Chapter 4: The Habitability of Earth
Chapter 4: The Habitability of Earth ASTR 115
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Chapter 4 The Habitability of Earth Vocab Radioactive Nuclei of an atom is unstable and will change into other isotopes over time Fossils Relics of organisms that lived and died long ago Feedback Process A change in one property either ampli es or counteracts the rest of they system Geology and Life Habitability on Earth is because of Earth s size and distance from the Sun Geology is the study of the Earth or the study of any world with a solid surface Over a long time life and geology is tied together Earth s geology has allowed for the process of evolution 3 Aspects of Earth s Geology 0 1 Volcanism Volcanic activity releases gases into Earth s atmosphere and were the original source or the atmosphere and ocean Also creates heat and chemical environments 0 2 Plate Tectonics Earth s surface has been formed by the movements of large sections of rocks called plate tectonics Gradually rearranges the continents Responsible for keeping the climate stable 0 3 Earth s Magnetic Field The magnetic eld keeps our atmosphere from degrading through solar wind Some animals use it as a reference Important to understand these 3 things if we want to nd life elsewhere Reconstructing the History of the Earth Humans have only been around for a few thousand years out of Earth s 45 billion years We reconstruct history using rocks and fossils Solid objects are able to offer insight into the past because they preserve information about the time they formed Must look at rocks and fossils that form the geological record 0 A record that refers to the relics of life Types of Rocks Rocks are classi ed into 3 types 0 1 Igneous Rocks Rock made from molten rock that cools and solidi es o 2 Metamorphic Rocks Rock that has been changed either chemically or structurally due to high pressure or heat o 3 Sedimentary Rocks Rock made from gradual compression of other sediment Rocks can change what type they are through different processes Because they are able to transform the composition doesn t tell us much The transformation between the three types of rocks is called the rock cycle graph below gtj hE JUE Firsqu 7 FEi nal TAT Types of Rocks A mineral is used to describe the crystal of a speci c chemical composition Over 4300 types of minerals So many that we usually group them by their primary constituents 0 Eg Silicates contain silicon and oxygen 0 Eg Carbonates contain carbon and oxygen The type of rock tells us how it was made while the mineral tells us what it is made of We also have subcategories to further classify rocks Two subtypes of igneous rock form the planets crust o Basalt Dark rock produced by undersea volcanoes with lots of iron and magnesium based silicate material 0 Granite Lighter less dense rock common in mountains Sedimentary Strata Sedimentary rock is important for 2 reasons 0 1 Most fossils are found there 0 2 They form in a way that keeps a record of time Sediments pile of from different time periods and can tell us what happened in the world during that time Distinct layers in sedimentary rock are called strata 0 We can view these layers when rocks become worn down over millions of years 0 More deeply buried layers are older Helps us identify animals and environments in certain time pedods Rock Analysis We can normally tell what a type a rock is from its appearance 3 types of analysis are important to nd out a rock s history 0 1 Mineralogical Analysis Identifying the minerals in a rock Tells us about temperature and pressure it formed at o 2 Chemical Analysis Determining the elemental and chemical composition Tells us what it is made of o 3 lsotopic Analysis Determining the ratio of different isotopes of elements in a rock Can tell us if the ratios are different from what we would expect Also allows us to see if the rock is radioactive Can also be applied to fossils and other solids Learning the Ages of Rocks and Fossils Radiometric Dating Measuring an objects proportions of various atoms and isotopes Radioactive decay is when a nucleus of a radioactive isotope undergoes spontaneous change such as breaking down 0 Eg Carbon 14 breaks down into carbon 13 or 12 The original nucleus is the parent nucleus and the changed is the daughter Alpha decay is when a large atom ejects a helium nucleus Beta decay is when an atom emits an electron Electron capture is when an electron is captured by the nucleus and combines with a neutron to form a proton This decay happens over timed interval so to measure the time we look at the parent to daughter ratio of isotopes in a given sample The Probability of