Published on January 16, 2008
Slide1: Lecture L7 - AST3020 Understanding dust 1. Clearing stage: do planets clear dust? 2. Comets 3. Asteroids 4. Planetoids 5. Zodiacal light 6. IDPs (Interplanetary dust particles) Clearing the junk left at the construction site:: Clearing the junk left at the construction site: Oort cloud formation Kuiper belt 10th planet(s) Slide3: Two-body interaction: a small planetesimal is scattered by a large one, nearly missing it and thus gaining an additional velocity of up to ~vesc (from the big body with mass Mp) The total kinetic energy after encounter, assuming that initially both bodies were on nearly-circular orbits is (we neglect the random part depending on the angle between the two components of final velocity). If the total energy of the small body after encounter, E=Ek + Epot, is positive, then the planetesimal will escape from the planetary system. Gravitational slingshot Slide5: Planet Earth 0.14 Mars 0.04 Jupiter’s core 5 Jupiter 21 Saturn 14 Uranus 10 Neptune 19 Conclusions: Terrestrial planets in the solar system cannot eject planetesimals Giant planets (even cores) can eject planetesimals out of the solar system Any cleared region may be seen as a gap in SED. So far no firm detection of exoplanets this way… but the process definitely happened in the solar system, leaving behind the Oort Cloud. Slide6: E=0 Jan Oort (1902-1992) found that a~ (2-7)*1e4 AU for most new comets. Typical perturbation by planets ~ 0.01 (1/AU) Slide7: Oort cloud of comets: the source of the so-called new comets size ~ Hill radius of the Sun in the Galaxy ~ 260,000 AU inner part flattened, outer elliptical Q: Porb = ? Slide8: Out of 152 new comets ~50 perturbed recently by 2 stars (one slow, one fast passage) excess of retrograde orbits, aphelia clustered on the sky Slide9: Fomation of Oort cloud Slide10: Kuiper belt, a theoretical entity since 1949 when Edgeworth first mentioned it and Kuiper independently proposed it in 1951, was discovered by D. Jewitt and J. Lu in 1993 (1st object), who later estimated that 30000 asteroid-sized (typically 100 km across) super-comets reside there. Slide11: Smith & Terrile (1984) Gerard Kuiper (1905-1973) Interestingly, we now observe that Kuiper belt apparently ends at r ~ 50 AU, so the original drawings were incorrect! The Kuiper belt is home to quite a Zoo of planetoids or plutinos, some of which are larger than the recently demoted (former) planet Pluto. Slide12: 10th planet(s): super-Pluto’s: Sedna, “Xena” also starring: Plutinos! Don’t worry… it’s hard to see! Better image on the next slide. Slide13: The 10th planet (temp. name “Xena” or UB313) first seen in 2003. It has a moon (announced in Sept. 2005) See the home page of the discoverer of planetoids, Michael Brown http://www.gps.caltech.edu/~mbrown/ Images of the four largest Kuiper belt objects from the Keck Observatory Laser Guide Star Adaptive Optics system. Satellites are seen around all except for 2005FY9; in 75% of cases! In comparison, only 1 out of 9 Kuiper belt objects, also known as TNOs (Trans-Neptunian Objects) have satellites. Slide14: On October 31 2005, 2 new moons of Pluto have been found by the Hubble Space Telescope/ACS Pluto Charon IDPs: Interplanetary Dust Particles: IDPs: Interplanetary Dust Particles 10 -100 km Comet Hale-Bopp (1997) Slide17: European (ESA) Giotto mission saw comet Halley’s nucleus in 1986, confirming the basic concept of comet nucleus as a few-km sized chunk of ice and rocks stuck together (here, in the form of a potato, suggesting 2 collided “cometesimals”) The bright jets are from the craters or vents through which water vapor and the dust/stones dragged by it escape, to eventually spread and form head and tail of the comet. Slide18: Borrely-1 imaged by NASA in 2001 Slide19: Why study comets? Comet Wild-2 is a good example: this 3km-planetesimal was thrown out in the giant impacts era from Saturn-Neptune region into the Oort cloud, then wandered closer to Uranus/Jupiter & has recently been perturbed by Jupiter (5 orbits ago) to become a short-period comet (P~5 yr) Comet Temple1, on the other hand, is a short-period comet that survived >100 passages - so we are eager to study differences between the more and the less pristine bodies. Comet Hale-Bopp Gas tail Dust tail Slide20: Stardust NASA mission - reached comet Wild-2 in 2004 Storeoscopic view of comet Wild-2 captured by Stardust http://stardust.jpl.nasa.gov/index.html and in particular: http://stardust.jpl.nasa.gov/mission/index.html http://stardust.jpl.nasa.gov/science/details.html Slide22: Stardust NASA mission - reached comet Wild-2 in 2004 The probe also carried aerogel - a ghostly material that NASA engineered (like a transparent, super-tough styrofoam, 2 g of it can hold a 2.5 kg brick - see the r.h.s. picture). Aerogel was used to capture cometary particles (l.h.s. picture) which came back and landed on Earth in Jan. 2006. Slide25: Tracks in aerogel, Stardust sample of dust from comet Wild 2. That comet was residing in the outer solar system until a close encounter