Lecture 6

Mass of the solar system

99.8% sun

terrestrial planets

The rocky inner planets (Mercury, Venus, Earth, and Mars)

Jovian Planets

The gaseous outer planets (Jupiter, Saturn, Uranus, and Neptune)

Between inner and outer planets

asteroid belt

Pluto

once considered a planet, has been reclassified as a dwarf planet.

Kuiper Belt

A region of the solar system that is just beyond the orbit of Neptune and that contains small bodies made mostly of ice

Trans-Neptunian Objects (TNOs)

Objects orbiting the Sun beyond the orbit of Neptune like Pluto, Eris

Planet

must be massive enough:
- (1) for its gravity to pull it into a roughly spherical shape, and
- (2) for it to have cleared out the neighborhood of its orbit of comparable mass objects

Oort Cloud

The Solar System is surrounded by a cloud of cometary bodies
- Gravitational influences from passing stars occasionally send comets into the Solar System

Mercury

orbit is tipped by 7 degrees.

Planets Rotation

counterclockwise as viewed from above
- Venus rotates clockwise as viewed from above.
- Uranus' rotational axis is tipped significantly.

Planet's Density

Calculate the planet's mass (M) by observing its satellite's orbital distance (d) and period (P).

If we know the distance to the planet, we can measure its angular diameter and calculate its linear diameter (or radius, R), and then its volume:

Average Density Inner Planets

• Inner planets have high average densities (~5 kg/liter):
- small bodies
- mostly rock and iron

Average Density Outer Planets

Outer planets have lower densities (~1 kg/liter):
- larger bodies
- gasses, ices and other volatiles

age of solar system

4.6 billion years
radioactive dating

half-life

length of time required for half of the radioactive atoms in a sample to decay

Model of Solar System Formation

Any model of the solar system's formation must account for all observations:
- Planets must revolve around the sun more or less in the same plane.
- Planets must rotate about their axes in the same direction as they revolve around the sun.
- Rocky, dense planets must be found close to the sun, and gaseous bodies must be farther from the sun.

Solar Nebula Theory

The solar system originated from a rotating, disk-shaped cloud of gas and dust, with the outer part of the disk becoming the planets, and the inner part becoming the sun.
4.5 billion years ago, the immense cloud of gas and dust that would become our solar system began to contract.
- As it contracted, it flattened into a disk and began to spin faster (Conservation of Angular Momentum).
- Most of the material in the cloud moved to the center to become the sun.

Condensation and Formation of Planets

As the material in the center gathered, its temperature increased.
The rest of the disk began to cool, and the gasses present began to condense, like steam rising out of a kettle.
- Close to the center, where the temperature was highest, only silicates and metals could condense.
- Farther out, volatile gasses could condense.

Silicate

a member of the mineral group that has silicon and oxygen in its crystal structure

rocky planetesimals

In the inner solar system, silicate (crystals of silicon and oxygen) and metal grains accreted (stuck together) over time, to form rocky planetesimals. These would become the terrestrial planets.

Icy planetesimals

In the outer solar system, icy planetesimals formed.

Planeresimals

grew through accretion into protoplanets, which were heated by collisions and by radioactive decay.
collided and gathered over millions of years to form the planets.

Differentiation

Denser material sank toward the center of the bodies, and lighter material floated toward the surface.

Atmosphere

The atmospheres of the terrestrial planets formed last, by either (or both):
Outgassing
Collisions

Outgassing

Volatiles trapped inside the planet escape through volcanoes or other processes.

Collisions

Volatiles could have been freed from the planet's crust by collisions, or via direct delivery by comets.

Atmospheric Retention

Small planets will have low escape velocities.
• Atmospheres around planets close to the sun will be very warm, giving the gas atoms a high thermal velocity.
• If the thermal velocity of atmospheric gasses is close to the escape speed for the planet, the atmosphere can escape.

protoplanetary disk

disks of dark, dusty material orbiting young stars.

doppler shift method

-A planet and its stars revolve around a common center of mass.
- We cannot detect the planet directly, but we can detect the resulting "wobble" in the central star. - As the star approaches us in its motion around the center of mass, its spectrum will be blueshifted.
- As it recedes, the spectrum will be redshifted.

Transit Method

Look for dimming of light from the central star.

Gravitational Lensing

Gravity bends the path of light when it passes near a massive object.
If that object is a star with companion planets, we can use the lensing effect to detect them.

Brown Dwarfs

stars too low in mass to fuse hydrogen.

Composition of Earth

surface is composed mostly of silicon and oxygen.
- These two combine to form silicates.