Radioactive Decay We are able to give a probability to the process but we can t exactly determine it We can predict an amount of expected radioactive isotopes in a sample HalfLife Concept We normally describe the decay rate of a substance by its halflife o The time it takes for half of the atoms to decay If we are con dent that a rock sample had 0 of the daughter atom present then we can simply take the halflife and work backwards to gure out how old the rock is Radiometric dating is similar in that we compare the ratio of parent to daughter isotopes and then use the decay rate to gure out how old the rock is The halflives of different elements vary greatly so we use certain elements to measure certain time periods Biggest issue with radiometric dating is knowing the rocks original composition 0 We must know if there was originally some of the daughter isotope present Scientists can normally gure out the original composition however if melting or major shock during the rocks life occurred then there is the possibility that our estimate of the rocks age is wrong This process only works with solids that have been unchanged since their formation To measure sedimentary rocks we have to look at the igneous rocks around them Reliability of Radiometric Ages Radiometric dating is highly reliable Scientists can use multiple isotopes in a single sample to get an accurate time prediction Usually a 12 uncertainty Geological Record The further into the past we look the more uncertainty there is Fossil Formation A fossil is any evidence of past life 0 Many contain no organic matter Minerals gradually replace the organic material 0 Bones teeth and shells are left behind because they are mineralrich Sometimes organic material can be preserved and studied even though the organism has been dead for millions of years 0 Preserved in tree resin and frozen in ice Majority of organisms never become fossils and instead decay into the Ea h Fossils become rarer as we look further into the past for 2 major reasons 0 1 Fossils can suffer the same fate as rock so they can be destroyed o 2 Most of early life was microscopic which means their fossils must be microscopic Geological Time Scale Scientists have divided Earth s history in distinct time intervals which form the geological time scale First major division is set into 4 eons o Hadean Archean Proterozoic Phanerozoic Phanerozoic 542 MYA Present 0 Marks the time period that we have evidence for visible life Proterozoic 25 BYA 542 MYA 0 Contains fossils of single cell organisms simple life Archean 385 BYA 25 BYA o Called ancient life after the discovery of fossils from the rst half of Earth s history Hadean 45 BYA 385 BYA o Called hellish Earth because nothing could survive in the environment More recent time periods have a richer geological record The Phanerozoic eon is divided into 3 subsections called eras o Paleozoic Mesozoic and Cenozoic 0 These mean old lifequot middle lifequot and quotrecent lifequot These 3 eras are then further divided into sections called periods These periods follow no consistent naming scheme The Age of Earth Oldest intact rock is 402 billion years old We study tiny mineral grains of zirconium silicate or zircons These are found in young sedimentary rock however some solidi ed as old as 438 BYA By looking at the oxygen isotope scientists have been able to determine that liquid water and continents were on the surface of the Ea h Also moon rocks tell us that Earth is over 44 billion years old To set a maximum age for the Earth we measure the age of meteorites that have fallen to Earth 0 These rocks date back to about 457 BYA In conclusion scientists determined that Earth and its moon were formed by 45 BYA Hadean Earth and the Dawn of Life Before life could survive Earth needed an atmosphere and liquid water In the beginning Earth was too small and ward to capture enough gases Other planetesimals with ice from the asteroid belt collided with Earth Gas from these planetesimals were trapped under Earth s surface When volcanoes erupted gases trapped in the Earth shot out o This process is called outgassing This released water vapor that eventually condensed to form our oceans Impacts can also result in outgassing as melted rock allows gas to escape C02 N2 and H2 were primarily the gases that made up Earth s atmosphere Zircons suggest that the Earth s oceans had formed about 44 BYA The early atmosphere was very different from today as there was no oxygen Could Life Have Existed During Earth s Early History Earth may have been habitable within 100 million years of its formation Early in the solar system many planetesimals were crashing into the young planets in a time called