Download Week 8

Leftovers
Gas is
eventually
captured or
pushed out by
wind from the
star, but dust
and
planetesimals
are left around
Formation of the Solar System
STEPS:
CLOUD
COLLAPSE
EVIDENCE:
•young stars seen in collapsing gas clouds
•planets orbit in same direction and same plane
ROTATING
DISK
•Sun and planets rotate in same direction
•disks seen around other stars
•terrestrial planets and asteroids found near Sun
CONDENSATION
•jovian planets, icy moons, comets found farther
away
•many meteorites are made of smaller bits
ACCRETION
•heavy cratering on oldest planet surfaces
•asteroids, comets are “leftovers”
GAS
CAPTURE?
•Jupiter, Saturn are mostly hydrogen and helium
•gaps in disks around other stars
Illustration of
Kepler planet
candidates
blocking
their stars
SUN
+JUPITER
+ EARTH
Transits - A Kind of Eclipse
• planet crosses in front of
a star, making star
appear fainter
1193 planets detected so
far!
Planet Transits
• orbit period and estimate of star mass tells us about
semi-major axis of planet:
æ P ö æ M Sun öæ A ö3
֍
÷
ç
÷ =ç
è 1 yr ø è M øè 1 AU ø
2
VIEW FROM EARTH:
star
planet
• amount of starlight blocked tells us
about size of planet:
Rstar
2
light blocked by planet p Rplanet æ Rplanet ö
=
=ç
÷
2
starlight
p Rstar è Rstar ø
2
Rplanet
Thought Question
Most planets that have been discovered around other stars are thought
to be like Jupiter. The Sun is about 11 times the size of Jupiter. What
fraction of the Sun’s light would get blocked if we were watching
Jupiter transit the Sun from far away?
A. 99%
B. 90%
star
C. 10%
D. 1%
E. 0.1%
F. 0.01%
planet
Questions!!!
Why are some jovian planets found near their stars?
(“Hot Jupiters”)
•
Do jovian planets form differently than we think?
• Did jovian planets “migrate” in toward their stars?
Why doesn’t everything have the “special direction”?
• Why are Venus and Uranus rotating differently?
• Why do some planets orbit in the opposite way that their stars rotate?
What are “super Earths” and “mini-Neptunes”?
Will terrestrial planet discoveries reveal something completely new?
“Hot Jupiters”
• orbit in as little as
0.8 Earth days!
• cloud-top temperatures
up to 1300 K
• some are “puffy” – up to
30% larger than Jupiter
HOW DID THEY GET
THERE?
“Super Earths”/”Mini Neptunes”
• a variety of densities
• about 2-10 Earth’s
mass, and up to 4x
Earth’s radius
How common are they?
How do they form?
What could they be
like?
still looking for
these!!
Comparative Planetology
Questions:
• How are planets similar and different?
 surface
 atmosphere
• Why are they different?
Uncovering Planet
History
CRATERING:
All planets heavily bombarded in past…
If a planet has craters today:
• surface wasn’t completely protected
(by atmosphere or oceans)
• surface wasn’t “cleaned” recently
(erosion, lava flows, or tectonics)
Look at:
 how heavily cratered surface is
 where craters are
Thought Question:
C (crater)
D (lava flow from
another volcano)
B (crater)
A (volcano)
What is the order from oldest to youngest?
Mercury:
heavily cratered, but
how long ago?
Impact Craters
Radiometric Dating of Rocks
A small fraction of atoms are radioactive - breaking up and
forming atoms of a different chemical element
Potassium-40:
Uranium-238:
 When rock solidifies, radioactive atoms and their products are
“frozen” into it …
no escape!
Radiometric Dating
•Nuclei of parent species (like potassium-40) decay to become
daughter products (like argon-40)
TOTAL K+Ar
N æ1ö
=ç ÷
No è 2ø
t / t half
N : current amount
N o : original amount
t half : half - life
half-life: time for half of radioactive atoms to decay
potassium-40: 1.25 billion yr
uranium-238: 4.468 billion yr
Thought Question:
Each atom of element Y will eventually decay to form one
atom of element X (with a half-life of 800 million years).
When the planets solidified, there was no element X. If
you find a rock where element Y is currently has 1/8th the
total element X plus element Y,
…what fraction of the
original Y is left today?
…how old is the rock?
(Enter your answer in
millions of years.)
N æ1ö
=ç ÷
No è 2ø
t / t half
N : current amount
N o : original amount
t half : half - life
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
FRACTIONS:
Chemical X
Chemical Y
Start 1 half- 2 half- 3 half- 4 halflife
lifes lifes lifes
1: 0
PROPORTIONS:
1 1
:
2 2
1 3
:
4 4
1 7
:
8 8
1 15
:
16 16
1:1
1:3
1:7
1:15
Oldest Known Crystal
4.374±0.006 billion yr old
Ages of Surfaces
•Radiometric dating of Moon rocks allow us to measure when
the rock was last melted…
…when were most craters made?
Thought Question:
This is a picture of Saturn’s
moon Enceladus. Which of
the following statements
about its surface is
probably true? (Enter the
letters for all correct
answers.)
A. The lower left part of the
surface probably froze
more recently than the
upper right.
B. The oldest parts are about
as old as the oldest parts
of the Moon.
C. None of the above is true.
History of Cratering
• most cratering in
first billion years of
solar system
• heavily cratered
surfaces have
changed little in last
3 billion years
A Scatter Plot
Can this help us
predict terrestrial
planet properties?
