Lecture L7 - AST3020

Understanding dust

1. Clearing stage: do planets clear dust?2. Comets 3. Asteroids4. Planetoids5. Zodiacal light6. IDPs (Interplanetary dust particles)

Clearing the junk left at the construction site:

Oort cloud formationKuiper belt

10th planet(s)

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

KvvE

Gravitationalslingshot

12

02

2

2

2

2

22

22

2

22

p

p

K

esc

Kesckpot

escKk

Kpot

Kp

pesc

Rr

MM

vv

escpeofConditionescapeE

vvEEE

vvE

vrGME

rGMv

RGM

v

:

.

,

,

,

Planet

Earth 0.14Mars 0.04

Jupiter’s core 5Jupiter 21Saturn 14Uranus 10Neptune 19

2

2

K

esc

vv

Conclusions:

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.

aGME2

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

Fomation of Oort cloud

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

IDPs: Interplanetary Dust Particles

m1010 -100 km

Comet Hale-Bopp (1997)

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.

Borrely-1 imaged by NASA in 2001

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 particle

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 toolkitDefinitions 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)

Or, as a bare minimum, an empirical model of dust (e.g., stolen from comets)

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, Qscam=1.343 + 0i

Air bubble in seawater, Qsca

Carbon in air, Qscam=1.95 - 079i

Carbon in air, Qabsm=1.95 - 079i

``Resonant’’ scattering from Mie theory

Scattering of red light (0.65 um) on water droplets of radius r

How Mie theory works in terms of reflection and/or surface electromagnetic waves.

GLORY

RAINBOW

Laboratory-measured optical constants

These peaks are causedby Si=O bond vibration

Wavelength (um)

silicatess=1 um

s=9 um s=20 um

s=3 um

H2Os=1 um

s=9 um s=20 um

s=3 um

Radiation pressure coefficient depends oncomposition, as wellas porosity

Radiation pressure on mixture may be stronger than on pure components

Radiation pressure onISM dust in three prototype debris disks.

Notice the logarithmic scale! ISM particles are absorbent, which enhances the effect.

Radiative Rutherfordscattering off a star

(the same Coulomb+1/r potential applies!)

A good candidatematerial can be foundfor the beta Pictoris diskSED and broadband photometry modeling

Choosing the plausible material

and

Calculating the temperature of solids

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

However, this may backfire.

T vs. r beta Pictoris

Temperature-distancerelationship and Ice boundarylocation in beta Pic

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 greenhouseasteroid (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)

)()(rr

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

Mirza Ahmic’s best fit to HST/ACS data (b Pic)

Why the differences??

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

HD142527

cold outerdisk

warm disk

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.

~90% amorphous

~95% crystalline

~45% amorphousco

mpa

re

~60% amorphous

Beta Pic,

Why?

We do not at present see in our statistics of Vega-typestars 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??