the heavy bombardment Impact craters were left from these massive collisions Earth probably had more collisions than the moon did because of its larger mass 0 Earth s collisions have mostly been washed away by erosion and volcanoes 0 To learn we then study the moon Lunar highlands are the most heavily cratered regions 0 These rocks tend to be older than other lunar rocks Lunar maria are regions with few or no craters 0 These exist probably from molten lava covering the impacts 0 Impacts after the lava ow are very rare There is evidence that there was a late heavy bombardment where there was a spike in impacts around 41 billion years ago and ending 38 billion years ago 0 Some scientists think this occurred because the young planets had orbital changes Allows us to apply our knowledge of craters to objects in the solar system 0 Eg Planets with lots of craters have undergone little change since they formed Large Impacts and Early Life Life could have arisen in the Hadean period because the impacts were still separated by a few thousand years Scientists simulate impacts and measure how much energy an impact would give off Large impacts would vaporize the oceans and melt the crust 0 These are sterilizing impacts because they would have killed all life Geology Despite the massive impacts that occurred on Earth s surface the impacts had little effect on shaping the geology of Earth 3 main factors that shape geology are volcanism plate tectonics and erosion Volcanism and plate tectonics have also helped life survive over time What is Earth Like on the Inside We learn about the interior of Earth by studying seismic waves 0 Waves that travel through Earth s interior and along the surface after an earthquake We can learn about the composition density pressure temperature and phase of the material by tracking how the wave moves Earth s interior Structure Earth is divided into 3 major sections based on density 1 Core 0 Located at the center of the Earth and is the densest layer containing mostly iron and nickel o Divided into the inner core solid and outer core molten 2ManUe o Moderately dense with mostly silicate minerals o Thickest layer that makes up most of Earth s volume 3 Crust 0 Lowest density that contains igneous rocks like granite and basa 0 Forms a thin outer skin around Earth Most of Earth s interior is solid rock Rock is not completely solid and sometimes can ow given enough pressure or temperature Lithosphere is Earth s outermost layer of rigid cool rock that oats on warmer rock beneath o It covers the crust and upper part of the mantle The mantle ows in a pattern called convection 0 Hot material rises and then cools and falls 0 When the material falls it is then heated again by the core and continues to cycle Differentiation and Internal Heat Earth underwent a process known as differentiation 0 The materials separated based on density The impact that created the Moon occurred after differentiation For differentiation to occur the Earth must have been almost all molten when it was rst formed The heat the melted the rock came from 3 main sources 0 1 Impacts of accretion melted outer layers 0 2 The gravitational potential energy of the denser layer sinking turned into thermal radiation 0 3 Radioactive decay released heat All early planets in our solar system underwent this process but they cool at different rates 2 things determine a planets ability to cool 0 1 The size large planets stay hotter longer 0 2 The ongoing heat deposition Planets with a constant heating source will stay hot Plate Tectonics Earth is the only planet with on going plate tectonics Tectonics means any surface being reshaped due to stretching or other forces Fractured pieces of the lithosphere move due to mantle convection Evidence for Plate Tectonics 3 main lines of evidence that support plate tectonics o 1 Evidence of past continental arrangements 0 2 Evidence that plates spread apart on sea oors o 3 Difference between the Earth s crust on continents and sea oors Alfred Wegner proposed an idea of continental drift 0 The idea that planets slowly drift across the ocean o The idea was rejected because there was no evidence People began to look at continental motion from mid ocean ridges Sea oor spreading explains how continents move apart Earth is made of 2 distinct types of crust o 1 Sea oor Crust High density igneous rock called basalt Young crust o 2 Continental Crust Lower density rock like granite Can be old or young and is much thicker than sea oor crust Mechanism of Plate Tectonics Plates move due to underlying mantle convection Molten rock erupts from sea oor ridges and cools to form new sea oor Crust eventually gets recycled back