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Phys 214. Planets and Life Dr. Cristina Buzea Department of Physics Room 259 E-mail: [email protected] (Please use PHYS214 in e-mail subject) Lecture 11. Geology and life Part 2 (Page 124-145) January 30 Contents Textbook: Pages 124-145 Plate tectonics Retention of planetary atmospheres The magnetic field of Earth Climate regulation and change The carbon dioxide cycle, ice ages & snowball Earth Acknowledgments: Images from NASA, ESA, Hubble Plate tectonics The Earth’s lithosphere has broken up into about dozen plates because of mantle convection. Plates - moving a few cm per year (fingernail growth) Evidence – GPS measurements, evidence of past continental drift (puzzles), seafloor spreading, difference between the nature of crust on the seafloors and the continents. Plate tectonics Seafloor crust - made of high-density igneous rock (basalt) , 5-10 km thick, quite young (average 70 millions years) Continental crust – lower density rock (granite), much thicker 2070 km, wide range of ages Seafloor spreading on the Earth is the process by which molten lava comes out between two tectonic plates that are slowly moving apart. Plate tectonics Subduction on the Earth is the process by which oceanic crust sinks below continental crust at a tectonic plate boundary. Plate tectonics Two continental plates collide into each other – creating mountains (the Himalayas) The Andes were formed by pieces of oceanic and continental crust colliding. Plate tectonics The East African rift zone was formed by two pieces of continental crust separating. Plate boundary faults occur where plates are sliding past each other. (San Andreas fault between the Pacific plate and North American plate). Los Angeles and San Francisco bay – 20 million years. Earthquakes several meters in a few seconds. Not all earthquakes occur near plate boundaries. Plate tectonics Volcanic activity may arise where hot mantle material rises up to make a hot spot. The Hawaiian Islands were formed when volcanic material flowed out from the interior at a hot spot beneath the Pacific Ocean. Plate tectonics carries the pacific plate over the hot spot, forming a chain of volcanic islands. Continental hot spots – geysers and hot springs. Plate tectonics through times 150 million years ago Earth’s map would be quite different because plate tectonics would have caused a shift in the position of the continents. Plate shift of 2 cm/year = >200 km in 100 million years 225 million years ago – a single continent - Pangea Plate tectonics in the solar system Earth the only planet with ongoing plate tectonics! Moon, Mercury, and Mars do not have plate tectonics - they have cooled so rapidly that they do not have enough internal convection to break up their thick crusts. Even though Venus is almost the same size as the Earth it does not have plate tectonics. Probably because due to the high temperatures at its surface, its crust has dried up and thickened so it cannot break up into plates. Retention of planetary atmospheres Not in the textbook! If gas molecules move fast enough and exceed the escape velocity – they escape into space. Escape velocity = the minimum speed needed before a body has enough kinetic energy to escape from the surface of a planet (overcome gravitational field). 2 mvesc mM Ec = E p , =G 2 r • The escape velocity from a planet of mass M and distance R is ! vesc 2GM = R where G – gravitational constant of the planet The extent of thermal escape in a planetary atmosphere is dependent on the temperature (T) and molecular weight of the gases. The gas escapes if k BT = 2 mvthermal 2 2 mvesc 2k BT " # vesc < 2 m kB - Boltzmann constant, T - temperature, m - molecular mass of gas ! Retention of planetary atmospheres Not in the textbook! • Different gases have different molecular masses – their average speeds are different at a given temperature. • For a planetary body to retain a particular gas in its atmosphere for a period of time = the age of the solar system, the average speed of the molecules in the gas should be less than 1/6 of the escape velocity. Retention of planetary atmosphere Volcanically active planets and satellites How to loose a planetary atmosphere? • • • 1. Thermal escape 2. Impacts 3. Solar wind stripping Smaller worlds are more prone to atmospheric loss via impacts than large worlds because they have weaker gravities and smaller escape velocities. The sweeping of atmospheric gas particles into space by the Sun is referred to as solar wind stripping. The magnetic field of Earth In order to have a global magnetic field, a planet must have: 1. an electrically conducting fluid in its interior 2. which is undergoing convection 3. and a reasonably rapid rotation. The magnetosphere and the solar wind The Earth’s magnetosphere is a cavity carved out in the solar wind by the Earth’s magnetic field. The magnetosphere deflects most of solar wind particles while chaneling a few towards the poles – auroras; charges particles trapped within are forming Van Allen belts. The magnetosphere protects the atmosphere, preventing the solar wind from stripping it away. Requirements for a global magnetic field Only Earth and Mercury among terrestrial planets have magnetic fields. Mercury –slight enigma – small & has a slow rotation (1 rotation in 59 Earth days), but a very large metal core. Mars & Moon have no magnetic field probably because of core solidification. Mars lost much of its atmosphere when its interior cooled. Venus rotates too slow (one rotation in 243-day Earth days). Climate regulation and change • Long-term habitability – volcanism, plate tectonics, magnetic field. • • • The climate on earth has been sufficiently stable to exist continually for nearly 4 billion years. Life on Earth needs liquid water => oceans should be at least partially liquid Temperature range for water to be liquid