Transcript NATS1311_112008_bw

Accretion and Formation of Planets Animation
Origin of the Asteroids
The Solar wind cleared the leftover gas, but not the leftover planetesimals.
Those leftover rocky planetesimals which did not accrete into a planet, crash into
inner planets, or get ejected from solar system are the present-day asteroids.
Most inhabit the asteroid belt between Mars & Jupiter. Jupiter’s gravity prevented
a planet from forming there.
Origin of the Comets
The leftover icy planetesimals
are the present-day comets.
Those which were located
between the Jovian planets, if not
captured, were gravitationally
flung in all directions into the
Oort cloud.
The nebular theory predicted the
existence of the Kuiper belt 40 years
before it was discovered!
Those beyond Neptune’s orbit
less likely to be destroyed by
collision or flung off - remained
in the ecliptic plane in the Kuiper
belt. Didn’t grow as large as
Jovian planets - material density
too low - still managed to grow to
large sizes- hundreds of
kilometers in diameter or more Pluto probably a Kuiper belt
comet
Exceptions to the Rules
So how does the nebular theory deal with exceptions, i.e. data which do not
fit the model’s predictions?
There were many more leftover planetesimals than we see today.
Most of them collided with the newly-formed planets and moons during
the first few 100 million years of the Solar System.
This was the heavy bombardment period.
Close encounters with and impacts by planetesimals could explain:
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Why some moons orbit opposite their planet’s rotation
– captured moons (e.g. Triton)
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Why rotation axes of some planets are tilted
– impacts “knock them over” (extreme example: Uranus)
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Why some planets rotate more quickly than others
– impacts “spin them up”
•
Why Earth is the only terrestrial planet with a large Moon
– giant impact
•
Source of water on Earth
– impact of icy planetesimals flung into the inner solar system by
gravitational encounters with Jovian planets
Formation of the Moon
(Giant Impact Theory)
• The Earth was struck by a Marssized planetesimal
• A part of Earth’s mantle was
ejected
• This coalesced into the Moon.
– it orbits in same direction as
Earth rotates
– has lower density than Earth
- formed from Earth’s outer
layers
– has smaller amounts of
easily vaporized ingredients
(e.g., water)
– Earth was “spun up”
Extrasolar Planets
• Planets which orbit other stars are called extrasolar planets.
• Over the past century, we have assumed that extrasolar planets exist, as
evidenced from our science fiction.
• We finally obtained direct evidence of the existence of an extrasolar
planet in the year 1995.
– A planet was discovered in orbit around the star 51 Pegasi.
– Over 200 such extrasolar planets are now known to exist.
Detecting Extrasolar Planets
• Can we actually make images of extrasolar planets?
– No, this is very difficult to do.
• The distances to the nearest stars are much greater than the distances
from a star to its planets.
– The angle between a star and its planets, as seen from Earth, is too
small to resolve with our biggest telescopes.
• A star like the Sun would be a billion times brighter than the light
reflected off its planets.
• As a matter of contrast, the planet would be lost in the glare of the star.
•
Improved techniques of interferometry may solve this problem
someday.
Detecting Extrasolar Planets
•
We detect the planets indirectly by observing the star.
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Planet gravitationally tugs the star, causing it to wobble.
•
This periodic wobble is measured from the Doppler Shift of the star’s spectrum.
Stellar Motion and Planets Animation
Doppler shift allows
detection of slight motion
of star caused by orbiting
planet
Determining Star’s Velocity Animation
A plot of the radial
velocity shifts forms a
wave.
–Its wavelength
tells you the period
and size of the
planet’s orbit.
–Its amplitude tells
you the mass of the
planet.
Doppler shift in spectrum of star 51 Pegasi - shows presence of large planet
with orbital period of about 4 days.
Determining Planet Mass and Orbit Animation
Remember - Doppler shift only tells us radial motion. If plane of orbit
perpendicular to our line of sight - no shift seen. If we view it from edge on,
maximum Doppler shift seen. Orbit generally tilted at some angle - star’s full
speed not measured. So mass derived from Doppler technique is minimum
possible. If changing velocity and varying position in sky measured (as in one
case - Gliese 876) orbital tilt can be determined and mass measured
accurately. Gliese 876 is only about 15 LY away.
Planetary Transit
•
The Doppler technique yields only planet masses and orbits.
•
Planet must eclipse or transit the star in order to measure its radius.
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Size of the planet is estimated from the amount of starlight it blocks.
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We must view along the plane of the planet’s orbit for a transit to occur.
– transits are relatively rare
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They allow us to calculate the density of the planet.
– extrasolar planets we have detected have Jovian-like densities.
