ASTR 150

‣ Homework 2 due Monday

‣ Planetarium: Last shows next week (M-Th)

‣ Next Monday/Wednesday no lectures‣ Time for asteroid lab

‣ Last time: Solar System Formation

‣ Today: Solar System Formation 2

Music: It Overtakes Me/The Stars Are So Big, I Am So Small... Do I Stand A Chance? – Flaming Lips1

Taking The Solar System for a SpinSolar nebula competition:

Gravity vs Angular Momentum

• If fall perpendicular to spin axisNeed to speed up

resistance: centrifugal force

• If fall parallel to spin axis: same speed, so no resistance forms protoplanetary disk

swirling gas and dustraw material for planets

– Origin of planet’s orbits!– Origin of Ecliptic plane– Organizes spins along initial spin axis

Q:  how  can  we  test  if  these  ideas  are  right?

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Why do large bodies in our solar system have orderly motions?

The solar nebula began to collapse due to its own gravityAs it collapsed, it began to spin faster and faster Sun forms at the center, surrounded by a flattened diskPlanets form from gas and dust in the disk

3The orderly motions of our solar system are a direct result of the solar system’s birth in a spinning, flattened cloud of gasNot all mass falls in directly (radially). Why?All gas has a small spin that preferentially causes the formation of a flattened structure

Disks around Young Stars are Common

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Young stars are surrounded by dense disks of gas and dust

Disks have been imaged with

HST’s infrared camera

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From dust grains to planetesimals to planets

http://www.metacafe.com/watch/1111454/formation_of_the_solar_system_great_animation/6

When the temperature is low enough, molecules condense (solidify) from the solar nebula into dust grainsGrains collide and stick: get larger by accretionGrow into planetesimals (~1 km across)Planetesimals collect into planets

From dust grains to planetesimals to planets

Dust grains are the ‘seeds’ of planet formation7

When the temperature is low enough, molecules condense (solidify) from the solar nebula into dust grainsGrains collide and stick: get larger by accretionGrow into planetesimals (~1 km across)Planetesimals collect into planets

iClicker Poll:What made what?

Solar nebula collapse leads to ‣ pre-Sun (“protostar”) forming in center

‣ surrounded by disk of gas and dustwhich will lead to planets, comets, asteroids

What will make what?

A. Earth mostly made of disk dust, Jupiter mostly made of disk gas

B. Earth and Jupiter mostly made of disk gasC. Earth and Jupiter mostly made of disk dust

D. Earth mostly made of disk gas, Jupiter mostly made of disk dust

8Answer A

What are the ‘seeds’ made of?

‣ The ingredients of the solar nebula fell into four major categories

‣ Hydrogen/helium gas (98% of mass) do not condense to form solids or ices

‣ Other components can condense at the right temperature - i.e., planet seeds!

9Everything astronomers know about the solar system and star formation suggests that the solar nebula was a fragment of an interstellar gas cloud. Such a cloud would have been mostly hydrogen with some helium and minor traces of the heavier elements. The original chemical composition of the solar nebula should have been roughly the same throughout the nebula.

Planet Formation in the DiskHeavy elements clump

1. Dust grains collide, stick, and form planetesimals– about 1012 of them, sort of like asteroids! All orbit in the same direction and in the same plane.

2. Gravity Effects: Big planetesimals attract the smaller planetesimals. So, fewer and fewer of large objects (100’s). Collisions build-up inner planets and outer planet cores.

3. Collisions can also account for odd motions of Venus (backwards), Uranus (rotates on its side), and Pluto (high inclination of orbit). Proof of period of high collision evident on moon

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Why  are  there  two  types  of  planets?

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The inner nebula was hot:‣ and only metals and rock could condense there‣ no ices can form

The outer nebula was cold, ‣ ices could condense ‣ in addition to metal and rock

Boundary: “snow line” or “frost line”‣ closest place where ice can survive

Temperature Controls Planet Formation

ices can existtoo hot for ice!

