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1/13/2014 PLANETS FROM A DISTANCE GLG‐190 ‐ The Planets Chapter 1 LECTURE OUTLINE Planetary temperatures Electromagnetic spectrum and composition Planetary sizes, masses, and densities Differentiation, moment of inertia, planetary interiors Textbook web site: http://ase.tufts.edu/cosmos/ 1 1/13/2014 PLANETARY TEMPERATURES Temperature of planet controlled by distance from Sun Amount of solar radiation hitting distant planets is much less than for inner planets Intensity of solar radiation striking given area (flux) decreases moving away from Sun (falls off as 1/r2) Internal heat and greenhouse effect can change planet’s temperature TEMPERATURE SCALES Fahrenheit (used in USA) Centigrade (used in rest of world) Water freezes at 32 F and boils at 212 F Water freezes at 0 C and boils at 100 C Kelvin (scientific usage) Size of degree same as for Centigrade scale Water freezes at 273.15 K and boils at 373.15 K All molecular motion stops at 0 K Very little difference between Centigrade and Kelvin at very high temperatures 2 1/13/2014 ELECTROMAGNETIC SPECTRUM Electromagnetic radiation can be considered either particles (photons) or waves Classified by… Energy Wavelength Inverse relationship between wavelength and energy Shorter wavelength more energy Longer wavelength less energy COMPOSITION OF SUN Prism spreads “white” light into color spectrum (right) Sun’s light has same range of colors but with thousands of superimposed black lines Lines catalogued by Joseph von Fraunhofer (1787‐1826) Solar spectrum for visible light showing dark Fraunhofer absorption lines (spectrum wraps around edges of image: each row is part of total spectrum) 3 1/13/2014 SPECTROMETRY Use prism or diffraction grating to spread white light into constituent colors, revealing dark lines Spectroscopy is only way to learn composition of something from distance FORMATION OF SPECTRAL LINES Interaction of light (photons) with electrons of atoms and molecules Absorption lines Wavelengths where energy is removed from spectrum Emission lines Wavelengths where energy is added to spectrum 4 1/13/2014 ATOMIC ABSORPTION LINES Photons strike atoms kicking electrons into higher energy orbitals Only certain photon energies will work Photon energy used up absorption spectrum Electron is higher orbitals fall back and emit energy emission spectrum MOLECULAR ABSORPTION LINES Light (photons) also interacts with molecules Molecules respond by changing… Vibrational modes (movement of atoms within molecule) Rotational modes (movement of molecule in space) Photon energy used up dark lines in spectrum 5 1/13/2014 DETERMINING SOLAR COMPOSITION Solar abundances determined by spectroscopy Match spectral lines of Sun’s photosphere with those determined in lab Intensity of lines is function of abundance (larger darker lines higher abundances) SOLAR COMPOSITION Why do we care? Sun contains almost all mass in solar system representative of starting composition from which planets formed Differences between composition of Sun and planets result from processes that formed and modified planets Most planets have compositions very different from the Sun (exceptions: Jupiter and Saturn) Z Element Atoms per million H 1 Hydrogen (H) 1,000,000 2 Helium (He) 97,000 6 Carbon (C) 360 7 Nitrogen (N) 110 8 Oxygen (O) 850 10 Neon (Ne) 120 12 Magnesium (Mg) 40 14 Silicon (Si) 40 16 Sulfur (S) 20 26 Iron (Fe) 32 All others <5 6 1/13/2014 TYPES OF PLANETS Terrestrial Giant Mercury, Venus, Earth, Mars Closest to Sun Rocky (like Earth) with high densities Few or no moons Solid surfaces Jupiter, Saturn, Uranus, Neptune Far from Sun Large, gaseous, low densities Many moons and rings No solid surfaces PLANETARY SIZES Direct observation of planet’s disk Measure angular size Angular size combined with distance actual size Occultation Result of syzygy (alignment of three astronomical bodies) Useful for bodies too small to directly observe disk Body blocks star’s light when it passes in front of it Duration of occultation combined with orbital information yields angular size and actual size 7 1/13/2014 Sun COMPARISON OF PLANETARY SIZES PLANETARY MASSES Mass determined by observing gravitation force body exerts on other bodies For example, orbits of planets can be used to determine mass of Sun Using data for Earth 107 P = 1 