into the mantle through subduc on Volcanoes tend to be located on mountain ranges near the coast where the mantle is releasing hot magma Plates can also crash into each other and push up to form mountain ranges Faults are plate boundaries where the plates are sliding against each other 0 This causes earthquakes Some earthquakes occur far from the plate where old boundaries were Volcanoes can also exist on hotspots where there is a week section of the crust 0 Eg Hawaiian Islands 0 Older islands of hotspots get eroded by the ocean and currents Plate Tectonics Over Time We can predict where continents will be in the future Earth s continents were once together and formed one big land mass called Pangea Why Does Earth Have Plate Tectonics Moon mars and Mercury are to small so they probably cooled to fast We don t know why Venus doesn t have plate tectonics or if it ever had them 0 Thought that it was because the high heat baked out the water and mantle o It caused the lithosphere to become rigid and not break into plates Earth s Magnetic Field Atmospheric gas can be released into space in 3 ways 0 1 Gas molecules move fast enough that they escape the atmosphere Called Thermal Escape Particles move faster when heated Lightweight particles also move faster than heavier ones 0 2 Impacts can blast atmospheric gas into space More likely in smaller worlds 0 3 Gas can be lost by solar wind Particles from the sun sweep gas from the atmosphere into space Requirements for a Global Magnetic Field Magnetic elds can affect charge particles near them Liquid metal in the outer core moves because of convection and the charged particles create a magnetic eld 3 basic requirements for a planet to have a magnetic eld 0 1 Interior are with electrically conducting uid 0 2 Convection in that layer 0 3 Moderately rapid rotation of the planet Earth meets all three which is why it is the only planet in our solar system with all 3 The Magnetosphere and Solar Wind Magnetosphere is a protective sphere that de ects most solar wind particles Some particles de ected towards poles where they form auroras Prevents Earth s atmosphere from being stripped away Climate and Regulation Change Earth s atmosphere slightly uctuates over time 0 But compared to other planets Earth is very stable Greenhouse Effect Global average temperature is the average temperature of the whole planet 0 Expected would be 16C but actual is 15C This change in temperature is due to the greenhouse effect Light and infrared waves come into our atmosphere and are absorbed by the ground The ground then heats up and releases some of the energy back into the atmosphere Gases trap some of the energy released by the ground and heat up our planet 0 These gases that absorb infrared light are called greenhouse gases Greenhouse gases slow the escape of the radiation and make the lower atmosphere hotter The more greenhouse gases the more warming there is Global Warming is due to humans increasing the amount of greenhouse gases in our atmosphere What Regulates Earth s Climate Most of Earth s carbon is locked in rocks which is important for not overheating our planet Carbon Dioxide Cyle The process by which carbon dioxide is removed from the atmosphere while keeping the atmosphere stable The cycle s process 1 Atmospheric C02 is dissolved in rain 2 Rain falls to Earth and the C02 is broken down into materials 3 The ocean breaks it down into carbonate rocks 4 Plate tectonics carry the rocks to subduction zones to be recycled o 5 As they are heated up some release C02 back into the atmosphere C02 Cycle as a Thermostat Built in negative feedback loop which will always turn Earth s temperature back towards normal The higher the temperature the more C02 is removed from the atmosphere Earth s Climate Over Time Ice Ages are when the global average temperature drops multiple degrees 0 This increases snow and ice on the planet Tilt change of Earth causes more extreme seasons Snowball Earth Periods in Earth s history when it was completely covered in ice 0 C02 cycle prevents Earth from staying in snowball periods Formation of the Moon 3 models about how the Moon was formed 0 1 The Moon formed alongside the Earth through accretion o 2 The Moon was captured by Earth 0 3 Earth split into two pieces All three models had little evidence and were all rejected When Apollo mission brought back rock samples from the moon all three were con rmed to be wrong Giant Impact Model 2 pieces of evidence supported this 0 1 Moon s average density is less than Earth s O O O O o 2 Composition of Moon s surface is similar to that of Earth s mantle A Mars sized object crashed into Earth and a small chunk ew off and became the Moon