12The important factor was temperature. The inner nebula was hot, and only metals and rock could condense there. The cold outer nebula could form lots of ices in addition to metals and rocks. The frost line seems to have been between Mars and Jupiter—it separates the formation of the dense terrestrial planets from that of the low-density Jovian planets.

i>clicker question

13Answer B

Terrestrials vs. JoviansInner solar system

‣ Metal & rock seeds

‣ Less material

‣ Small, rocky planets

Outer solar system

‣ Ices, rock & metal seeds

‣ More material

‣ Proto-planets grow big

‣ Gravity captures large amounts of H and He gas

‣ Large, gaseous planets

Jovian planets grew massive enough to

gravitationally capture gas from

disk14

Where did the asteroids and comets come from?

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Planetesimals

The young solar system was filled with trillions of planetesimalsMany were swept up to become parts of the forming planetsWhat happened to the rest?

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Leftover planetesimals bombarded other objects in the late stages of solar system formation

Evidenced by the cratered surfaces of the Moon & Mercury!

Period of heavy bombardment

Lasted for about 800 million years

Heavy Bombardment

The Moon’s surface shows the scars of the heavy

bombardment17

Fates of the planetesimalsSwept up by forming planets or ejected from solar systemNear terrestrials - swept upBut some remain...

Asteroids  and  comets  are  le6over  planetesimals!

Comets are icy because they formed outside the

frostline

Asteroids are rocky because they formed

inside the frostline

18Ceres is the largest of the rocky leftoversPluto & Eris are the largest of the icy leftovers

Origin  of  Asteroid  Belt

‣ Planetesimals between Mars and Jupiter did not gravitationally clump into a planet

‣ Jupiter’s gravity stirred up these planetesimal orbits and prevented clumping

‣ Asteroid belt is leftover planetesimals prevented from ever forming a planet

The asteroid belt is not the remains of a destroyed

terrestrial planet

19Jupiter ejected most asteroids from the Solar System or sent them on orbits to collide with the forming terrestrials

Kuiper Belt Objects:Formed beyond the orbit of Neptune

Not enough material to form another Jovian planet

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Oort Cloud Comets:Ejected to deep freeze

Oort Cloud comets formed in the Jovian planet region

Gravitational interactions with the proto-Jovians changed their orbits

‣ Launched them to the Oort Cloud

OR

‣ Sent them into the inner solar system!

f rom choos to the kuiPer bel t

etes imals and warped the i rnear: ly circular orbits so thatsorne - a mere few bi l l ion -

leaked out o f th is reservo i r andfel l i rrward into the gravitat ionaic l a w s o i t h e g r a n t p l a r r e t s . T h a l s e tof i a 5oo-mil l ion-year-iong snowballf ight. The giant planets f iung cotnet- to Pluto-size objects at one anothet:, in toward the snrai ler 'plariets and out towar:d thc edge of the solar svste nr- and sorle escaped conrpleteiv.

Fun for the g iant p lanets , you might sav, but L- l r r t so r )1 i 'as-

ant for : the innunrerab ie bodies that ceased to ex is t e1 ' r r ' i ' r ' c

exi led to roam interstel lar space. Yet Jupiter, Satul ' l l , 1.1' ' , ' t

nus, and Neptune didn't emet:ge {rom that t t n*5"11 irulr l

unscathed. By the t imc l t was over' they'd migrated to r( 'n/

orb i ts .

Herd ing PlonetsIn 1984 planetary scientists Jul io Fernandez and Winq

Huen Ip used computers to simulate dyranrical pt:oc|s-tcsin the ear iy so lar systern. Based on condi t ions thoLrg l r i to

exist as the Sun {ormed 4' l : bi l l ion vears ago' theY watrtcdto see i f Uranus and Neptune wouid grow f rom the p iar r

etesimals avai lable beyond Jupiter and Saturn' Sureenough, they would.