year = 3.156 s a = 1.496 1011 m G = 6.67300 10‐11 m3 kg‐1 s‐2 MSun = 1.9886 1030 kg M 4 2 a 3 GP 2 P = orbital period a = semi‐major axis of planet G = Newton’s gravitational constant 8 1/13/2014 planet M planet 4 2 D 3 GP 2 moon Use same method for planets and their moons Know distance, d determine orbital radius, D Observe period, P Calculate mass of planet Mass of Jupiter using Ganymede’s orbit D = 1,070,400 km = 1.070 109 m P = 7.15455296 d = 6.181 105 s MJupiter = 1.8967 1027 kg (1/1000 of Msun) PLANETARY DENSITIES Density () is mass (m) divided by volume (V), which is determined from size… m V Usually expressed as grams per cubic centimeter (g/cm3) or kilograms per cubic meter (kg/m3) 1 g/cm3 = 1000 kg/m3 Density (g/cm3) Density (kg/m3) Water 1.00 1000 Ice 0.92 920 Air (at 0 C) 0.00129 1.29 Granite 2.75 2750 Basalt 3.0 3000 Iron 7.87 7870 Material 9 1/13/2014 UNCOMPRESSED DENSITY Compression increases density of material Gravity of massive objects like planets will squeeze materials in interior increasing density Less squeezed less dense Image pile of mattresses; bottom ones will be flatter than those on top Flatter ones have higher density (more flat ones higher overall density) Thus, larger of two piles has higher density, even though both contain same mattresses Planetary materials like mattresses more squeezed at bottom (in center) Need to account for this effect uncompressed density Studying uncompressed density allows comparison of the compositions of planets More squeezed more dense DENSITIES OF PLANETS Planet Density (g/cm3) Uncompressed Density Mercury 0.06 5.44 5.4 Venus 0.85 5.24 3.97 Earth 1.00 5.52 4.03 Moon 0.012 3.34 3.3 Mars 0.11 3.93 3.71 Jupiter 318 1.3 0.3 Saturn 95 0.7 0.3 Uranus 15 1.3 0.5 Neptune 17 1.6 0.5 Terrestrial Planets Giant Planets Mercury, Venus and Earth have very similar observed densities Mass (Earth 1) Densities of larger terrestrial planets (Venus and Earth) significantly increased by compression Giant planets have total densities doubled or tripled by pressure 10 1/13/2014 DIFFERENTIATION Planets not made of just one material Planetary bodies heated in early history Contain metal, rock, ice, gas These materials have different densities (complicates problem of determining uncompressed density) Heat from accretion and impacts Radioactive heat Hot accretion melting magma ocean Density separation of materials Metal sinks to form core (generates additional heat) Metal‐rock‐ice layers on icy moons Large planets retain heat better Volume R3, heat generation Surface area R2, heat loss Ganymede PLANETARY INTERIORS Difficult to determine thickness of layers inside planet Possible to estimate types and thicknesses of layers if planet’s original composition is known Original compositions estimated by starting with solar composition Account for effects of various processes such as gas loss to get theoretical starting composition Composition estimates can be combined with moment of inertia to better constrain the distribution of mass inside a planet 11 1/13/2014 MOMENT OF INERTIA Measure of an object's resistance to changes in rotation rate Depends upon mass distribution inside planet Measured by observing how planet interacts gravitationally with other bodies (moons, spacecraft) Coefficient of moment of inertia, k, reflects distribution of mass, regardless of total mass or radius MOMENTS OF INERTIA Object k Implication Mercury 0.33 large dense core Venus 0.33 large dense core Earth 0.33 large dense core Moon 0.393 Homogeneous interior; almost no core Mars 0.366 small dense core 0.254 dense core; extended gas shell (rotationally distorted) 0.210 small dense core; extended gas shell (rotationally distorted) Uranus 0.23 small dense core; extended gas shell Neptune 0.29 larger dense core; extended gas shell 0.06 very dense core; extended gas shell (rotationally distorted) Jupiter Saturn Sun 12 1/13/2014 SUMMARY Kepler’s and Newton’s discoveries allowed scientific study of planets Developments in spectroscopy allowed determination of solar composition Establishing distances allowed determination of many planetary and solar characteristics Density, in turn allowed estimates of composition Size, mass, density Internal layering, differentiation Moment of inertia (determined largely by spacecraft interactions with planets) confirmed layering inside planets 13