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The Earth
General Features
Mass: MEarth = 6 x 1027 g
Radius: REarth = 6378 km
Density: ρ = 5.5 g/cm3
Age: 4.6 billion years
Tides
A feature of oceans (but solid material has small tides too).
Two high and two low tides per day.
Tides are due to Moon's gravitational pull being stronger on
side of Earth closest to it (Sun causes smaller tides).
Earth-Moon gravity keeps them orbiting each other. But side of Earth
closest to Moon has slightly stronger pull to Moon => bulges towards it.
Other side has weaker pull => bulges away compared to rest of Earth.
The Earth spins once a day while the bulge always points towards and away
from the Moon => high and low tides.
We also see tidal forces at work in other places in the Universe.
This is an example of the "tidal force". Can be
important for other planets, moon and pairs of stars or
galaxies.
"The Antennae" used to be two normal spiral galaxies,
but tidal forces distorted their shapes badly.
Tidal Locking
The tidal bulge in the solid Moon elongates it slightly (2-3 km)
along axis pointing to Earth.
If spin period faster than orbit period, tidal bulge would have to move around
surface (like Earth’s ocean tides), creating friction, which slows the Moon’s
spin down until tidal bulge stops migrating.
Atmospheric Structure
78% Nitrogen
21% Oxygen
300
T, K
gas is ionized by
solar radiation
Ionosphere
Thermosphere
Aurora
ozone is O3 , which
absorbs solar UV
efficiently, thus
heating stratosphere
200
100
commercial jet
altitudes
Stratosphere
Troposphere
Original gases disappeared. Atmosphere is mostly
due to volcanoes!
Turbulence
Mesosphere
Altitude
[km]
0
0
500
1000
Temperature [K]
room temperature (at ground)
Convection
Convection is the turbulent motion of a fluid caused by heat exchange. The transfer of
heat causes the fluid (either a gas or a liquid) to circulate and move in turbulent streams
and eddies similar to those we see in boiling, or sometimes in flowing, water.
Convection causes both small-scale turbulence and large scale circulation patterns. It
also occurs within Earth, on other planets, and in stars.
The Greenhouse Effect
Sunlight hits the Earth; some is immediately reflected back to space by clouds. Some gets
through the atmosphere and is absorbed at the Earth’s surface. The Earth then re-radiates in
the infrared (“IR” or “Heat radiation”). Certain gases absorb IR, then reradiate back into the
Earth’s atmosphere, trapping it there.
Main greenhouse gases are CO2 and H2O . Methane (CH4) also.
If no greenhouse effect, surface would be 40 oC cooler!
Why is the Sky Blue?
Air molecules tend to let reddish light through while scattering blueish light in all
directions, from where it gets scattered again to your eye.
Earthquakes and Seismology
Earthquakes create two types of vibrations in the solid Earth, or seismic
waves.
They are P-waves ("primary") and S-waves ("secondary").
P-waves are waves of compression.
S-waves are waves with transverse motion (undulation is perpendicular to the
direction of propagation).
They travel at different speeds from each other, and at different speeds
through different types of rock.
Like all waves, seismic waves bend when they encounter changes in
density. If density change is gradual, wave path is curved.
S-waves are unable to travel in liquid.
Thus, measurement of seismic waves gives info on density of Earth's
interior and which layers are solid/molten.
Earth's Interior Structure
Average density
5.5 g/cm3
Crust
Mantle
Core
3 g/cm3
5 g/cm3
11 g/cm3
Density increases with depth => "differentiation"
Earth must have been molten once. When it started to
cool and solidify, denser material sank.
Aurora
Near Fox, Alaska
(just outside Fairbanks)
Aurora -- explanation
• Charged particles (electrons mainly) from the Sun travel to the Earth. As they get
closer, the Earth’s magnetic field exerts a force on the particles, causing them to
spiral around the magnetic field lines.
• The magnetic field lines converge near the North and South poles -- actually, at
the North and South Magnetic poles, which are near but not at the geographic
poles.
• The particles spiral along the lines into the atmosphere near the poles. They
collide with molecules in the atmosphere and ionize them. The ionized gases then
relax to the un-ionized state, releasing the excess energy as light.
