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Geological Time Scale & Global Properties ASTR 4: Life in the Universe Outline • • • • Radiometric Dating Global Properties Geologic Time Scale & Evolution of Life Tree of Life Radiometric Dating • Isotopes which are unstable are said to be radioactive. • They spontaneously change in to another isotope in a process called radioactive decay. – protons convert to neutrons – neutrons convert to protons • The time it takes half the amount of a radioactive isotope to decay is called its half life. • By knowing rock chemistry, we chose a stable isotope which does not form with the rock…its presence is due solely to decay. • Measuring the relative amounts of the two isotopes and knowing the half life of the radioactive isotope tells us the age of the rock. The Age of our Solar System • Radiometric dating can only measure the age of a rock since it solidified. • Geologic processes on Earth cause rock to melt and resolidify. Earth rocks can’t be used to measure the Solar System’s age. • We must find rocks which have not melted or vaporized since the condensed from the Solar nebula. – meteorites imply an age of 4.6 billion years for Solar System • Radioactive isotopes are formed in stars & supernovae – suggests that Solar System formation was triggered by supernova – short half lives suggest the supernova was nearby Inside the Terrestrial Worlds • After they have formed, the molten planets differentiate into three zones: • core - made of metals • mantle - made of dense rock • crust - made of less dense rock • Lithosphere - the rigid, outer layer of crust & part of the mantle which does not deform easily Inside the Terrestrial Worlds active geology inactive geology Heating the Terrestrial Worlds • Planetary interiors heat up through: • accretion • differentiation Supplies all the heat at the beginning • radioactivity Supplies heat throughout the planet’s life Cooling the Terrestrial Worlds • Planets cool off through: • conduction - heat flowing on the microscopic level • convection - heat flowing on the macroscopic level (bulk motions) • eruptions - hot lava bursts through crust • the larger the planet, the longer it takes to cool off! Cooling the Terrestrial Worlds Magnetic Fields • Electric charges moving via convection in a molten iron core and spinning acts like an electromagnet magnetic field • Earth has a magnetic field • Venus, Mars, & the Moon do not • Mercury surprisingly has a weak magnetic field ?? Shaping Planetary Surfaces • Major geological processes that shape planetary surfaces: • impact cratering: excavation of surface by asteroids or comets striking the planet • volcanism: eruption of lava from interior • tectonics: disruption of lithosphere by internal stresses • erosion: wearing down by wind, water, ice Impact Cratering • objects hit planet at 10 – 70 km/s • solid rock is vaporized • a crater is excavated • matter is ejected in all directions • craters are circular – large craters have a central peak Counting Craters to find Surface Age • Cratering rate decreased as Solar Systems aged. • The older the surface, the more craters are present. Volcanism • Underground, molten rock, called magma, breaks through crack in the lithosphere. • Trapped gases are released: • H2O, CO2, N2 • Viscosity of lava (typically basalt) determines type of volcano Tectonics & Erosion • convection cells in the mantle causes both: • compression in lithosphere • mountains are produced • extension in lithosphere • valleys are produced • mountains & valleys appear on the surface • movement of rock by ice, liquid, or gas • valleys shaped by glaciers • canyons carved by rivers • sand blown by wind • erosion not only wears down features, it also builds them: • sand dunes • river deltas • sedimentary rock Atmosphere • A layer of gas which surrounds a world is called an atmosphere. • they are usually very thin compared to planet radius • Pressure is created by atomic & molecular collisions in an atmosphere. • heating a gas in a confined space increases pressure • number of collisions increase • unit of measure: 1 bar = 14.7 lbs/inch2 = Earth’s atmospheric pressure at sea level • Pressure balances gravity in an atmosphere. Effects of an Atmosphere on a Planet • greenhouse effect • makes the planetary surface warmer than it would be otherwise • scattering and absorption of light • absorb high-energy radiation from the Sun • scattering of optical light brightens the daytime sky • creates pressure • can allow water to exist as a liquid (at the right temperature) • creates wind and weather • promotes erosion of the planetary surface • creates auroras • interaction with the Solar wind when magnetic fields are present Planetary Energy Balance • Solar energy received by a planet must balance the energy it returns to space • planet can either reflect or emit the energy as radiation • this is necessary for the planet to have a stable temperature What Determines a Planet’s Surface Temperature? • Greenhouse Effect cannot change incoming Sunlight, so it cannot change the total energy returned to space. • it increases the energy (heat) in lower atmosphere • it works like a blanket • In the absence of the Greenhouse Effect, what would determine a planet’s surface temperature? • the planet's distance from the Sun • the planet’s overall reflectivity • the higher the albedo, the less light absorbed, planet cooler • Earth’s average temperature would be –17º C (–1º F) without the Greenhouse Effect Magnetospheres • The Sun ejects a stream of charged particles, called the solar wind. • it is mostly electrons, protons, and Helium nuclei • Earth’s magnetic field attracts and diverts these charged particles to its magnetic poles. • the particles spiral along magnetic field lines and emit light • this causes the aurora (aka northern & southern lights) • this protective “bubble” is called the magnetosphere Earth’s Magnetosphere Weather and Climate weather – short-term changes in wind, clouds, temperature, and pressure in an atmosphere at a given location climate – long-term average of the weather at a given location • These are Earth’s global wind patterns or circulation • local weather systems move along with them • weather moves from W to E at mid-latitudes in N hemisphere • Two factors cause these patterns • atmospheric heating • planetary rotation Four Major Factors which affect Longterm Climate Change Gain/Loss Processes of Atmospheric Gas • Unlike the Jovian planets, the terrestrials were too small to capture significant gas from the Solar nebula. • what gas they did capture was H & He, and it escaped • present-day atmospheres must have formed at a later time • Sources of atmospheric gas: • outgassing – release of gas trapped in interior rock by volcanism • evaporation/sublimation – surface liquids or ices turn to gas when heated • bombardment – micrometeorites, Solar wind particles, or highenergy photons blast atoms/molecules out of surface rock • occurs only if the planet has no substantial atmosphere already Gain/Loss Processes of Atmospheric Gas • Ways to lose atmospheric gas: • condensation – gas turns into liquids or ices on the surface when cooled • chemical reactions – gas is bound into surface rocks or liquids • stripping – gas is knocked out of the upper atmosphere by Solar wind particles • impacts – a comet/asteroid collision with a planet can blast atmospheric gas into space • thermal escape – lightweight gas molecules are lost to space when they achieve escape velocity gas is lost forever! Tree of Life • Three Domains of Life – Prokaryotes (without nucleus) • Archaea • Bacteria – Eukaryotes (with nucleus) • Eucarya • Phylogenetic Tree of Life – Carl R. Woese, 1977 – 16S ribosomal RNA
