Conditions for lifeConditions for life

http://photojournal.jpl.nasa.gov

Mercury Earth Jupiter Uranus

Venus Mars Saturn Neptune

Our solar systemOur solar system

Earth, our home

http://visibleearth.nasa.gov

Earth: Earth: Goldilocks ZoneGoldilocks Zone

Earth’s 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

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%

Venus

http://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-venus.html

http://blog.thomaslaupstad.com/2007/04/12/moon-venus-and-earthshine/

moon

Venus!

http://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-mars.html

Mars

Earth, Venus, and MarsEarth, 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

Runaway greenhouse warming in VenusRunaway 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

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 => temperature rises => more liquid water evaporates => 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

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

… … and the problem with Marsand 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

Earth’s shieldsEarth’s 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

Earth’s magnetic fields shield us from the Sun’s 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

The water molecule

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

WaterWater

WaterWater

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)

O

H H104.5

positive side

negative side

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

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

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

Why is water still on Earth?Why is water still on Earth?

Earth’s 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

Hydrological cycleHydrological cycle

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)

(T)

(E)

Environmental soil physics by Daniel Hillel, 1998, Academic Press

Water balance within the root zoneWater 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)

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., > 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

Carbon cycleCarbon cycle

Carbon cycle has a long term efffect on Earth’s 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%

Long term carbon cycleLong term carbon cycle

Carbonate rocks Organic carbon rocks

Surface carbon reservoirs:oceans (40)

atmosphere (0.75)biota (0.6)soil (1.6)

rockdegassing

rockdegassing

rock

wea

ther

ing

rock

wea

ther

ing

rock

bur

ial

rock

bur

ial

Rocks (75,000)

figures in trillion tonnes or teratonnes

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

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

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

Coquina, a limestone composed of fossilized shell debris cemented together by calcite

http://geology.about.com/od/more_sedrocks/ig/sedrocksgallery/coquina.--2t.htm

http://gccweb.gccaz.edu/earthsci/imagearchive/chemical1.htm

http://www.hunstantonfossils.co.uk/Hunstanton-Fossils-Geology/geology-guide.htm

Chalk, composed of fossilzed shells of microscopic organisms such as foraminifera

Role of photosynthesisRole 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

http://gccweb.gccaz.edu/earthsci/imagearchive/chemical1.htm

Coal Shale

http://www.geologytimes.com

Plate tectonicsPlate tectonics

CO2 can also be released from activities of Earth’s 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 Earth’s mantle this convection is, in turn, caused by the decay of

radioactive elements, mainly potassium (isotope 40K), thorium (Th), and uranium (U)

http://www.geography-site.co.uk/pages/physical/earth/tect.html

Plate tectonics akin to a jigsaw puzzle

http://www.crystalinks.com/platetectonics.html

MANTLE

http://facstaff.gpc.edu/~pgore/Earth&Space/GPS/platetect.html

Carbon regulation of climateCarbon 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

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)

Short term carbon cycleShort term carbon cycle

Atmospherecarbon

Soilcarbon

Biotacarbon

Oceancarbon

Gas exchange

Rivertransport

Marine respiration

DegassingTerrestrialrespiration

Terrestrialphoto-synthesis

Marine photosynthesis

Litter fall, root decay,calcification

http://www.sheepdrove.com/312.htm

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)

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

http://www.globalchange.umich.edu/globalchange1/current/lectures/kling/carbon_cycle/Archive/carbon_cycle_2004.html

NET CHANGE = INPUT – OUTPUT

3.2  = (6.3 + 1.6) – (1.4 + 1.7)

3.2  = 4.8 ??!!

Carbon imbalance:

Where’s the rest of 1.6 Gt C?

The case of the “missing” carbonThe 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