1/3 Earth
95% Earth
1/2 Earth
1/4 Earth
What Made Them This Way?
PLANET
MASS/RADIUS
STRENGTH OF
GRAVITY
HOLDS ONTO
GASES?
VOLCANIC
ACTIVITY
HOW MUCH
OUTGASSING?
DISTANCE FROM
SUN
TEMPERATURE
CONDENSATION?
Uncovering Planet History
VOLCANOES:
If a planet has volcanoes:
• interior was hot enough for rock to
move
• crust was thin enough to allow lava
to reach surface
Clues:
number of volcanoes
their pattern
are they active?
very few
volcanoes, but
they are VERY
large…
Mars
volcanoes
probably not
active
Venus
 largest number of volcanoes among terrestrial planets
 evidence of active volcanoes
Thought Question:
Suppose a terrestrial planet the same age as Earth is
discovered orbiting another star, and it is your job to
predict what it is like. If the planet is known to have a
mass and size smaller than Venus but larger than Mars,
what would be the best prediction?
A. It should have no volcanoes.
B. It should have volcanoes, but they may or may not be
active.
C. It should have a moderate number of active volcanoes
spread evenly over the surface.
D. It should have a large number of active volcanoes found
only in small areas of the surface.
Moon Oceans?
Enceladus
(Saturn moon)
Ganymede
(Jupiter moon)
Terrestrial Planet Interiors
Temperature of interior involves competition between:
• thermal energy added
LEAKY BUCKET ANALOGY:
by radioactivity:
radioactive atoms release heat
when they decay
SOURCE OF HEAT
HEAT NOW IN
PLANET
• thermal energy lost
to space:
heat can only escape from planet’s
surface
HEAT LOST
FROM
SURFACE
Terrestrial Planet Interiors
• thermal energy
added by
radioactivity:
every bit of mass contains a
small fraction of radioactive
atoms:
• thermal energy lost to
space:
heat can only escape from
planet’s surface
energy
planet
added per
mass
second is related
to
energy
lost per
second
4 3
Eadded µ M »average
D ×V = D × p R
3
density
E lost µ A = 4pR
is related
to
planet
area
2
Loss of Internal Heat
Terrestrial planets are losing heat (cooling) because
radioactive heating is smaller, but exactly how small
affects how quickly the inside cools off:
E added
E lost
4 3
p
R
V 3
µ =
µR
2
A 4pR
Thought Question:
Based on the rates of energy being added and lost,
which terrestrial planet should cool off most slowly?
(Hint: compare rates of adding and losing thermal
energy by thinking about the ratio.)
A. Mercury
B. Venus
C. Earth
D. Moon
E. Mars
4 3
Eadded µ p R
3
2
Elost µ 4p R
Thought Question:
Mars is about half the diameter of Earth and
1/10th the mass of Earth. Based on the
energy added and lost per second, Mars
probably cooled off…
A. about 10 times as fast as Earth.
B. about 2 times as fast as Earth.
C. at about the same rate as Earth.
D. about 1/2 as fast as Earth.
E. about 1/10th as fast as Earth.
4 3
Eadded µ p R
3
2
Elost µ 4p R
The Heat Inside:
• As planets formed, collisions and radioactivity melted
them into sphere shape:
• large planets take longer to cool off and solidify inside:
Earth, Venus:
Mars:
Mercury, Moon:
still active today
active volcanoes
once, but not many
dead for a long
time
Venus from the
Ground
Atmosphere Conditions
Average Temperature:
850 F
737 K
Atmospheric Pressure:
90x Earth’s
Chemicals:
96% CO2
60 F
288 K
-60 F
210 K
0.007x Earth’s
78% N2
95% CO2
Terrestrial Planet Atmospheres
Addition of Gases
• Outgassing by volcanoes
• Bombardment
Loss of Gases:
• Gas escape into space
• Condensation
• Chemical reactions
Where Did It Come From?
Comet impacts bring “ices”:
water vapor (H2O)
carbon monoxide (CO)
ammonia (NH3)
methane (CH4)
Volcanoes release gas from
molten rock:
carbon dioxide (CO2)
nitrogen (N2)
water vapor (H2O)
Escape Speeds
v
• Larger planet mass  gas molecules have to
move faster to escape
2GM planet
escape speed:
v =
esc
Rplanet
• Higher temperature  faster gas molecules move
 easier to escape
Temperature
• Temperature relates to average speed of motions of atoms
• absolute zero (0 K) is when thermal motion stops
box increases the
average particle speed
Gas animation
Speeds of Gas Molecules
E K = kT
3
2
Average particle kinetic energy in a
gas only depends on temperature:
-23
k = 1.38 ´10
J
K
(Boltzmann’s constant)
3
2
Average particle speed in a gas:
1 2
kT = EK = mv
2
3kT
2
v =
m
3kT
v=
m
Gas of a particular chemical has a
chance to escape over the history of
the solar system if:
1
v > vesc
6
Thought Question:
What combination of factors makes it more likely
to lose a gas from a planet’s atmosphere?
(Enter a three letter answer.)
A. High mass
planet
B. Low mass planet
C. High temperature planet
D. Low temperature planet
E. High-mass gas
molecules
F. Low-mass gas molecules
Thought Question:
How does the average speed of a hydrogen
molecule (H2) compare to the average
speed of an oxygen molecule (O2) at the
same temperature?
(H2 has twice the mass of hydrogen atom, and O2
has 32 times the mass of a hydrogen atom.)