Download Lecture (Powerpoint)

Announcements
●
●
●
Mar 31st – last day to withdraw from course with a
`W' grade
If you don't know your current mark in this course,
email Jonathan ([email protected])
Less than ½ of total marks assigned so far in this
course; still time to make up points
●
Reading Quiz Marks:
●
Assignment Marks:
Review: Multicellular life vs. Colonies
●
Some confusion in assignment:
`when did multicellular life
first arrive?'
●
One common answer: 2.7 BYA
●
Stromatalite fossils?
●
●
●
●
Colonies of prokaryotes, not
multicellular organisms
Like mold on bread, yeast
Colony of independent singlecelled organism
Could take any organism and
transplant it – doesn't depend
on others
The Search: Our Solar System
●
History Of The Solar System
–
●
●
Planet formation
The Moon
–
History
–
Exploration
Venus
–
Greenhouse effect
–
Exploration
History of the Solar System
●
Planets, Comets, Asteroids, Meteorites
●
Protoplanetary disk
●
Instabilities
Planets
●
●
●
All but Pluto inhabit a plane --the ecliptic
From Earth's point of view
(tilted axis), Sun, Moon, planets
all trace out a single arc along
the sky
Dense, rocky planets near the
sun, large fluffy gas giants
further out
Orbits
●
●
●
●
●
Planets are falling towards Sun
due to gravitational acceleration
Moving toward the side fast
enough that they miss
Moving too fast – escape entirely,
leave Sun
Move too slowly – fall into Sun
Same with satellites circling Earth,
or Sun orbiting in our galaxy, or...
Gravity
●
●
●
●
●
Gravity acts between all massive
objects
Gravitational force is equal on
both objects
If orbiting, both objects move, not
just one, since both are being
acted on by gravity
Both orbit the center of mass of
the system
Equal mass objects; center of mass
is at the center of the two objects
Gravity
●
●
●
●
●
If one body is more massive,
then gravitational force is
increased
Center of mass tilts towards
more massive body
Forces still equal
Equal force on lighter body
moves it more than the same
force on the heavier body
Lighter object moves larger
distance than heavier object
Gravity
●
●
Force of gravity also increases
as objects get nearer
Inverse Square Law (same as
light)
Orbits
●
Kepler's Laws: (EMPERICAL)
–
Planets travel in ellipses,
with sun at one focus of
ellipse
–
Area swept out by radius is
equal over any equal amount
of time
–
Square of the planet's period
(the `year' for that planet)
proportional to the distance
to the sun cubed.
–
P2 ~ a3
Planets
●
●
●
●
●
Almost all planets are in same
plane
All planets (except Uranus)
rotate more or less in the same
plane, as does Sun
Very suggestive of the idea that
planets, Sun formed from a
disk, as we discussed before
Suggested by Laplace in 1600s.
Disk near star is depleted in
Hydrogen, Helium by
evaporation
Planet Formation
●
●
●
●
As disk cools, gas/dust disk can begin
condensing
Grains form, which themselves
agglomerate to larger particles
Regions where disk is originally dense
condense faster, gravitationally attract
more material
Process of continued agglomeration can
form planets
Instability
●
●
●
Some processes are naturally stable
–
Burning in main sequence stars
–
Core heats up – outer layers puff
up – core cools down
–
Automatically stabilizes itself
–
Ball in a right-side-up bowl
Once there's a region of high density in a
gas cloud or disk, increase in
gravitational attraction to that region...
Unstable
–
Ball on an up-side-down bowl
Planet Formation
●
●
●
●
Proto-planetary-core starts
sweeping out material and
planetesimals at its radius
Accrete material streams in from
just outside or inside its radius
There is a limit to this process; if
there are planets forming on either
side, eventually the gaps collide –
no more new material
This process of slowly sweeping
up and accreting material can take
millions of years
Mystery: `Hot Jupiters'
●
●
A Jupiter couldn't form at 1AU;
evaporation would prevent such a
gas giant from forming
Many of the extra-solar planets
observed are gas giants at
distances ~ 1AU
●
What happened?