BuiFernandez and Ip not iced someth ing e1se. A l t l roLrqht h c s j n r u l a t e d g i a n t p l a n e t s s t a r t e d a t c e r t a i r l d i s t a n ( ' t ' i l o l x

the Sun, they inevitably ended up at very dif ferent plat 'cs.This discovery planted the seed that ult imately grew rir tcl

the Nice model.Take Neptune as an exalnple, says Levison' Whenetcr '

the planet, with i ts strong gravitat ional f ield, encountcrcda t iny p lanetes imal , i t had about au even chance o i s t 'a t ter -

i .g i t towar :d o f awav t l 'on l t l rc Sr ' tn . Every t ime Ncptuneaccelerate c1 sor t re thr r rg outwat :d , thc p lanet decelerate d

and droppcd sunwar :d ius t a tad. Corrvet 'se lv every t in re

the p ianet f luLng s6nrc th i r l { 5Lrnwal :d , i ts own orb i t edgedfa r t hc r : i r o r l t he Su t r sec t he box bc low) ' Th i s i s j us t

N e w t o n ' s t h i r . d l a l t : i , , r . e v c l ' v a t ' t i o t t . t l t e l c ' s a l l C q r - r a l a n doppos i t e r ca t ' l i o t t

On ave rage, you 'd e \pcc ' t cqt ra l t ru t lbet :s o f inward

and outward t r - rgs, le ar ing Neptnne 's orb i t re la t ive ly un-

chapged. But as po l rcr i r r l a - ' t l te pLapet 's grav i tV is , i t ' s not

s t rong enough to k ick prau\ t i t i r rqs orLt o i the so lar s 'Stem.

So a1n-rost eve rV t i l re \ . l l t1 t tc i rgotcd ap icy c t runk out -w a r - d . t l r C , b i C C t r e t r r r ' 1 , r l r , ' . \ ' ( r ' ( i ' t ' t t t l l d C r r C o u n t e r f h C

planet again .on the other hand. \ \ rrr- ir . \ r l ' \cptrtnc tk:cek:t 'ated a plan-

e tes ima l , caus ing 1113 , i l r r t ' i ' ' r L i c l ' ) se l ' t o t he Sun ' Nep -

tune i tse l f t rcc t lc ra te t la I i i t \ i - - i - i l c l i t toved jn to an or :b i t jus t

a s k o s h f a r t l r c r i t ' o l l t t t , \ l r

I(

.ro1\ jPlqnetory Slingshots

When o smol l ob iect Possesc lose to o p lonet , grov i to t iono linteroctions chonge both tro'

iectories. The orbit of the muchmore mossive bodY olters ominuscule omount , but o f termony b i l l ions o f encounters ,the chonges ore signif icont.

Despite o 5O'5O chonce ofeither of the encounters Pic'tured ot r ight, the net result forour solor system wos to sendSoturn, Uronus, ond Neptuneoutword, ond luPi ter inword.To understond whY, consider ocoin toss - with o quirk.

Let 's ossociote heods withf l ipp ing the p lonetes imol uP

ond owoy f rom the Sun, ondto i ls wi th sending i t c loser tothe Sun. Every t ime the co incomes down toi ls, however,we ' l l le t i t fo l l to the ground,where we con't PloY with i togo in . Whenever the co in reg is-ters heods, we ' l l f l iP i t ogo in .

So i f we stort our tossingwi th , soy, 1OO co ins, we ' l lf l ip obout on equol number o fheods snd to i ls . But soon oursupp l y w i l l be gone ; o l l t heco ins wi l l l ie on the ground.Simi lor ly , NePtune ended uPf l ipp ing more Plonetes imsls intoword the Sun, thus movingi ts orb i t outwqrd.

