Lecture L7 - AST3020
Understanding dust
1. Clearing stage: do planets clear dust?2. Comets 3. Asteroids4. Planetoids5. Zodiacal light6. IDPs (Interplanetary dust particles)
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.
2
22escK
K
vvE
Gravitationalslingshot
12
02
2
2
2
2
22
22
2
22
p
p
K
esc
Kesckpot
escKk
Kpot
Kp
pesc
Rr
M
M
v
v
escpeofCondition
escapeE
vvEEE
vvE
vrGM
E
rGM
vR
GMv
:
.
,
,
,
Planet
Earth 0.14Mars 0.04
Jupiter’s core 5Jupiter 21Saturn 14Uranus 10Neptune 19
2
2
K
esc
v
vConclusions:
Terrestrial planets in the solar systemcannot eject planetesimals
Giant planets (even cores) can eject planetesimals out of the solar system
Any cleared region may be seen as a gapin SED.
So far no firm detection of exoplanetsthis way… but the process definitely happened in the solar system, leavingbehind the Oort Cloud.
aGM
E2
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)
Oort cloud of comets: the source of the so-called new cometssize ~ Hill radius of the Sun in the Galaxy ~ 260,000 AU
inner part flattened, outer elliptical
Q: Porb = ?
pcpcrrL 311038500310 43311 .~~/,
Out of 152 new comets
~50 perturbedrecently by 2 stars(one slow, one fastpassage)
excess of retrogradeorbits, aphelia clustered on the sky
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 30000asteroid-sized (typically 100 km across) super-comets reside there.
Smith & Terrile (1984)
Gerard Kuiper (1905-1973)
Interestingly, we now observethat Kuiper belt apparently endsat r ~ 50 AU,so the original drawings wereincorrect!
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.
10th planet(s):
super-Pluto’s:Sedna, “Xena”
also starring:Plutinos!
Don’t worry…it’s hard to see!Better image onthe next slide.
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 seenaround 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.
On October 31 2005, 2 new moons of Pluto have beenfound by the Hubble Space Telescope/ACS
Pluto
Charon
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 throughwhich water vapor and thedust/stones dragged by itescape, to eventuallyspread and form head and tail of the comet.
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
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.htmlhttp://stardust.jpl.nasa.gov/science/details.html
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 ofit 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.
Tracks in aerogel, Stardust sample ofdust from comet Wild 2. That comet was residing in the outer solar system until a close encounter with Jupiter in 1974.
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
"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)
Deep Impact NASA probe - impacted comet Tempel1 on July 4, 2005 (v =10.2 km/s) - see the movie frames of the actual impactof 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.
See http://stardust.jpl.nasa.gov/science/feature001.htmlabout the differences between comets Wild-2 and Temple 1.
Here is the Deep Impact description http://deepimpact.jpl.nasa.gov/home/index.html
CometTemple 1nucleus
~10m resolution
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 cometChuryumov-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
These particles have been delivered to Earth for $free$
IDP (cometary origin?) Chonditic meteorite
Brownlee particlescollected in the stratosphere Donald Brownlee, UW
Brownlee particleA 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.
Glass with Embeded Metals and Sulfides - found in IDPs
Nano-rocks composed of a mixtureof materials, some pre-solar
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 thisworld
(pre-solarisotopes,compositionof GEMS)
Dust modeler’s toolkit
Definitions of Qsca, Qabs, QextSimplified case of no diffraction
Mie theory Mie theory program online at
http://omlc.ogi.edu/calc/mie_calc.htmlTemperature calculation with Mie theoryScattering patternsPolarizationRadiation coefficients
How Mie theory helped understand beta Pictoris+ other systems
The physics of dust and radiation is very simple
In the past the amount of dust hidden by coronograph maskhad to be reconstructed usingMEM= maximum entropy methodor other models. Today scattered light data often suffice (e.g., Mirza’s 1501 project!)
tau = optical thickness perpendicularto the disk (vertical optical thicknass)
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)
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
Radiation pressure onISM dust in three prototype debris disks.
Notice the logarithmic scale! ISM particles are absorbent, which enhances the effect.
A good candidatematerial can be foundfor the beta Pictoris diskSED and broadband photometry modeling
What minerals will precipitate from asolar-composition,cooling gas? Mainly Mg/Fe-rich silicates and water ice. Planets are made of precisely these things.
Silicatessilicates
ices
T(K)
Chemical unityof nature… and it’sthanks to stellar nucleosynthesis!
EQUILIBRIUM COOLING SEQUENCE
Equilibrium temperature of solid particles (from dust to atmosphereless planets)A = Qsca = albedo (percentage of light scattered)Qabs = absorption coefficient, percentage of light absorbedQabs + Qsca = 1 (this assumes the size of the body >> wavelength
of starlight, otherwise the sum, called extinction coefficientQext, might be different)
total absorbing area = A, total emitting area = 4 A (spherical particle)
Absorbed energy/unit time = Emitted energy /unit timeA Qabs(vis) L/(4 pi r^2) = 4A Qabs(IR) sigma T^4L = 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 timeQabs(vis) - in the visible/UV range where starlight is emitted/absorbedQabs(IR) - emissivity=absorptivity (Kirchhoffs law!) in the infared,
where thermal radiation is emitted
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 433Venus 0.72 240 540 huge greenhouse Earth 0.45 235 280 greenhouse Moon 0.15 270 270Mars 0.25 210 220 weak greenhouse
asteroid (typical) 0.15 160 160Ganimede 0.3 112 112Titan 0.2 86 90(?)Pluto 0.5 38 38
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)
)(
)(
r
r
eq
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
)(r
Vertical optical thickness
Vertical profile ofdust density
Radius r [AU] Height z [AU]
STIS/Hubble imaging (Heap et al 2000)
Modeling (Artymowicz,unpubl.):parametric, axisymmetric diskcometary dust phase function
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
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?
Microstructure of circumstellardisks: identical with IDPs(interplanetary dust particles)
mostly Fe+Mg silicates(Mg,Fe)SiO3
(Mg,Fe)2SiO4
The disk particlesare made of the Earth-type minerals!
(olivine, pyroxene, FeO, PAH= PolycyclicAromatic Hydrocarbons)
Crystallinity of minerals
Recently, for the first time observations showed the differencein the degree of crystallinity of minerals in the inner vs. the outer diskparts. This was done by comparing IR spectra obtained with single dishtelescopes with those obtained while combining several such telescopesinto an interferometric array (this technique, long practiced by radioastronomers, 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 differentstructure 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 bysome heating (annealing) effect at temperatures up to ~1000 K.
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