon cycle has a long term efffect on Earths
  climate
• Carbon cycle has two cycles
• short-term cycle
• long-term cycle
• Carbon exists mainly as
• gas CO2 in atmosphere
• dissolved bicarbonate ions (HCO3-) in oceans
• various organic compounds in soil
• Carbon is a major component in all living
  organisms
• plants 50
• animals 19

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Long term carbon cycle
Rocks (75,000)
Carbonate rocks
Organic carbon rocks
rock burial
rock burial
rock weathering
rock weathering
Surface carbon reservoirs oceans (40) atmosphere
(0.75) biota (0.6) soil (1.6)
rock degassing
rock degassing
figures in trillion tonnes or teratonnes
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• Weathering (chemical breakdown) of rocks remove
  CO2 from the atmosphere
• CO2 reacts with water and silicate and carbonate
  minerals to form, in water, Ca, Mg, bicarbonate
  ions, and silicia
• 4CO2 6H2O CaSiO3 MgSiO3
• Ca2 Mg2 4HCO3- 2H4SiO4
• or
• atmospheric CO2 water Ca and Mg silicate
  minerals
• ions and species dissolved in river water

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• The dissolved ions wash into rivers which
  eventually flows into the sea. In the ocean,
  organisms use the dissolved Ca and bicarbonate
  ions to make shells
• Ca2 2HCO3- CaCO3 CO2 H2O
• or
• Ca and bicarbonate ions in seawater
• calcite CO2 and water
• Dissolved silicate precipates to opal
• H4SiO4 SiO2.H2O H2O
• or
• dissolved silica opal water
• Mg is removed from seawater mainly by reacting
  with hot rocks to form clay minerals

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• The carbonate shells accumulate at the ocean
  bottom and eventually form carbonate-bearing
  sedimentary rocks, such as coquina and chalk
• With burial, these rocks heat up and compress,
  and the carbonate minerals break down to release
  CO2
• the CO2 percolates out of the crust and escape
  into the atmosphere, completing the cycle
• SiO2 CaCO3 CaSiO3 CO2
• or
• silicate minerals carbonate minerals
• calcium silicate minerals carbon dioxide

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Coquina, a limestone composed of fossilized shell
debris cemented together by calcite
http//gccweb.gccaz.edu/earthsci/imagearchive/chem
ical1.htm
http//geology.about.com/od/more_sedrocks/ig/sedro
cksgallery/coquina.--2t.htm
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Chalk, composed of fossilzed shells of
microscopic organisms such as foraminifera
http//www.hunstantonfossils.co.uk/Hunstanton-Foss
ils-Geology/geology-guide.htm
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Role of photosynthesis

• Another route for CO2 to return to atmosphere
• removal of CO2 from the atmosphere by
  photosynthesis
• then the burial of organic matter (OM) to make
  organic-rich rocks, primarily coal and
  carbonaceous shale
• CO2 H2O (CH2O)n O2
• where (CH2O)n represents carbohydrates, starches,
  and other organic compounds in plants
• oxidation of sedimentary rocks as they are
  exposed by erosion or other physical breakdown
  returns CO2 into the atmosphere, completing the
  cycle
• (CH2O)n O2 CO2 H2O

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Coal
Shale
http//www.geologytimes.com
http//gccweb.gccaz.edu/earthsci/imagearchive/chem
ical1.htm
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Plate tectonics

• CO2 can also be released from activities of
  Earths plate tectonics
• plate tectonics allow degassing of CO2 from rocks
  into the atmosphere
• without plate tectonics, very difficult to return
  CO2 into the atmosphere
• Earth would be frozen over
• Plate tectonics is caused by the convection in
  Earths mantle
• this convection is, in turn, caused by the decay
  of radioactive elements, mainly potassium
  (isotope 40K), thorium (Th), and uranium (U)

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Plate tectonics akin to a jigsaw puzzle
http//www.geography-site.co.uk/pages/physical/ear
th/tect.html
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MANTLE
http//www.crystalinks.com/platetectonics.html
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http//facstaff.gpc.edu/pgore/EarthSpace/GPS/pla
tetect.html
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Carbon regulation of climate

• As CO2 increases, temperature increases due to
  greenhouse effect, so weathering of rocks
  increases.
• Why?
• higher temperature may lead to higher rainfall
  (higher ET), so higher rate of weathering
• higher CO2 or temperature may increase plant
  photosynthesis, so plants produce more organic
  acids and other compounds to increase rock
  weathering
• As rate of rock weathering increases, more CO2 is
  removed from atmosphere, so this leads to cooling

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• But as Earth cools, the rate of rock weathering
  now slows down, and CO2 builds up in the
  atmosphere because of degassing by solid Earth
• Almost all carbon on Earth is sequestered in
  rocks
• 6.5 x 1016 tons of C are in rocks
• but only 4.1 x 1013 tons of C are in other
  surface reservoirs (1000 times less than in
  rocks)
• this balance is important it keeps Earth from
  being too cold (too much CO2 locked up in rocks)
  or too hot (too much CO2 released into atmosphere)

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Short term carbon cycle
Marine photosynthesis
Gas exchange
Ocean carbon
Atmosphere carbon
River transport
Degassing
Terrestrial photo- synthesis
Terrestrial respiration
Soil carbon
Biota carbon
Litter fall, root decay, calcification
Marine respiration
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http//www.sheepdrove.com/312.htm
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• Short term carbon cycle refers to the circulation
  of carbon among the surface reservoirs (oceans,
  atmosphere, soil, and biota)
• Photosynthesis removes carbon from atmosphere,
  and respiration returns it
• Oceans absorb carbon from the atmosphere and
  releases it in smaller quantities
• colder the ocean, carbon can be absorbed easier
• warmer the ocean, harder to hold on to the carbon
• analogy cold Coke drink
• Soil adds carbon via degassing
• decay of organic matter (higher the temperature,
  faster decaying rate)

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• Flow of carbon from one reservoir to another is
  well established
• but the amount and mechanisms of transport is not
  well established yet
• Human activities, through burning of fossil
  fuels, add 6.3 Gt (gigatonnes or billion tonnes)
  carbon per year

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Carbon imbalance Wheres the rest of 1.6 Gt C?
NET CHANGE INPUT OUTPUT
3.2  (6.3 1.6) (1.4 1.7)
3.2  4.8 ??!!
http//www.globalchange.umich.edu/globalchange1/cu
rrent/lectures/kling/carbon_cycle/Archive/carbon_c
ycle_2004.html
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The case of the missing carbon

• Not all the amount of C added by human activities
  is found in the atmosphere
• The ocean plays a critical role in determining
  amount of CO2 in the atmosphere on a long term
  time scale
• Some of the anthropogenic C is stored in oceans
  and biosphere
• Critical to know all the sinks if to accurately
  predict the expected climate change