●
Two possibilities:
–
Migration
–
Different formation
mechanism
Planet Formation
●
●
●
Migration is possible
As planets form and
accrete material, they
experience a drag force
Drag takes energy from
planets motion and they
fall inwards
Planet Formation
●
●
●
Fast formation is also
possible
In sufficiently massive disk,
instabilities can occur much
faster, and on larger scales
Can happen quickly enough
that perhaps giants can form
near star
Asteroids
●
●
●
Failed planet formation
between Mars and Jupiter
Remnants are thought to be
similar to the planetesimals
that existed in the rockyplanet zone
Orbit in the same plane as
the planets
Comets
●
●
●
`Dirty snowballs'
Similar to planetesimals of outer
solar system
Two regions: Kuiper Belt
–
●
`Asteroid Belt' past Neptune
Oort Cloud
–
Much much further out
(100,000 AU)
–
Perhaps objects kicked out of
solar system in early
formation??
Meteors, Meteorites
●
●
●
●
●
●
Meteors: `falling stars'
Caused by incoming object
burning up in Earth's atmosphere
20,000 – 150,000 mph
Sometimes remnant survives:
meteorite
Easiest form of space exploration:
bits of space come to us!
Meteors have contained amino
acids
Meteorites
●
●
●
●
Different kinds of meteorites
Carbonaceous Chondrite: very
nearly solar composition, minus
volatiles
–
Most common
–
Most primitive?
Achondrites: likely from Mars,
Moon
Chondrites: like crusts of
terrestrial planets
●
Stony-Iron
●
Iron
Meteorite Impacts
Meteorite-House impact
Park Forest, IL, Mar 2003
●
●
●
●
Occasionally Earth, other planet
hit by meteor/asteroid/comet
Chances of two solar system
objects colliding very small, but
there are a lot of objects out there
Even more objects in early solar
system; many have been swept
up/accreted/kicked out since then
Earth's atmosphere protects
against smaller objects
Meteorite Impacts
●
●
●
●
Often does damage to far more
than just drywall
Large objects rare but if they do
collide, can be catastrophic
Effect weather on surface of
planet
Even tear planet in two?
The Moon
●
What the Moon is like
●
How it formed
●
Exploration of the moon
The Moon
●
No atmosphere
●
No geological activity
●
No water
●
-> no erosion
●
Can provide information
about formation of solar
system that is absent from
Earth
Mercury
●
Similar to moon
●
Similar size
●
Small, empty, simple
●
Very close to Sun
●
No atmosphere to mediate
temperature swings:
–
+750o F in sun
–
-230o F in shade
Moon's Cratering
●
●
●
Nothing to alter surface
Complete history of
cratering in Moon's history
From predicted cratering
rate, one expects that crust of
moon formed very quickly in
solar system history
Exploration of Moon
●
●
●
●
●
Closeness means best explored
extraterrestrial body
USA,USSR began sending
flyby probes, impact probes to
moon in 1959 (Luna, Ranger)
In 1960s, began sending
landers (Luna, Surveyor),
return probes (Zond)
1968, Apollo 8 – Manned
mission
1969, Apollo 11 – Manned
landing
Exploration of Moon
●
●
Exploration continued through
the 1970s, with probes sent
with increasingly sophisticated
science equipment, cameras
Return probes, astronauts
brought many lunar samples
back for analysis
Exploration of Moon
●
●
●
●
●
Earliest probes took better pictures of surface,
determined no magnetic field
Once landed on surface, could observe
abundances
Difference between deep craters and surface
Much lower than even Earth's crust in volatiles
(Hydrogen, Carbon, Nitrogen...)
Oxygen isotopes very similar to those in Earth's
Crust
Exploration of Moon
●
●
●
●
●
Earliest probes took better pictures of surface,
determined no magnetic field
Once landed on surface, could observe
abundances
Difference between deep craters and surface
Much lower than even Earth's crust in volatiles
(Hydrogen, Carbon, Nitrogen...)