• Aurora occur both at high northern latitudes (Aurora Borealis) and at high
southern latitudes (Aurora Australis).
• Aurora have been seen and photographed from space by unmanned spacecraft
and by Shuttle astronauts.
• Aurora have been detected on at least two of the Jovian planets.
The Moon
Mass = 7.4 x 1025 g
= 0.012 MEarth
Radius = 1738 km
= 0.27 REarth
Density = 3.3 g/cm3
(Earth: 5.5 g/cm3)
Gravity = 1/6 that of Earth
Distance = 384,400 km
The Lunar Surface
- Large, dark featureless areas:
"maria" or "seas".
- Lighter areas at higher
elevation: "highlands".
- Many craters (due mostly to
meteoroid impacts). No winds
to erode them away.
- Highlands have 10x the crater
density of maria.
Note: texts refer to “maria” as “mare” (this is actually
singular).
Cratering Rates
Small meteoroids common, large ones rare. So same true for craters:
Crater size
Occurrence
10 km
every 10 million years
1m
every month
If no erosion, the number of craters in an area tells you
the age of the surface. So highlands older than maria.
Radiometric dating of moonrocks confirms this.
Lunar Volcanism (long ago)
Remember: volcanism is a way of losing internal heat
Evidence:
- Maria: result of old,
widespread lava flows
(filled in largest, early
impact craters)
- "Rilles": ditches
indicating old lava flows
- Linear chains of craters
(not due to impacts),
probably marks
ancient fault, collapsed
lava domes
Moon's History
Age: 4.5 billion years
3.9 billion years ago:
heaviest meteoritic bombardment
ended
3.9 - 3.2 billion years ago:
volcanism created maria. Maria
are just the largest craters, filled in.
3.2 billion years -> present
no volcanism, cratering continued
at lower rate, geologically dead!
Water Ice on the Moon – Sept 2009
Data from Cassini mission
Absorption of infrared
beam by water and OH ice.
About 32 ounces of water
per ton.
How did the Moon form?
We're not quite sure! Three older theories:
1) "Fission": The material that would be the Moon
was thrown off the Earth and coalesced into a single
body. Problem: Earth not spinning fast enough to
eject large amount of material.
2) "Coformation": The Moon and Earth formed out
of the same material at the beginning of the Solar
System. Problem: Moon has different density and
composition.
3) "Capture": The Moon was a stray body captured
into orbit around Earth. Problem: an extremely
unlikely event, given Moon's size is a substantial
fraction of Earth's.
So now, Impact theory preferred:
Early in Solar System, when many large planetesimals around, a
Mars-sized object hit the forming Earth, ejecting material from the
upper mantle which went into orbit around Earth and coalesced to
form Moon. Computer simulations suggest this is plausible.
Mars
Mass = 0.11 MEarth
eccentricity = 0.093
Radius = 0.53 REarth
Range in distance from Sun = 1.38 1.66 AU
Density = 3.9 g/cm3
Average distance from Sun = 1.52 AU
Rotation Period = 24.6 hours
Orbital Period = 687 days
The Martian Atmosphere
- 95% CO2
- Surface Pressure 0.006 that of Earth's atmosphere (thin air!)
- Surface Temperature 250 K.
- Dust storms sometimes envelop most of Mars, can last months.
A “Reverse Runaway Greenhouse Effect”?
During volcanic phase (first two billion years), thicker
atmosphere, warmer surface, possibly oceans. Gradually most
CO2 dissolved into surface water and combined with rocks, then
atmospheric and surface water froze (creating ice caps and
probable permafrost layer).
The Martian Surface
Olympus Mons
Tharsis Bulge
Valles Marineris
Southern Hemisphere ~5 km higher
elevation than Northern, and more
heavily cratered.
South is like lunar highlands, surface
~4 billion years old, North like maria,
~3 billion years old.
Tharsis Bulge - highest (10 km) and
youngest (2-3 billion years) region.
(Mars Global Surveyor radar data)
Valles Marineris - 4000 km long, up to
7 km deep. Ancient crack in crust.
Reasons not clear.
Olympus Mons - shield volcano, highest
in Solar System, 3x Everest in height.
100 km across.