Earthquakes generate

Pressure waves and Shear waves

P waves (pressure waves)

act like sound waves and travel through rock and liquid equally well.

Detected everywhere on the planet

S waves (shear waves)

are like the waves on a guitar string (moving sideto-side), and only travel through rock.

liquid core of earth casts a shadow in s-waves revealing itself

Earth 4 layers

The Crust, mantle, liquid core, and solid inner core

Crust

outermost layer, composed of mostly silicates, like the skin of an apple

Mantle

region of hot but not quite molten rock; flows under great pressure and extends halfway to the center of the Earth

Liquid Core

dense liquid mixture of iron, nickel, maybe sulfur

Solid Inner Core

probably composed of iron and nickel

When did layering occurr?

during differentiation, early in the Earth's history.

earth core temperature

around 6500 K, as hot as the surface of the sun

Convection

Mantle material heated near the hot core rises, and cooler material near the top of the mantle sinks. This is very similar to water in

continental plates

Lie beneath landmasses. Contains more silicon dioxide. Plates less dense than oceanic.

Subduction

As plates are pushed together, one plate can slide under another.

plate tectonics

A theory stating that the earth's surface is broken into plates that move.

plate boundaries

the edges of tectonic plates where volcanoes and earthquakes are

Earth's magnetic field

Magnetic lines of force from Earth's polar north and south, acting like a giant magnet.
generated by currents flowing in the molten iron core.
flips every few hundred thousand years
protect from harmful rays

Atmosphere

The Earth is surrounded by a thin envelope of gas, its atmosphere, which is composed mostly of nitrogen

Hydrosphere

All the water at and near the surface of the earth, 97% of which is in oceans

atmospheric pressure

The atmosphere of the Earth pushes downward on the surface and everything on it.
- The closer you are to the surface, the greater the weight.
- At the surface (sea level), atmospheric pressure is called one atmosphere.

Troposphere

• Lowermost layer, extending to around 12 km upward.
• Temperature decreases rapidly with altitude.
• Most clouds are found here

Stratosphere

• It extends from the troposphere to about 50 km up.
• Temperature increases with altitude.
• Ozone layer is here.

Ionosphere

It is located above 80 km or so.
• Most atoms are ionized - one or more electrons have been removed from each atom.
• These ionized particles reflect AM radio signals back down to Earth.

Aurora Borealis (Northern Lights)

located in the ionosphere. They are:
• electrical currents flowing through the atmosphere.
• created by energetic particles colliding and exciting ionospheric particles.

global warming

ultraviolet light absorbed by ozone
carbon dioxide absorbs infrared and warms the surface

greenhouse effect

Natural situation in which heat is retained in Earth's atmosphere by carbon dioxide, methane, water vapor, and other gases

Water on Earth

covers 71%

cuts through rock

stores carbon dioxide

change chemical makeup of rocking making them melt at lower temp

Coriolis effect

The effect of Earth's rotation on the direction of winds and currents.

Maria

Dark, flat areas on the moon's surface formed from huge ancient lava flows.

Craters

impact sites where planetesimals and other bodies struck

Highlands

impact sites where planetesimals and other bodies struck

Rilles

structures that look like narrow canyons or stream courses

most likely ancient lava tubes that ran just beneath the surface.

Regolith

The outermost layer is powdered rock

Moon's crust

Thicker on far side and thinner on earth side

Moon's Mantle

thick, but too cool to be stirred by convection.

Moon's iron core

may be molten, and is located not quite at the center of the moon.

Mercury

least explored

Dayside temp 700K

Nightside temp 70K

very eccentric orbit

Scarps

Running across Mercury's
surface are scarps, kilometer-
tall cliffs that are the result of
the shrinking of the planet as
it cooled.
• Scarps are similar to wrinkles
in an apple's skin that form
as it dries.
• The presence of these cliffs
suggests that the core of
Mercury is large and metallic
- something that would
shrink significantly as it
cools.

Caloris Basin

A large "bulls-eye" impact crater andis the largest feature in the surface of Mercury

Interior of Mercury

Mercury's density is unusually high, 5.4 kg/liter.
• This indicates a very large
iron core covered by a thin
silicate mantle.
• Mercury may have a molten
iron core (mixed with
sulfur), despite its size.
• This core may be the cause
of the weak magnetic field
(similar in shape to the
Earth's, but much weaker).

Why Mercury so dense

It is possible that Mercury once had a thicker mantle, but a collision
removed most of the material, leaving only the core and a little lighter
mantle material behind.