with Jupiter in 1974. Slide28: OLIVINES, Mg-Fe silicate solid state solutions (also found by Stardust) are the dominant building material of both our and other planetary systems. Forsterite, Mg2SiO4 Fayalite, Fe2SiO4 Slide29: "I would say these materials came from the inner, warmest parts of the solar system or from hot regions around other stars," "The issue of the origin of these crystalline silicates still must be resolved. With our advanced tools, we can examine the crystal structure, the trace element composition and the isotope composition, so I expect we will determine the origin and history of these materials that we recovered from Wild 2." D. Brownlee (2006) Slide30: Deep Impact NASA probe - impacted comet Tempel1 on July 4, 2005 (v =10.2 km/s) - see the movie frames of the actual impact of the probe taken by the main spacecraft, taken 0.83s apart. The study showed that Temple1 is porous: the impactor dug a deep tunnel before exploding. Slide31: See http://stardust.jpl.nasa.gov/science/feature001.html about the differences between comets Wild-2 and Temple 1. Here is the Deep Impact description http://deepimpact.jpl.nasa.gov/home/index.html Comet Temple 1 nucleus ~10m resolution Slide32: Other missions are ongoing…. Rosetta mission by ESA (European Space Agency) will first fly by astroids Steins and Lutetia near Mars after the arrival at the comet Churyumov-Gerasimenko in 2014, the spacecraft will enter an orbit around the comet and continue the journey together. A lander will descend onto the surface. http://rosetta.esa.int Slide33: These particles have been delivered to Earth for $free$ IDP (cometary origin?) Chonditic meteorite Brownlee particles collected in the stratosphere Donald Brownlee, UW Slide34: Brownlee particle Slide35: Brownlee particle A few out of a thousand subgrains shows isotopic anomalies, e.g., a O(17) to O(16) isotope ratio 3-5 times higher than all the rest - a sign of pre-solar nature. Slide37: Glass with Embeded Metals and Sulfides - found in IDPs Nano-rocks composed of a mixture of materials, some pre-solar Slide38: Figure 1. Transmission electron micrographs of GEMS within thin sections of chondritic IDPs. (A) Bright-field image of GEMS embedded in amorphous carbonaceous material (C). Inclusions are FeNi metal (kamacite) and Fe sulfides. (B) Dark-field image. Bright inclusions are metal and sulfides; uniform gray matrix is Mg-rich silicate glass. (C and D) Dark-field images of GEMS with "relict" Fe sulfide and forsterite inclusions. Out of this world (pre-solar isotopes, composition of GEMS) Dust modeler’s toolkit: Dust modeler’s toolkit Definitions of Qsca, Qabs, Qext Simplified case of no diffraction Mie theory Mie theory program online at http://omlc.ogi.edu/calc/mie_calc.html Temperature calculation with Mie theory Scattering patterns Polarization Radiation coefficients How Mie theory helped understand beta Pictoris + other systems Slide41: The physics of dust and radiation is very simple In the past the amount of dust hidden by coronograph mask had to be reconstructed using MEM= maximum entropy method or other models. Today scattered light data often suffice (e.g., Mirza’s 1501 project!) tau = optical thickness perpendicular to the disk (vertical optical thicknass) Slide44: Or, as a bare minimum, an empirical model of dust (e.g., stolen from comets) Mie theory of scattering (+absorption, polarization,Qrad): Mie theory of scattering (+absorption, polarization,Qrad) C. F. Bohren and D. R. Huffman (Editors), Absorption and Scattering of Light by Small Particles (Wiley-Interscience, New York, 1983). Gustav Mie (1869-1957) Slide46: Wavelength = 0.55 um Ocean water in air, Qsca m=1.343 + 0i Air bubble in seawater, Qsca Carbon in air, Qsca m=1.95 - 079i Carbon in air, Qabs m=1.95 - 079i ``Resonant’’ scattering from Mie theory Slide47: Scattering of red light (0.65 um) on water droplets of radius r Slide48: How Mie theory works in terms of reflection and/or surface electromagnetic waves. GLORY RAINBOW Slide50: Laboratory-measured optical constants These peaks are caused by Si=O bond vibration Wavelength (um) Slide54: silicates s=1 um s=9 um s=20 um s=3 um Slide55: H2O s=1 um s=9 um s=20 um s=3 um Slide56: Radiation pressure coefficient depends on composition, as well as porosity Slide57: Radiation pressure on mixture may be stronger than on pure components Slide58: Radiation pressure on ISM dust in three prototype debris disks. Notice the logarithmic scale! ISM particles are absorbent, which enhances the effect. Slide59: Radiative Rutherford scattering off a star (the same Coulomb +1/r potential applies!) Slide60: A good candidate material can be found for the beta Pictoris disk SED and broadband photometry modeling Slide61: Choosing the plausible material and Calculating the temperature of solids Slide63: What minerals will precipitate from a solar-composition, cooling gas? Mainly Mg/Fe-rich silicates and water ice. Planets are made of precisely these things. Silicates silicates ices T(K) Chemical unity of nature… and it’s thanks to stellar