Planetary Transit Animation
Orbital distances and
approximate masses of
first 77 planets
discovered. There have
been 313 extrasolar
planets discovered to
date.
Gliese 581 d - the third planet of the red dwarf star Gliese 581
(approximately 20 light years distance from earth)
- appears to be the best example yet discovered of a possible
terrestrial exoplanet which orbits close to the habitable zone of
space surrounding its star
- appears to reside outside of the "Goldilocks" zone, but greenhouse
effect may raise the planet's surface temperature to that which would
support liquid water.
False-color infrared image of the
brown dwarf 2M1207 (blue) and its
planetary companion 2M1207b (red),
as viewed by the Very Large
Telescope.
- only confirmed extrasolar planet
to have been directly imaged.
Atmospheres
Atmosphere - a layer of gas which surrounds a world
– usually very thin compared to planet radius
Pressure is created by atomic and molecular collisions in an atmosphere.
– heating a gas in a confined space increases pressure
– number of collisions increase
– unit of measure: 1 bar = 14.7 lbs/inch2 = Earth’s atmospheric pressure at sea
level
Pressure balances gravity in an atmosphere
– atmospheric pressure equal to weight of a column of gas extending upward
Earth’s atmosphere extends several hundred kilometers into space
– no official boundary
Low-Earth orbiting satellites (a few hundred kilometers) experience atmospheric drag
– slows them down so they spiral downward, eventually burning up as they
reenter the dense lower atmosphere
– Space Station and Hubble space telescope have to be periodically boosted to
higher orbits
– larger satellites don’t completely burn up - space debris
Atmospheres
Atmosphere - layer of gas surrounding a
world
Atmospheric pressure - collisions of individual
atoms or molecules in atmosphere
Air molecules in a balloon exert pressure as
they collide with the walls pushing outward.
Air molecules outside balloon collide with wall
and exert pressure inward. Balloon stays
inflated when pressures are balanced.
Adding molecules to balloon (blow it up)
causes balloon to expand (increases its
volume) until pressures are balanced again.
Heating it also increases pressure (increases
the speed of the molecules). The balloon
expands until pressures are equalized again
Gas in an atmosphere is held down by gravity.
Atmosphere above presses downward,
compressing atmosphere below.
At the same time, fast moving molecules exert
pressure in all directions, including upward tends to make atmosphere expand.
Planetary atmospheres exist in balance
between downward weight of their gases and
upward push of their gas pressure
The higher you go, the less the weight of gas
above you, and the less the atmospheric
pressure.
1 bar - atmospheric pressure at sea level equal to weight of a column of gas extending
upward from Earth’s surface from sea level
Effects of an Atmosphere on a Planet
greenhouse effect
- makes the planetary surface warmer than it would be otherwise
extreme on Venus
just right for life on Earth
weak on Mars
- distributes heat around planet
scattering and absorption of light
- absorb high-energy radiation from the Sun
- scattering of optical light brightens the daytime sky
creates pressure
- can allow water to exist as a liquid (at the right temperature)
creates wind and weather
promotes erosion of the planetary surface
creates magnetosphere
- caused by interaction of atmosphere with the Solar wind when magnetic fields
are present
- protects atmosphere from loss of gases
- protects surface from high-energy solar particles
- leads to aurora
The Greenhouse Effect
Visible Sunlight passes through a
planet’s atmosphere.
Some of this light is reflected, some is
absorbed by the planet’s surface - heats
surface up
Planet re-emits this energy (heat) as
infrared (IR) light - blackbody (thermal)
spectrum
IR light is “trapped” by the atmosphere.
- absorbed and re-emitted in
random directions by greenhouse
gases - H2O, CO2, CH4 (methane)
- surrounding air is heated
This causes the overall surface
temperature to be higher than if there
were no atmosphere at all.
The Greenhouse Effect Animation
Greenhouse Gases
Key to Greenhouse Effect…gases which absorb IR light effectively:
- water [H2O]
- carbon dioxide [CO2]
- methane [CH4]
These are molecules which rotate and vibrate easily.
- they re-emit IR light in a random direction
The more greenhouse gases which are present, the greater the amount of surface
warming.
Greenhouse Gases Animation
Planetary Energy Balance
Solar energy received by a planet must balance the energy it returns to
space
- planet can either reflect or emit the energy as radiation
- this is necessary for the planet to have a stable temperature
What Determines a Planet’s Surface Temperature?
Greenhouse Effect cannot change incoming Sunlight, so it cannot
change the total energy returned to space.
- it increases the energy (heat) in lower atmosphere
- it works like a blanket - it slows the escape of heat
In the absence of the Greenhouse Effect, what would determine a
planet’s surface temperature?