View From the Surface
Dry, desert-like. Red => high iron content. Mars didn't differentiate as
completely as Earth. Sky has butterscotch hue due to dust.
Viking 1 site (1976)
Sojourner robot from Pathfinder (1997)
Opportunity panorama: inside Victoria Crater
Deepest crater explored by far (230 feet) => apparently it was the top
of an underground water table. Now heading to deeper Endeavour
Crater.
Evidence for Past Surface Water
"runoff channels" or
dry rivers
"outflow
channels"
standing water erosion in craters?
South
North
teardrop "islands" in
outflow channels
Pathfinder (1997) site was an outflow channel
Red arrows: rounded boulders indicating water erosion?
White arrows: "conglomerate" rock, like in Earth's riverbeds?
Blue arrows: sharp-edged boulders, volcanic rock?
Did Mars once have a huge ocean?
Long stretches along border are very
even in elevation, like a coastline
Ocean fed by outflow channels from
higher elevation southern hemisphere?
Evidence for "Permafrost" layer beneath surface
"Splosh" craters suggesting
liquefied ejecta.
Something flowed in this crater wall between 2001 and 2005
Mars' History
Smaller than Earth, Mars cooled faster.
Atmosphere and surface water in first 1.5 billion years. Life?
Most volcanic activity ended two billion years ago.
Differentiation less complete than on Earth.
No evidence for plate tectonics.
Atmosphere mostly froze out into subsurface ice, polar ice
caps and surface rocks.
Phoenix mission – icy soil at the poles!
Phoenix analyzing scooped up
dirt – was Mars ever favorable
for microbial life? Organic
compounds?
Mars' Moons Phobos and Deimos
Phobos: 28 x 20 km
Deimos: 16 x 10 km
Properties similar to asteroids. They are probably asteroids captured
into orbit by Mars' gravity.
Next mission – Mars Science Laboratory – fall 2011 launch
http://marsprogram.jpl.nasa.gov/msl/
Venus
Mass = 0.82 MEarth
Radius = 0.95 REarth
Density = 5.2 g/cm3
Average distance from Sun = 0.72 AU
Orbital period = 225 days
Rotation period = 243 days (longer than
orbital period, and retrograde!)
Venus' Atmosphere
- Pressure at surface is 90 x that of Earth's => much more gas in
atmosphere.
- Consequence - meteoroids burn up easily. No impact craters less
than about 3 km.
- 96.5% CO2
- Yellowish color from sulfuric acid clouds and haze.
- Hot at surface - 730 K!
- Why so hot? Huge amount of CO2 leads to strong greenhouse
effect.
Early on, T may have been much lower (but still warmer than Earth).
Oceans?
But if warm enough, T would start to rise because of...
Runaway Greenhouse Effect
1) Water and CO2 evaporate from oceans into atmosphere.
2) Greenhouse effect more efficient.
3) Temperature rises.
4) More evaporation (back to #1).
=> complete evaporation of oceans. Thick atmosphere.
Alternatively, volcanic outgassing may never have produced oceans,
like on Earth, because too hot. All gases stayed in atmosphere.
Missions to Venus
Soviet Venera 4 -18 (1967 - 1983)
Mariner 2, 5 and 10 (1962, 1967 and 1974)
Pioneer Venus
(1978)
Magellan (1989)
Venera 13 photo of
surface. Rocks are
basalt and granite.
Color is due to
atmosphere.
Color corrected
for atmosphere.
Radar reflection from spacecraft orbiting Venus has been used to
measure the surface topography. This reveals altitude variations on the
surface.
Venus is, overall, flatter than the Earth. There is no evidence for plate
boundaries, i.e., no large scale plate tectonics.
Volcanoes are randomly distributed (Magellan).
1 km
But plenty of evidence of small-scale stresses
and fractures => much
small-scale shifting of crust. Crust may
be thinner or softer than Earth’s. All “hotspot” volcanism?
Impact Craters
Unlike Moon, impact craters distributed randomly over surface => all parts
of surface have about same age.
Paucity of impact craters => surface is young, about 500 million years.
Volcanism
100 km
3 km
Shield volcano elevation map from Magellan radar data.
Volcanism may be ongoing, based on sulfur dioxide variations in atmosphere.
But very little resurfacing in past 500 million years.