An Evolving Image of Earth

Transcription

1 Starting from Scratch With the assistance of observational and theoretical astronomy, and by studying meteorites, geologists have developed hypotheses about the origin of our solar system and the Earth An Evolving Image of Earth Ancients thought the universe was geocentric. Heavenly traced circular orbits around a flat central Earth. Ptolemy ( C.E.) was held to have proven geocentricity. This idea became doctrine, and Earth, the center of creation. Geocentricity held for 1000 years during the dark ages. A heliocentric (sun-centered) universe was proposed by the Greeks ~ 250 B.C.E. Earth s Rotation and Shape Earth s rotation axis centers on the star Polaris. Visible in a time-lapse photograph of the night sky. Foucault s pendulum experiment revealed rotation in The flat Earth concept dispelled by 250 B.C.E. Evidence suggested a spherical Earth. Curved shadow on the Moon during eclipses. Only the top of distant sailing ships was visible. Earth s Circumference Eratosthenes calculated the circumference of the Earth at 24,421 miles in ~ 200 B.C. In Alexandria, he noted the sun angle the same day there was no shadow in a deep well in Syene. He calculated that the distance between the two locations was 1/50th of Earth s circumference. He was correct!! 1

2 The Renaissance Revival of reason in 15 th century Europe brought new ideas. Copernicus Published evidence for heliocentricity. Kepler Calculated elliptical planetary orbits. Galileo Observed moons orbiting Jupiter. Contradicting geocentricity, these advances were heretical. The Earth wasn t the center of the Universe. Planetary orbits weren t circular (perfect). Not all heavenly bodies orbited Earth. Isaac Newton Devised Law of Universal Gravitation. Three Laws of Motion. The mathematics of change (calculus). Showed that natural laws govern events. Modern View of the Universe Earth is one of nine planets in the solar system. The Sun is a star on an arm of the Milky Way galaxy. The Milky Way contains more than 300 billion stars. The Universe contains more than a billion galaxies. The closest galaxy is 2.2 million light years from us. Distances in the Universe Immense expanses of space and time. The speed of light (c) is 186,000 mi/s (300,000 km/s). The Moon is 1.3 light seconds (~237,000 miles) away. The Sun is 8.3 light minutes (93 million miles) away. In 1 year, light travels 5.87 trillion miles (9.5 trillion km) A light year, therefore, is a measure of distance. Alpha Centauri, the closest star, is 4.3 light years away. The visible universe is > 13 billion light years away The dimensions of the Universe are staggering! The Doppler Effect The Doppler Effect describes the compression or relaxation of waves with relative motion. The stopped train sounds the same at A and B. The moving train sounds differently at A and B. Approaching train has a higher pitch (A). Receding train has a lower pitch (B). Pitch drops (higher to lower) as train passes. All have heard this as cars whiz by on a road. 2

3 Red Shift Visible light is electromagnetic radiation. Visible wavelengths range from 400 and 700 nanometers (nm or 10-9 m). 400 nm Violet - Blue = Higher frequency 700 nm Red = Lower frequency Red Shift As with the sound from a train, a Doppler shift alters light from a moving star as observed from Earth. Light from an approaching star is compressed causing a shift to higher frequencies (toward blue). Light from a receding star is expanded yielding a shift to lower frequencies (red shift). A larger degree of spectral shift attends a faster star. Spectral shifts require very fast motion. The Expanding Universe Astronomers observed light from galaxies was red shifted. In 1929, Hubble recognized the red shift as a Doppler Effect. He concluded galaxies were moving away at great speed. Curiously, no galaxies were found heading toward Earth. That all galaxies have a red shift led to the development of the Expanding Universe Theory. Big Bang The concept of an expanding Universe led to the question When did it all begin? The best answer so far: All of the mass and energy in the universe was packed into a single microscopic point. It exploded 13.7 Ga and has been expanding ever since. TIME 3