Oxygen isotopes very similar to those in Earth's
Crust
Possible Moon Formation Scenario
Possible Moon Formation Scenario
●
Explains similar Oxygen abundances
–
●
Very different from meteorites
Explains fewer volatiles
–
If Earth's iron core had already settled,
impact would have dislodged crust material
–
Heat of impact would have vaporized
volatiles
Moon Effect on Earth Life
●
●
●
Tides:
–
Helps drive ocean life to land
–
Helps form tidal pools
(`cells')
Moon's effect on tides stronger
than Sun's
Orbital tilt:
–
●
Moon's presence helps
stabilize Earth's tilt
Both may be helpful, but neither at
moment seem crucial
Venus
●
What Venus is like
●
The Greenhouse Effect
●
The Runaway Greenhouse Effect
Venus
●
●
●
●
●
Closest to Earth
¾ as far away from Sun as
Earth is
Very similar to Earth's size,
density
Covered by thick, opaque
clouds
Clouds opaque to visible,
infrared light, but transparent to
radio; can use radar to map
surface
Venus: Inverse Square Law
●
●
●
●
Venus ¾ as far away from sun,
so gets (4/3)2 ~ 1.8 times as
much light
Blackbody radiation in radio
from surface -> 850o F on
surface!
Much hotter on surface than
inverse-square law accounts for.
Heat from surface/geologic
activity? Radar, Soviet probes
say no.
Planet Temperatures
●
●
Earth is continuously
bombarded by radiation from
Sun
Why doesn't Earth get hotter
and hotter?
Planet Temperatures
●
●
●
●
●
Earth is continuously bombarded
by radiation from Sun
Why doesn't Earth get hotter and
hotter?
As Earth warms, glows as a
blackbody
Heats up more, glows more
Equilibrium is reached when
energy coming from Sun equals
energy given off by Earth
●
(Stable or Unstable?)
●
Can calculate this eq: -22oF
Planet Temperatures
●
●
Not bad, but Earth is on average
warmer than that (~60oF)
What happens?
Planet Temperatures
●
●
●
●
●
●
Not bad, but Earth is on average
warmer than that (~60oF)
What happens?
Atmosphere traps some radiation
inside
Less heat escaping for a given
surface temperature
Pushes equilibrium higher
Planet is warmer than would be
without atmosphere
Greenhouse Effect
●
●
●
●
●
●
This happens in greenhouses, or in
cars on a hot day
Sunlight streams in through
transparent (to visible light) glass,
is absorbed by plants/car seats/etc
Hot material emits energy as a
blackbody; if outside, would cool
somewhat
But glass is opaque to infrared
light (where most blackbody
radiation would be emitted)
Energy is trapped inside
greenhouse/car
Gets hot
Greenhouse Effect
●
●
●
Earth's Atmosphere, too,
is transparent to visible
light but largely opaque
to infrared light
CO2, water vapor are
`greenhouse gases'
which trap some IR in
atmosphere
Much of early CO2 in
Earth's atmosphere
locked up in oceans
Greenhouse Effect
●
●
●
●
Venus' atmosphere has a huge
amount of Carbon Dioxide
Traps an enormous amount of heat
All water completely gone; not
merely evaporated, but broken
down
High temperatures make other
compounds more likely than on
earth – Sulphuric acid, etc.
Runaway Greenhouse Effect
●
Greenhouse effect on a planet with oceans is
unstable:
–
Planet gets warmer
–
Increased evaporation of oceans
–
More water vapor in atmosphere
–
(And more CO2)
–
Greenhouse gas!
–
Planet gets warmer....
Runaway Greenhouse Effect
●
●
●
●
●
●
If Earth were moved to Venus' orbit, runaway
greenhouse effect would occur
And that explains biggest difference between the
two today
Difference in surface temperature, atmosphere
composition,...
Much larger effect than naïve estimate from
distance to Sun
Large effect of atmosphere
Planets as near a Sun-like star as Venus can't
have liquid water
Assignment for Next Class (Apr 2)
●
●
●
Very short (happy break!)
Come up with and consider three processes (at
least one of each) that are either stable or
unstable.
Once something starts happening, either:
–
There's a compensating effect which
pushes the system back to where it was,
(stable) or
–
There's an effect which pushes whatever's
happening even faster (unstable)
Reading for Next Class (Apr 2)
●
Chapter 13, 14: Mars, and Life on Mars?
–
History of Mars Exploration
–
Search for Water
–
Search for Biology