may seem wide to humans, but compared to temperatures on other worlds -> Earth’s climate remarkably stable • Why Earth long-term climate is stable and warm enough for water to be liquid? Climate regulation The right distance from the Sun is not enough for a body to have liquid water Moon – daytime 125oC (above boiling point of water), night-time temperature -175oC. The Sun gradually has brighten with age (30% brighter today than when Earth formed) Earth average temperature based solely on its distance from the Sun = -1oC. The actual global average temperature today = 15oC. The greenhouse effect = the trapping of infrared radiation from the Earth’s surface by greenhouse gases like carbon dioxide, methane, and water. Without the greenhouse effect, the oceans on the Earth’s surface would be frozen The greenhouse effect Mechanism: trapping some of the infrared light emitted by the surface Greenhouse gases = some atmospheric gasses can absorb infrared light; water vapors H2O, carbon dioxide CO2, methane CH4. Diatom molecules like nitrogen N2, oxygen O2 do not absorb infrared. After a greenhouse molecule absorbs an infrared photon -> emits a new infrared photon in a random direction It slows the escape of infrared radiation from lower atmosphere – making it warmer – like a blanket. Venus – the greenhouse effect The greenhouse effect is not intrinsically bad – as portrayed in the news. Human activity may add too much greenhouse gases in the atmosphere – global warming The greenhouse effect on VENUS is responsible for the searing 470oC temperature! CO2 is less than 1% of Earth atmosphere, more than 96% of Venus atmosphere! Venus and Earth are nearly the same size – volcanic outgassing probably released the same amount of CO2 What happened with the CO2 on Earth? What regulates Earth’s climate? Most of the Earth’s carbon dioxide has dissolved in the oceans and is locked up in sedimentary carbonate rocks like limestone (containing about 170,000 times as much CO2 as our atmosphere). The carbon dioxide cycle regulates the surface temperature by varying the amount of carbon dioxide in the atmosphere. - Atmospheric CO2 dissolves in rainwater – mild acid - The acid rain erodes rocks, rivers carry the minerals to the oceans - In the oceans, calcium combines with dissolved CO2 and falls to the ocean floor making carbonate minerals – accumulate as limestone - Plate tectonics carry carbonate rocks to subduction zones into the mantle; - Some melt and release CO2 through volcanoes The carbon dioxide cycle as a thermostat If the temperature of the Earth warms up - the CO2 cycle speeds up the formation of carbonate minerals in the oceans, thus pulling more carbon dioxide out of the atmosphere. If the temperature of the Earth cools - the CO2 cycle slows the formation of carbonate minerals in the oceans, thus pulling less carbon dioxide out of the atmosphere. The CO2 cycle cannot easily correct the CO2 due to industrialization because it operates far too slowly to correct for any short-term changes (the feedback mechanism takes about 400,000 years). How did Earth’s climate change over the time? Ice ages = mild, short-term cooling periods during the Earth’s (global average temperature drops by a few degrees) During the past few million years, ice ages were MOST LIKELY caused by small changes in the Earth’s axis tilt. On long time scale - the sun brightening and the migration of continents influenced the climate On shorter time scale – small, cyclical changes in earth’s rotation and orbit – making season more or less extreme Greater tilt = more extreme seasons, warmer summers and cooler winters,prevents ice from building up – warmer planet Smaller tilt = the opposite Snowball Earth Long and deep ice ages between 750-580 million years ago, 2.4-2.2 billion years ago Glaciers advanced to the equator, oceans freezing worldwide 90% sunlight reflected by ice compared to 5% by water = > surface cooled more The CO2 cycle stops -> CO2 outgassed by volcanoes – heats the Earth Global average temperature –50oC!! Snowball Earth = severe, long-term cooling periods during the Earth’s history. The Earth is able to recover from a snowball phase by carbon dioxide gradually building up in the atmosphere from volcanism The end of the last snowball Earth phase roughly coincides with the beginning of the Cambrian period which produced an explosion in diversity of life. Earth’s long term habitability Major factors that kept earth habitable for the past 4 billion years • Volcanic outgassing (gases and water) • Protective effect of its magnetic field • The greenhouse effect warmed the planet for the water to be liquid • The moderate greenhouse effect is maintained by the self-regulating CO2 cycle • Earth axis tilt and rotation influences its climate • The climate regulation is not perfect, leading sometimes to snowball earth The formation of the Moon Models: 1. Moon formed along with Earth during accretion (calculations did not support this model; the Moon average density is much lower than Earth’s) 2. Moon has been an independent planet captured into Earth’s orbit (improbable to loose its orbital energy; only likely with small bodies – friction with gas surrounding the planet Mars) 3. Young molten Earth spinning so fast it split into 2 pieces (improbable to spin so fast) 4. Impact with a body the size of Mars The formation of the Moon • • The currently accepted theory for the formation of the Moon is the giant impact model in which material was blasted from the Earth after an impact with a body the size of the planet Mars The strongest of evidence that supports the giant impact model for the formation of the Moon is that the overall composition of Moon rocks returned from the Apollo program is similar to the composition of Earth’s mantle material, except for the lack of volatile elements Next lecture • Chapter 5. The nature of life on Earth