- the planet's distance from the Sun
- the planet’s overall reflectivity
- the higher the albedo (the reflectivity of the surface), the less
light absorbed, planet cooler
- Earth’s average temperature would be –17º C (–1º F) without the
Greenhouse Effect
The Inverse Square Law
The inverse square
law for light. At
greater distances
from the Sun, the
same amount of light
passes through an
area that gets larger
with the square of
the distance. The
amount of light per
unit area therefore
declines with the
square of the
distance. The closer
a planet it to the Sun,
the more light it
receives.
Albedo
Albedo - the fraction of light that is reflected or scattered by a body or
surface
Substance/Object
Albedo
Enceladus
0.8
Europa
0.6
Forest
0.05 - 0.10
Granite
0.30 - 0.35
Grass
0.05 - 0.30
Mars
0.25
Moon
0.12
Sand
0.20 - 0.40
Snow
0.6
Soil
0.05 - 0.30
Urban-areas
0.05 - 0.20
A = [total scattered light]/[total incident light]
Temperature vs Reflectivity Animation
Greenhouse Effect on the Planets
Greenhouse Effect warms Venus, Earth, and Mars
- on Venus: it is very strong
- on Earth: it is moderate
- on Mars: it is weak
- average temperature on Venus and Earth would be freezing without it
Light Scattering
Atmospheric gases are largely transparent to visible light
Most photons penetrate to the ground, warming it as the light is
absorbed
Small portion of light is scattered
- why our sky is bright
- light is not scattered on Moon, Mercury
- their skies are dark - stars are visible during day
- shadows extremely dark
Gas molecules scatter blue light more effectively than red light
Atmospheric gases scatter blue light more than red light. During most of the
day, you therefore see blue photons coming from most directions in the sky sky looks blue. Only red photons reach your eyes at sunrise or sunset - light
must travel a longer path through the atmosphere to reach you. Atmosphere
on Mars too thin to scatter light effectively - sky is reddish from presence in
the atmosphere of reddish dust from surface. On Venus, almost all blue light
scattered away - atmosphere dimly lit and appears reddish orange.
Atmospheric Structure
Atmospheric structure determined by
interactions of light from the Sun and
the atmospheric gases
X rays
- ionize atoms & molecules
- dissociate molecules
- absorbed by almost all gases
Ultraviolet (UV)
- dissociate some molecules
- absorbed well by O3 (ozone) and
H 2O
Visible (V)
- passes right through gases
- some photons are scattered
Infrared (IR)
- absorbed by greenhouse gases
Structure of Earth’s Atmosphere
pressure and density of atmosphere decrease with altitude
temperature varies “back and forth” with altitude
- these temperature variations define the major atmospheric layers
exosphere
- low density; fades into
space
thermosphere
- temp begins to rise at
the top
stratosphere
- rise and fall of temp
troposphere
- layer closest to
surface
- temp drops with
altitude
Reasons for Atmospheric Structure
Light interactions are responsible for the structure we see.
Troposphere
- absorbs IR photons from the surface
- temperature drops with altitude
- hot air rises and high gas density causes storms (convection)
Stratosphere
- lies above the greenhouse gases (no IR absorption)
- absorbs heat via Solar UV photons which dissociate ozone (O3)
- UV penetrates only top layer; hotter air is above colder air
- no convection or weather; the atmosphere is stratified
Thermosphere
- absorbs heat via Solar X-rays which ionizes all gases
- contains ionosphere, which reflects back human radio signals
Exosphere
- hottest layer; gas extremely rarified; provides noticeable drag on satellites
Structure of Terrestrial Planet Atmospheres
Mars, Venus, Earth all
- have warm tropospheres
(and greenhouse gases)
- have warm thermospheres
which absorb Solar X rays
Only Earth has
- a stratosphere - because it
contains a UV-absorbing
gas (O3)
.
All three planets have warmer
surface temps due to greenhouse
effect
Planets with very little gas like
Mercury only have an exosphere
Magnetospheres
The Sun ejects a stream of charged particles, called the solar wind.
- it is mostly electrons, protons, and Helium nuclei
Earth’s magnetic field diverts these charged particles and allows them to enter the
atmosphere only near the poles
- the particles spiral along magnetic field lines and impact the atmosphere
causing it to fluoresce
- this causes the aurora (aka northern and southern lights)
- this protective “bubble” is called the magnetosphere
Other terrestrial worlds have no strong magnetic fields and thus no
magnetosphere
- solar wind particles impact the exospheres of Venus and Mars
- solar wind particles impact the surfaces of Mercury and Moon
The Earth’s Magnetic Field
The rotating molten metallic core of the Earth generates a magnetic field
with magnetic field lines like those illustrated by the influence of a bar
magnet on iron filings.