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ASTR 150
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Homework 2 due
Monday
Planetarium: Last shows
next week (M-Th)
Next Monday/
Wednesday no lectures
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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 Lips
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Taking The Solar System for a Spin
Solar nebula competition:
Gravity vs Angular Momentum
• If fall perpendicular to spin axis
Need to speed up
resistance: centrifugal force
• If fall parallel to spin axis:
same speed, so no resistance
forms protoplanetary disk
swirling gas and dust
raw 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 gravity
As it collapsed, it
began to spin faster
and faster
Sun forms at the
center, surrounded by
a flattened disk
Planets form from gas
and dust in the disk
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The orderly motions of our solar system are a direct result of the solar system’s birth in a spinning,
flattened cloud of gas
Not 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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Disks have been
imaged with
HST’s infrared
camera
Young stars are
surrounded by
dense disks of gas
and dust
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From dust grains to planetesimals to
planets
http://www.metacafe.com/watch/1111454/formation_of_the_solar_system_great_animation/
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When the temperature is low enough, molecules condense (solidify) from the solar nebula into dust grains
Grains collide and stick: get larger by accretion
Grow into planetesimals (~1 km across)
Planetesimals collect into planets
From dust grains to planetesimals to
planets
Dust grains are the ‘seeds’ of planet formation
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When the temperature is low enough, molecules condense (solidify) from the solar nebula into dust grains
Grains collide and stick: get larger by accretion
Grow into planetesimals (~1 km across)
Planetesimals collect into planets
iClicker Poll:
What made what?
Solar nebula collapse leads to
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pre-Sun (“protostar”) forming in center
surrounded by disk of gas and dust
which will lead to planets, comets, asteroids
What will make what?
A.
B.
C.
D.
Earth mostly made of disk dust, Jupiter mostly made
of disk gas
Earth and Jupiter mostly made of disk gas
Earth and Jupiter mostly made of disk dust
Earth mostly made of disk gas, Jupiter mostly made
of disk dust
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Answer A
What are the ‘seeds’ made of?
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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!
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Everything 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 Disk
Heavy 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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Temperature Controls Planet Formation
The inner nebula was hot:
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and only metals and rock could condense there
no ices can form
The outer nebula was cold,
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ices could condense
in addition to metal and rock
Boundary: “snow line” or “frost line”
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closest place where ice can survive
too hot for ice!
ices can exist
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The 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
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Answer B
Terrestrials vs. Jovians
Inner solar system
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Metal & rock seeds
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Less material
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Small, rocky planets
Outer solar system
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Ices, rock & metal
seeds
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More material
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Proto-planets grow big
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Gravity captures large
amounts of H and He gas
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Large, gaseous planets
Jovian planets grew
massive enough to
gravitationally
capture gas from
disk
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Where did the asteroids and
comets come from?
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Planetesimals
The young solar system was filled with
trillions of planetesimals
Many were swept up to become parts of
the forming planets
What happened to the rest?
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Heavy Bombardment
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
The Moon’s surface shows
the scars of the heavy
bombardment
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Fates of the planetesimals
Swept up by forming planets or ejected from solar system
Near terrestrials - swept up
But some remain...
Asteroids and comets are le6over planetesimals!
Asteroids are rocky
because they formed
inside the frostline
Comets are icy because
they formed outside the
frostline
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Ceres is the largest of the rocky leftovers
Pluto & Eris are the largest of the icy leftovers
Origin of Asteroid Belt
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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
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Jupiter 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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This also changed the jovian planet orbits
Jupiter migrated inward
Other jovians migrated outward
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Earth’s Cosmic Water Source?
Interactions with Jovians may have sent waterbearing planetesimals to a young Earth
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Either icy planetesimals or rocky planetesimals with some water from near the frost line (beyond Mars’ orbit)
Recap:
The Violent Birth and Infancy
of the Planets
Heavy Bombardment
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planetesimals collide with young
planets
lasted 800 million years
Formation of the Moon: Smack!
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Mars-sized object collided with young
earth
orbiting debris clumped to form Moon
collision gave Earth its 23.5 degree
spin tilt
Early Earth
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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
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on your way to the Grand
Canyon: must-see detour!
Occurred 50,000 years ago
impactor:
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50 meter across
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impact speed
approximately 13 km/sec =
30,000 mph!
Energy of explosion equal to
25 megatons of TNT!
1.2 km across
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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!
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Space debris moving
at high speeds
Earth’s orbital speed:
30 km/s (67,000 mph)
Impactor speeds
entering the
atmosphere
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Range: 11-72 km/s
average asteroid speed:
17 km/s
average comet speed:
51 km/s
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Minimum 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/s
Why different averages? Asteroids orbit in the same direction as Earth, so the average net impact speed is lower
Comets orbits go every which way, so you can get head-on collisions!
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?
Proof in the Pudding:
Moon Pie
http://antwrp.gsfc.nasa.gov/apod/ap090206.html
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Proof in the Pudding:
Moon Pie
With even a small telescope, you can see
>30,000 craters on the Moon.
http://antwrp.gsfc.nasa.gov/apod/ap071120.html
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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
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the Moon has no atmosphere to
protect it
Earth’s surface is about 75% water
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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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