4 The Modern Universe Big Bang Big Bang Big Bang theory is rooted in the Laws of Physics. The Big Bang started a rapid cascade of events. Protons and neutrons formed within 1 second Hydrogen atoms formed within 3 minutes. Hydrogen atoms fused to form new elements (He, Be, Li, B) in the process called Big Bang Nucleosynthesis. With time, the Universe continued to Expand. Cool. Decrease in density. Big Bang With expansion and cooling, atoms started bonding. Hydrogen formed H 2 molecules; the fuel of stars. Atoms and molecules formed during Big Bang Nucleosynthesis coalesced into gaseous nebulae. Gravitational attraction caused gaseous nebular collapse, and... Increases in temperature, density and rate of rotation. Big Bang Condensed nebula formed flattened accretion discs. Continued nebular collapse led to such in increase in heat and mass that the protostar began to glow. Addition of mass led to initiation of nuclear fusion, igniting these newborn 1 st generation stars. The 1 st stars consumed their H 2 fuel rapidly, leading to... Collapse, heating and explosion as a supernova. 4

5 Eagle Nebula The Hubble Space Telescope has shown us some striking examples of nebulas in neighboring space--regions of active star formation. The Hubble is way past its life span. How to Make a Solar System, pt 1 Begin with a region of space that has a high proportion of gas and dust -- these are leftovers from earlier cosmic events Disturb this dusty region with something like a shock wave, perhaps from a nearby supernova: something to get the gas and dust to compress. How to Make a Solar System, pt 2 Gravity then takes over, with most of the matter in the region (which we call the presolar nebula) being compressed into the center of a rotating disk DR21 in constellation Cygnus (infrared image) NASA/JPL-Caltech Nebula are interstellar clouds of gas and dust and the birthplace of stars Stars are where heavy elements are made 5

6 How to Make a Solar System, pt 3 Dust in the disk starts to clump together, forming larger masses; meanwhile the central part of the nebula, where most of the mass is, starts to heat up The internal pressure in the center of the nebula eventually is great enough to initiate nuclear fusion: in this process light elements merge to form heavier elements, and a huge amount of energy is released (the familiar form to us is sunlight) Galactic Catastrophism We estimate that the entire Process of nebula formation and Star birth takes < 1 Myr. compare this to the ages of The Earth (~4.5 Byr) And the universe (~13 Byr) Fusion is how all of the heavy elements in nature are formed,although the most important reaction is where hydrogen atoms combine to form helium atoms Light Elements Periodic Table Heavy Elements 28 Si He 7( 43( C O 4 ( He) 1 + H) C-N-O O C He Ni 20 C Cycle Ne Mg S O + + energy + energy 56 Fe The End of the Line for Massive Stars Massive stars burn a succession of elements. Iron is the most stable element and cannot be fused further. Instead of releasing energy, it uses energy. 6

7 Supernova! Supernova Remnants: SN1987A a b c d a) Optical - Feb 2000 Illuminating material ejected from the star thousands of years before the SN b) Radio - Sep 1999 c) X-ray - Oct 1999 d) X-ray - Jan 2000 The shock wave from the SN heating the gas Supernova Remnants: Cas A Optical X-ray Elements from Supernovae All X-ray Energies Silicon Calcium Iron 7

8 Nebular Hypothesis Sol (our sun) Origin of the solar system is believed to have formed as a rotating cloud of gas and fine dust. As the dust clouds contract the pressure acting on the hydrogen and helium increases At sufficiently high pressures some of the hydrogen reacts to form helium - light is emitted The chemical elements are formed within the interiors of stars SOHO/Extreme Ultraviolet Imaging Telescope (EIT) consortium The Solar System The Solar System Condensed from Solar nebula about 5 billion years ago Contains Sun, planets, asteroids, comets, and dust Two main types of planets 1. Terrestrial 2. Jovian 8

9 Making Planets As the sun forms, the outer parts of the nebula have organized into the rotating protoplanetary disk Gas and dust in the disk rapidly clump together (mainly through a combination of graviational attraction and static forces) into planetesimals The clumps get bigger and bigger as each protoplanet sweeps material from its part of the disk As planetesimals grow in size, collisions can be destructive, but eventually plants grow large Based on meteorite ages, we think that building planets this way would take < 100 Myr to accomplish. Looking for Planets Outside our Solar System One reason that we think the Nebular Hypothesis is accurate is that we see evidence of planet formation around young stars elsewhere in the Universe. The Terrestrial (inner) Planets The inner planets of the solar system have some common features. They are broadly similar in size and considerably richer in elements like iron (Fe), silicon (Si), oxygen (O), aluminum (Al) and magnesium (Mg) [the elements that make up rocks] than the outer planets of the solar system. Comparative Densities of the Planets Although the compositions of the inner planets are broadly similar, in the outer part of the solar system most of the volatile elements are condensed, and thus these planets have very low densities. 9