Venus

Slightly smaller than earth
Thick clouds
Rotates once every 243 days
surface hot

Venus atmosphere

100 times more massive
- Surface pressure is 90
atmospheres.
- 96.5% carbon dioxide
- thick clouds of sulfuric
acid
- trace amounts of water
vapor
- Lightning has been
detected, but there is no
rain.

Runaway greenhouse effect

Venus surface temperature

750 K

Mars

most explored besides Earth

Mars: Valles Marineris

a canyon that stretches
across the face of the globe
• as wide as the United States

Mars: Olympus Mons

tallest mountain in the solar
system (26 km tall)
• probably no older than 250
million years
volcano

Mars: Polar Ice Caps

made of water, ice, and
frozen carbon dioxide
grow and shrink seasonally

Mars: Tharsis bulge

bulge may have been caused by an upwelling of hot material
from the interior of the planet. This created volcanoes in the region
(including Olympus Mons) and may have created Valles Marineris.

Mars: Dune Fields

Deserts ring Mars at
mid-latitudes.
• Winds blow surface
material into dunes,
similar to those found
on Earth.

Mars axial tilt

25 degrees seasons similar to earth
Southern winter worse than northern farthest from sun

Water on Mars

Many scientists think that
Mars was once warmer and
wetter than it is today.
• Surface features seem to
support this:
- dry riverbeds
- splash craters (craters that
form from impacts on
damp soil)
- gullies in crater walls, etc.
• Mars must have had a warmer
and denser atmosphere in the
past to support liquid water on
its surface.

gullies

deep ditches formed after heavy rainfall

So where water

Some may be trapped
just below the surface
as permafrost.
- Some may be locked
up in mineral
compounds.
- Most probably escaped
to space:
• evaporated from
the surface
• UV photons broke
apart H2O molecule
• H escaped to space

Surface of Mars

dry, dusty
place.
• Plains covered with
rocks ranging from
pebble-size to boulder
size.
• There is lots of
evidence for flowing
water, rivers, and salty
oceans.

Mars atmosphere

95% carbon dioxide,
3% nitrogen
- Surface pressure is very
low - same pressure as
Earth at 35 km.
- Clouds of water ice and
carbon dioxide ice float
through the sky.
- No rain ever falls, but it
sometimes snows dry ice
crystals.
• Temperatures range from just
above freezing to 180 K.
• The winds on Mars, though gentle, can
carry dust far into the atmosphere.
- These dust storms sometimes obscure
most of the surface.
• No erosion.

Moons of mars

Phobos and Deimos

Bode's Rule

(a
simple mathematical
formula that seemed to
predict where planets
would be found),

Asteroid Belt

The asteroids are not the
remains of a shattered
planet.
- All the asteroids
mass added together
is ~1% of the Earth's
mass.

Ceres Asteroid

Ceres is the largest, only 930 km
across.
- Ceres is massive enough to pull
itself into a sphere, and is
classified as a dwarf planet.

Eros asteroid

potato-shaped

Asteroid composition

Carbonaceous bodies
• carbon-rich, coal-like
substance
• located in the outer part of
the asteroid belt
• may have water ice

- Silicate bodies
• composed primarily of
silicates (low-density rock)

- Metallic iron-nickel bodies
• composed mostly of dense
metals
• located in the inner part of
the asteroid belt

Origin of Asteroids

Asteroids are probably fragments
of planetesimals.
• The planetesimal had collected a
mixture of rocks and metals, and
then differentiated, creating a
dense metallic core and a lighter,
silicate-rich outer shell.
• A collision with another asteroid
could have shattered the
planetesimal.
- Fragments of the inner core
would form the iron-nickel
asteroids.
- Fragments of the outer shell
would form the silicate
asteroids.

Asteroid Orbits

It is likely that the
asteroids were unable
to form a planet
(however small) due
to the gravitational
tidal influence of
Jupiter.
• Jupiter "stirs up" the
asteroids, keeping
them apart.
• There are empty
regions in the asteroid
belt, called Kirkwood
Gaps.
• These gaps are
present at orbital
resonances of Jupiter.
• Asteroids with an orbital resonance get periodic
tugs from Jupiter, pulling them out of position.

Kirkwood gaps

Regions in the main belt of asteroids where few or no asteroids are found. They were named after the scientist who first noticed them.

Small planets

planets cool
quickly, leading to
dead worlds with little
activity.
- Small planets also
have trouble holding
an atmosphere.

Larger planets

hold
on to their heat, and
have active interiors
and surfaces.