nucleosynthesis! EQUILIBRIUM COOLING SEQUENCE Slide65: However, this may backfire. Slide68: T vs. r beta Pictoris Slide69: Temperature-distance relationship and Ice boundary location in beta Pic Slide70: Equilibrium temperature of solid particles (from dust to atmosphereless planets) A = Qsca = albedo (percentage of light scattered) Qabs = absorption coefficient, percentage of light absorbed Qabs + Qsca = 1 (this assumes the size of the body >> wavelength of starlight, otherwise the sum, called extinction coefficient Qext, might be different) total absorbing area = A, total emitting area = 4 A (spherical particle) Absorbed energy/unit time = Emitted energy /unit time A Qabs(vis) L/(4 pi r^2) = 4A Qabs(IR) sigma T^4 L = stellar luminosity, r = distance to star, L/4pi r^2 = flux of energy, T = equilibrium temperature of the whole particle, e.g., dust grain, sigma = Stefan-Boltzmann constant (see physical constants table) sigma T^4 = energy emitted from unit area of a black body in unit time Qabs(vis) - in the visible/UV range where starlight is emitted/absorbed Qabs(IR) - emissivity=absorptivity (Kirchhoffs law!) in the infared, where thermal radiation is emitted Slide71: Equilibrium temperature of solid bodies falls with the square-root of r T^4 = [Qabs(vis)/ Qabs(IR)] L/(16 pi r^2 sigma) which can be re-written using Qabs(vis) = 1-A as T = 280 K [(1-A)/Qabs(IR) (L/Lsun)]^(1/4) (r/AU)^(-1/2) Theoretical surface temperature T of planets if Qabs(IR)=1, and the actual surface temperature Tp. Differences are mostly due to greenhouse effect Body Albedo A T(K) Tp(K) comments _____________________________________________________ Mercury 0.15 433 433 Venus 0.72 240 540 huge greenhouse Earth 0.45 235 280 greenhouse Moon 0.15 270 270 Mars 0.25 210 220 weak greenhouse asteroid (typical) 0.15 160 160 Ganimede 0.3 112 112 Titan 0.2 86 90(?) Pluto 0.5 38 38 Slide72: Optical thickness: perpendicular to the disk in the equatorial plane (percentage of starlight scattered and absorbed, as seen by the outside observer looking at the disk edge-on, aproximately like we look through the beta Pictoris disk) Slide73: What is the optical thickness ? It is the fraction of the disk surface covered by dust: here I this example it’s about 2e-1 (20%) - the disk is optically thin ( = transparent, since it blocks only 20% of light) picture of a small portion of the disk seen from above Examples: beta Pic disk at r=100 AU opt.thickness~3e-3 disk around Vega opt.thickness~1e-4 zodiacal light disk (IDPs) solar system ~1e-7 Slide74: Vertical optical thickness Vertical profile of dust density Radius r [AU] Height z [AU] STIS/Hubble imaging (Heap et al 2000) Modeling (Artymowicz,unpubl.): parametric, axisymmetric disk cometary dust phase function Slide75: Mirza Ahmic’s (2006) best fit to HST/STIS data (b Pic) Model of dust distribution uses empirical ZL scattering phase function and two overlapping disks, inclined by a few degrees Fitting method: multiparametric fit (~18 par.) using simplex algorithm model Slide76: Mirza Ahmic’s best fit to HST/ACS data (b Pic) Why the differences?? Slide77: Chemistry/mineralogy/crystallinity of dust All we see so far are silicate particles similar to the IDPs (interplanetary dust particles from our system) Ice particles are not seen, at least not in the dust size range (that is also true of the IDPs) Are all planetary systems made of the same material? Slide78: Microstructure of circumstellar disks: identical with IDPs (interplanetary dust particles) mostly Fe+Mg silicates (Mg,Fe)SiO3 (Mg,Fe)2SiO4 Slide79: HD142527 cold outer disk warm disk Slide80: The disk particles are made of the Earth-type minerals! (olivine, pyroxene, FeO, PAH= Polycyclic Aromatic Hydrocarbons) Slide81: Crystallinity of minerals Recently, for the first time observations showed the difference in the degree of crystallinity of minerals in the inner vs. the outer disk parts. This was done by comparing IR spectra obtained with single dish telescopes with those obtained while combining several such telescopes into an interferometric array (this technique, long practiced by radio astronomers, allows us to achieve very good, low-angular resolution, observations). In the following 2 slides, you will see some “inner” and “outer” disk spectra - notice the differences, telling us about the different structure of materials: amorphous silicates = typical dust grains precipitating from gas, for instance in the interstellar medium, no regular crystal structure crystalline grains= same chemical composition, but forming a regular crystal structure, thought to be derived from amorphous grains by some heating (annealing) effect at temperatures up to ~1000 K. Slide83: ~90% amorphous ~95% crystalline ~45% amorphous compare ~60% amorphous Beta Pic, Slide84: Why? We do not at present see in our statistics of Vega-type stars any simple time-evolution of dustiness or crystallinity of solids in circumstellar disks. Annealing could be thermal (in proximity to stars), while transport done by outflows. Does migration of dust explain observations??