PTYS 214 Spring 2018 Announcements Next midterm 3/1! 1
Previously Solar flux decreases as radiation spreads out away from the Sun Planets are exposed to some small amount of the total solar radiation A portion of that radiation can be used by photo(autotrophs/heterotrophs) (e.g. photosynthesis) Other biota (chemo-) can eat energy-rich organic molecules from photo-autotrophs or each other 2
Energy/food chain Photosynthesis Solar Radiation Respiration 3
Are there other sources of energy? 4
Are there other sources of energy? Earthquakes? Volcanoes? 5
Earth is geologically active Earthquakes, volcanoes and the slow motion of the continents (plate tectonics) do not depend on the energy from the Sun There should be an internal heat source! 6
Heat Flow from Earth s Interior Heat coming from the Earth s interior amounts to about 3.8 1013 W, or an average heat flow of 0.075 W/m2 7
Why is the interior of the Earth hot? 8
Sources of Energy in the Earth s Interior Energy remaining from accretion 50% Energy released from differentiation / compaction small Radioactive decay 50% 9
1. Radioactive decay Process in which an unstable atomic nucleus loses energy in the form of particles or electromagnetic waves and decays towards a more stable nucleus (also known as fission) Unstable: nucleus has an unbalanced number of protons and neutrons Stable: balanced number of protons and neutrons Energy is released during radioactive decay 10
Nuclides Atomic species characterized by the number of protons Z and neutrons N in the nucleus Some nuclides are stable, others are not Isotopes: Nuclides with the same atomic number (Z), but different mass number (A=Z+N) Example: 12C 13C 14C 12 C: 6 protons, 6 neutrons Stable 13 C: 6 protons, 7 neutrons Stable 14 C: 6 protons, 8 neutrons Unstable 11
Types of Radioactive decay α Alpha Decay U 238 Th + 4He 234 New nuclide: Z-2 and N-2 particle speed ~10,000 miles/sec (kinetic energy) Beta Decay Th 234 β Pa + e 234 neutron to proton: Z+1 proton to neutron: Z-1 Decay events are random! 12
Half-life (T½) Amount of time it takes for one-half of the radioactive atoms in a sample to decay 13
Chart of Nuclides 14
Typical natural radioactive decay A=Z+N 15
Radioactivity on Earth Slowly decaying (large half-life) radioactive isotopes are a constant heat supply for the Earth Important natural radioactive elements on Earth are U (92 p, 146 n) T½ ~ 4.5 billion years 235 U (92 p, 143 n) T½ ~ 0.7 billion years 40 K (19 p, 21 n) T½ ~ 1.25 billion years 232 Th (90 p, 142 n) T½ ~ 14 billion years They are still around today, after 4.6 billion years 238 What about 26Al, T1/2 ~ 0.7 million years? 16
2. Accretional Heat Accretional heating occurs in forming planets as a result of the transfer of kinetic energy of objects striking the surface of the proto-planet Once the planet is formed, it will start cooling down by slowly losing its accretional energy over geologic time 17
Nebular hypothesis Giant Molecular Cloud becomes gravitationally unstable formation of the proto-sun Infrared Remaining dust and grains grow to clumps (D ~10 m) 1 2 KE= mv 2 Clumps grow into planetesimals (D ~5 km) Planetesimals grow into planets Tremendous amount of energy is released when planetesimals run into each other ACCRETION 18
How much energy is in an impactor? Let s consider an impactor with radius ~50 km that collides with Earth at 20 km/sec How much energy will it release? Assume: Density 3000 kg/m3 Mass = Density Volume ( 4π 3 V sphere = R 3 ) KE = 3.1 1026 J Convert (J) to TNT using 1 Mton TNT (trinitrotoluene) = 4.184 1015 J E (Megaton TNT) =??? How many Tsar Bombe (50 MT)? 19
Accretion We still see the evidence of these early, huge collisions on the surface of the Moon Manicouagan, 100 km Meteor Crater, 1.2 km There are a few craters on the Earth s surface as well Why so few? 20
3. Internal Energy from Differentiation Early Earth heats up due to radioactive decay and impacts Enough energy is quickly accumulated that most of Earth becomes molten Iron (dense!) from impactors follows gravity and accumulate towards the core Lighter materials, such as silicate minerals, migrate upwards in exchange Result: A differentiated Earth & release of energy! 21
Internal Heat Summary Radioactive decay, accretion, and sinking of heavy metals provide energy in the Earth s interior (internal energy) Internal energy is the driver of volcanism, earthquakes, and plate tectonics in general Tectonics constantly brings fresh rocks (H,O,N,P,S) and volcanic gases to the surface where they can react with chemicals in the ocean releasing energy for life Hydrothermal vents Tectonics also participates in regulating climate 22
Earth, Mars, Venus: All have adequate sources of energy for photosynthesis Probably had similar delivery of organic molecules by comets and asteroids Why do we see life only on Earth? 23
Need for a Liquid Living systems need a medium in which molecules can dissolve and chemical reactions can take place In any living system, H2O: Dissolves organic molecules (hydrogen bond) Transports chemicals in and out of the cell Directly participates in metabolic reactions CO2 + H2O + energy CH2O + O2 Why water? 24
Elements in the Universe (by weight) Element H2O and NH3 are polar Parts per million Hydrogen 750,000 Helium 230,000 Oxygen 10,000 Carbon 5,000 Neon 1,300 Iron 1,100 Nitrogen 1,000 Silicon 700 Magnesium 600 Sulfur 500 All Others 500 Water, H2O Ammonia, NH3 O H H N H H H Methane, CH4 H C H Ethane, C2H6 H H H H H C H C H 25
Why water? Water is liquid over a broader range of temperatures and within Earth s surface temperature range a) Broader temperature range water stays liquid through climate changes b) Higher temperature range water allows faster rates of chemical reactions, but not hot enough to break important carbon bonds Other substances are liquid at temperatures that are problematic for biochemical reactions 26
Three states of water On Earth water can be present in all three states (phases): ice (solid), liquid water (liquid), water vapor (gas) Pressure and Temperature control which phase is the dominant in a particular planetary environment We have discussed Temperature What about Pressure? 27
What is Pressure? Pressure is a force applied over an area of a surface in the direction perpendicular to that surface F P= A where F = force; A = area SI Units: 1 Pa = 1 N/m2 28
lin Bo i ing Freez g Phase Diagram Saturation (Phase) Curve: line along which two phases are in equilibrium (liquid to vapor Condensation = Evaporation) Triple Point: temperature and pressure at which three phases (gas, liquid, and solid) of a given substance can coexist in thermodynamic equilibrium Critical Point: liquid and vapor phase cease to exist 29
Temperature and Pressure specify the phase of any substance 1 2 3 Conditions (1) solid phase Conditions (2) liquid phase Conditions (3) gas phase We can make a liquid boil by either: a) increasing temperature (at constant pressure) or b) decreasing pressure (at constant temperature) 30
H2O Earth Mars It is not possible to get liquid water to be stable below 0.006 atm pressure (average surface pressure on Mars is 0.007 atm) CO2 It is not possible to get liquid CO2 at 1 atm pressure (surface pressure on Earth) dry ice 1 atm 31
Any other advantage of water over other liquids? 32
Another H2O advantage: Ice floats! Most substances are denser as solids than as liquids Ice is less dense than liquid water, which is why ice floats The ice crust acts as a blanket, decreasing heat escape from the liquid water body below Lakes and oceans do not freeze out completely! Life can survive glaciations 33