PLANETS FROM A DISTANCE

Transcription

1 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: 1

2 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/r 2 ) Internal heat and greenhouse effect can change planet s temperature TEMPERATURE SCALES Fahrenheit (used in USA) Water freezes at 32 F and boils at 212 F Centigrade (used in rest of world) Water freezes at 0 C and boils at 100 C Kelvin (scientific usage) Size of degree same as for Centigrade scale Water freezes at K and boils at K All molecular motion stops at 0 K Very little difference between Centigrade and Kelvin at very high temperatures 2

3 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 ( ) Solar spectrum for visible light showing dark Fraunhofer absorption lines (spectrum wraps around edges of image: each row is part of total spectrum) 3

4 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

5 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 MOLECULARABSORPTION 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

6 DETERMINING SOLARCOMPOSITION 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) Nitrogen (N) Oxygen (O) Neon (Ne) Magnesium (Mg) Silicon (Si) Sulfur (S) Iron (Fe) 32 All others <5 6

7 TYPES OF PLANETS Terrestrial Mercury, Venus, Earth, Mars Closest to Sun Rocky (like Earth) with high densities Few or no moons Solid surfaces Giant 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

8 COMPARISON OF PLANETARY SIZES Sun 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 P = 1 year = s a = m G = m 3 kg 1 s 2 M Sun = kg 2 4 a M 2 GP P = orbital period a = semi major axis of planet G = Newton s gravitational constant 3 8

9 planet M planet 2 4 D 2 GP 3 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 = m P = d = s M Jupiter = kg ( 1/1000 of M sun ) 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/cm 3 ) or kilograms per cubic meter (kg/m 3 ) 1 g/cm 3 = 1000 kg/m 3 Material Density (g/cm 3 ) Density (kg/m 3 ) Water Ice Air (at 0 C) Granite Basalt Iron

10 UNCOMPRESSED DENSITY Compression increases density of material Gravity of massive objects like planets will squeeze materials in interior increasing density 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 Less squeezed less dense More squeezed more dense DENSITIES OF PLANETS Planet Mass (Earth 1) Density (g/cm 3 ) Uncompressed Density Mercury Venus Earth Moon Mars Jupiter Saturn Uranus Neptune Terrestrial Planets Giant Planets Mercury, Venus and Earth have very similar observed densities Densities of larger terrestrial planets (Venus and Earth) significantly increased by compression Giant planets have total densities doubled or tripled by pressure 10

11 DIFFERENTIATION Planets not made of just one material Contain metal, rock, ice, gas These materials have different densities (complicates problem of determining uncompressed density) Planetary bodies heated in early history 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 R 3, heat generation Surface area R 2, 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

12 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 Homogeneous interior; almost no core Mars small dense core Jupiter dense core; extended gas shell (rotationally distorted) Saturn 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 Sun very dense core; extended gas shell 0.06 (rotationally distorted) 12

13 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 Size, mass, density Density, in turn allowed estimates of composition Internal layering, differentiation Moment of inertia (determined largely by spacecraft interactions with planets) confirmed layering inside planets 13