Now he re ' s t he ke r : \ \ i ' , , ln r a l i r t w a r d , t l r a t o b ; c ' - r ' ' -

Because t he bodv f e l l r l - t , ' l l r 'the o ther g iant P lanei t T l lc tJup i t e r , wh i ch has roL t { r r l r ' J

\c1r t r - rne dropped a P lanetes i -: rre \-e r calr le back. Why?:r 'ar i tat ional griP o{ one of

.rrei plaver was (and st i l l is)t imes the mass of Uranus

"Heods"Plonet orbit

0bject

"Toi ls"

Septentbt:r 2oo7 SkY & TelescoPe21

This also changed the jovian planet orbitsJupiter migrated inwardOther jovians migrated outward

Earth’s Cosmic Water Source?

Interactions with Jovians may have sent water-bearing planetesimals to a young Earth

22Either icy planetesimals or rocky planetesimals with some water from near the frost line (beyond Mars’ orbit)

Recap: The Violent Birth and Infancy

of the PlanetsHeavy Bombardment‣ planetesimals collide with young

planets‣ lasted 800 million years

Formation of the Moon: Smack!‣ Mars-sized object collided with young

earth‣ orbiting debris clumped to form Moon‣ collision gave Earth its 23.5 degree

spin tilt

Early Earth‣ constant bombardments‣ no water until brought by comet

impacts

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Impacts and

the Earth

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Is  the  impact  threat  a  real  danger  or  just  media  hype?

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Meteors and Earth

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Meteor Crater Near Winslow, Arizona

‣ on your way to the Grand Canyon: must-see detour!

Occurred 50,000 years ago

impactor:

‣ 50 meter across

‣ impact speed approximately 13 km/sec = 30,000 mph!

Energy of explosion equal to 25 megatons of TNT!

1.2 km across27

Scientists now believe that the crater was created approximately 50,000 years ago. The meteorite which made it was composed almost entirely of nickel-iron, suggesting that it may have originated in the interior of a small planet. It was 150 feet across, weighed roughly 300,000 tons, and was traveling at a speed of 28,600 miles per hour (12 kilometers per second) according to the most recent research. The explosion created by its impact was equal to 2.5 megatons of TNT, or about 150 times the force of the atomic bomb that destroyed Hiroshima.

How can falling objects cause so much damage?Impacts occur at enormous speed!

‣ Space debris moving at high speeds

‣ Earth’s orbital speed: 30 km/s (67,000 mph)

‣ Impactor speeds entering the atmosphere‣ Range: 11-72 km/s‣ average asteroid speed:

17 km/s‣ average comet speed:

51 km/s

28Minimum speed - is escape velocity due to Earth’s gravity - an object falling into Earth’s gravity well will be accelerated to at least 11 km/sWhy different averages? Asteroids orbit in the same direction as Earth, so the average net impact speed is lowerComets orbits go every which way, so you can get head-on collisions!

Proof in the Pudding:Moon Pie

http://antwrp.gsfc.nasa.gov/apod/ap090206.html

Does Large Space Junk Ever Impact Objects in the Solar System?

Image of the Moon with the Space Station in the

foreground. Cool or what?

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Proof in the Pudding:Moon Pie

http://antwrp.gsfc.nasa.gov/apod/ap071120.html

With even a small telescope, you can see >30,000 craters on the Moon.

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• Full Moon (telescope view) with lighter highlands and darker basalt plains, filling multi-ringed basins

• Apollo 16 view of Descartes Highlands, with impact craters on all scales

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Group Discussion

The Moon clearly has had some LARGE impacts over its lifespan of ~4.5 billion years. Why didn’t the Earth?

– I didn’t fall into a crater on the way over here today.

More than one reason!

a) When your group has a good answer click A on your iClicker.

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Earth-Moon Cratering Differences

Most impactors burn up in Earth’s atmosphere

‣ the Moon has no atmosphere to protect it

Earth’s surface is about 75% water

‣ most meteorites go to bottom of ocean

Erosion due to wind and water slowly covers up craters on land

But still, the Earth does have some craters...

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