10 The Outer Planets: Gas Giants The compositions of Jupiter, Saturn, Uranus and Neptune are dominated by hydrogen, helium, methane -- gases/ice. They are 4-10 times larger than the Earth, But have much lower densities. Terrestrial Planets Why is the solar system chemical segregated the way it is? Small and rocky Composed of Oxygen, Silicon, Aluminum, Calcium, Iron, and Magnesium Mercury Venus Earth Mars Mercury Davies, M. E., S. E. Dwornik, D. E. Gault, and R. G. Strom, Atlas of Mercury, NASA SP-423 (1978). Venus 10

11 Mars NASA and the Hubble Heritage Team Jovian Planets Large, gaseous Composed mainly of hydrogen and helium Jupiter Saturn Uranus Neptune Jupiter and Io NASA/JPL/University of Arizona 11

12 First ever color picture of Titan s surface January 14, 2005 Water and hydrocarbon ice Saturn NASA/JPL/GSFC/Ames ESA/NASA/Univ. of Arizona Neptune Alan Stern (Southwest Research Institute), Marc Buie (Lowell Observatory), NASA and ESA 12

13 Earthrise on the Moon Moon Facts Large mass relative to Earth Large angular momentum of Earth-Moon system Moon is depleted in volatile elements and iron Earth and moon have similar oxygen isotope ratios Moon was once entirely molten (magma ocean) Moon origin hypotheses The Apollo Program Capture from independent heliocentric orbit Co-accretion Fission from rapidly rotating Earth Collisional ejection Disintigrative capture Resolving moon origin was a scientific objective of the Apollo program Major scientific and technological advances resulted 13

14 Collisional Ejection Hypothesis Explains the data best Earth hit by Mars-sized object Rocky mantle of object and Earth were partially vaporized and ejected into Earth orbit. Dust and vapor coalesced to form the Moon Meteorites Fragments of early formed bodies called planetesimals 3 principal types of meteorite Chondrite earliest solar system material Stony Iron All have the same age early solar system 4.5 Bya Chondrite Meteorites Allende Chondules Iron Meteorite Canyon Diablo 14

15 Differentiation of the Solar System There is a strong thermal gradient across the solar system, meaning that it is much hotter close to the Sun than far away. In the early solar nebula, objects close to the young Sun were heated to the point that some elements were largely boiled off. Widmanstätten pattern of Fe-Ni alloys Early Solar System Timeline You don t need to commit this to memory, but having a general idea of the scales involved is important (i.e., billions v. millions of years, which came first, etc.). Times are Gya solar system assembled accretion of Earth complete Earth s core formed age of oldest lunar rocks Earth s crust stabilized Earth s hydrosphere in place rapid decline in meteorite bombardment end of activity on Mercury volcanic activity on Moon end of activity on Moon end of activity on Mars Summary The Universe is about 13+ billion yrs old. The Solar System consists of 9 planets, 4 terrestrial and 5 Jovian, that revolve around the sun (heliocentric orbit). The sun and planets formed by accretion of particles and gases 4.5 billion years ago. The Moon formed by collision of a Mars-sized body with the proto-earth (collisional ejection hypothesis) 15

16 Summary Many natural processes, such as the origin of the Earth and Moon, cannot be observed directly and must be inferred from indirect observation, measurement, and calculation. Example: The origin and early history of Earth is inferred from analysis of meteorites, the Moon, and other planets. Solar System Formation Synopsis Solar System Formation The Nebular Theory of Solar System Formation is supported by the configuration of planets. The orbital plane of 8 planets lie within 3 of the Sun's equator. Pluto orbits at an oblique angle to the plane of the ecliptic, suggesting its origin as a captured planet. 16