Transcript The Terrestrial Planets

The Terrestrial Planets
A Study in Contrasts
• Mercury in many ways is similar to
Earth’s moon.
• Venus and Mars both have properties
more like Earth’s, as well as drastic
differences.
• Earth - vibrant, teeming with life.
• Venus - uninhabitable inferno.
• Mars - dry, dead world.
Which object is pictured
below?
A.
B.
C.
D.
E.
Mercury
Venus
Earth
Mars
Earth’s Moon
Which object is pictured
below?
A.
B.
C.
D.
E.
Mercury
Venus
Earth
Mars
Earth’s Moon
Which object is pictured
below?
A.
B.
C.
D.
E.
Mercury
Venus
Earth
Mars
Earth’s Moon
Which object is pictured
below?
A.
B.
C.
D.
E.
Mercury
Venus
Earth
Mars
Earth’s Moon
Which object is pictured
below?
A.
B.
C.
D.
E.
Mercury
Venus
Earth
Mars
Earth’s Moon
Physical Properties
• Surface gravity - strength of the gravitational force at
the body’s surface.
• Escape speed - speed required for any object to
escape forever from the body’s gravitational pull.
• The moon’s gravitational pull is much weaker than
Earth’s (Moon’s gravity is 1/6 that of Earth).
• Using radar and lasers, the distance of the moon
from the earth is known to within about 2 cm. The
semi-major axis of the moon’s orbit about the earth is
roughly 384,000 km.
Overall Structure
• Earth has 6 main regions:
– Core - central region of Earth,
surrounded by the mantle.
– Mantle - layer of Earth just interior to
the crust.
– Crust - layer of Earth which contains
the solid continents and seafloor.
– Hydrosphere - layer of Earth that
contains the liquid oceans and
accounts for roughly 70% of Earth’s
total surface area.
– Atmosphere - layer of gas confined
close to planet’s surface by force of
gravity.
– Magnetosphere - zone of charged
particles trapped by planet’s magnetic
field, lying above the atmosphere.
Overall Structure
• Moon:
– Lacks a hydrosphere,
an atmosphere, and a
magnetosphere.
– Not as well studied as
Earth’s since less
accessible.
– Crust, mantle, and
core are present but
differ from those of the
earth.
Tides
• Definition - the daily fluctuation in ocean level.
• The magnitude of the fluctuation (height of the tides) can vary
from a few centimeters to many meters depending on location
and time of year. Average 1 m in the open sea.
• An enormous amount of energy is contained in the daily tidal
motions of the oceans.
• Tidal bulge - elongation of the earth caused by the difference
between the gravitational force on the side nearest the moon
and the force on the side farthest from the moon. The long axis
of bulge points toward the moon.
• The tidal bulge effect is greatest in the oceans since liquid can
move around easier.
• Tidal force - the variation in one body’s gravitational force from
place to place across another body (doesn’t necessarily have
anything to do with ocean tides!).
Is the moon the only object
that causes the tides on
Earth?
A. Yes, tides are caused only by the
moon.
B. Tides are also caused by the sun.
C. Tides are caused by all objects that
exert a force on the earth.
Tides
• We also have a
tidal bulge that
points in the
direction of the
sun.
• The interplay
between the tidal
force due to the
sun and the tidal
force due to the
moon accounts
for the varying
tide heights over
the course of a
month or year
(depends on
alignment of sun,
earth, and moon).
Tidal Locking
• The moon rotates once on its axis in 27.3
days - the exact time it takes to complete one
revolution around the earth. This is what’s
called a synchronous orbit.
• So, the same side of the moon always faces
the earth.
• This is due to the tidal interaction of the moon
and Earth.
• Tidal locking is a common occurrence with
moons in our solar system.
Earth’s Atmosphere
• Composition (% by volume)
– Nitrogen - 78%
– Oxygen - 21%
– Argon - 0.9%
– Carbon dioxide - 0.03%
– Water vapor content varies, ranging from
0.1 to 3%.
Mercury’s Atmosphere
• Has no appreciable atmosphere.
• Can trap gas from the solar wind (hydrogen and
helium) for a matter of weeks at a time.
• High surface temperatures (up to 700 K at noon on
the equator) are reason for lack of atmosphere.
• Surface temperature falls to 100 K at night.
• Largest temperature range of any planet or moon in
the solar system.
• Polar regions remain cold (as low as 125 K) and may
contain thin sheets of permanently frozen water ice.
Venus’s Atmosphere
• Spacecraft measurements indicate a surface temperature near 730 K.
• Venus’s atmosphere is much more massive than Earth’s and extends
further out from the planet’s surface.
• The surface pressure is 90 times that of Earth - equivalent to being 1 km
under water on Earth (humans can only dive to 100 m safely).
• 96.5% atmosphere is carbon dioxide.
• Remaining 3.5% mostly nitrogen.
• Venus is similar to Earth in mass, radius, and location in the solar
system. So we assume it must have started out much like Earth.
• Venus however has no water - so if it had some at its beginning,
something has happened to it.
• Clouds are composed not of water vapor, but sulfuric acid droplets.
• Upper level winds reach speeds of 400 km/h relative to the planet.
• Below the clouds is a layer of haze extending down to an altitude of 30
km. Below 30 km, the air is clear.
Mars’ Atmosphere
• Atmosphere is quite thin.
• Atmosphere composed primarily of carbon dioxide
(95.3%), 2.7% nitrogen, 1.6% argon, plus small
amounts of oxygen, carbon monoxide, and water
vapor.
• Surface pressure is 1/150 that on Earth.
• Average surface temperatures are about 70 K cooler
than on Earth.
Earth’s Atmosphere
• Layers
– Troposphere - portion of the atmosphere from the surface to
about 15 km.
– Stratosphere - lying above the troposphere, extending up to
an altitude of 40 - 50 km.
– Mesosphere - lying between the stratosphere and the
ionosphere, 50 - 80 km above Earth’s surface.
– Ionosphere - above 100 km where the atmosphere is
significantly ionized and conducts electricity.
• Convection occurs in the troposphere. It is the constant
upwelling of warm air and the concurrent downward flow of
cooler air.
• The ozone layer is contained in the stratosphere. Ozone is a
form of oxygen. In this layer, incoming solar ultraviolet radiation
is absorbed by atmospheric oxygen, ozone, and nitrogen. It
acts as a “planetary umbrella,” protecting us from damaging
radiation.
The Development of Earth’s Atmosphere
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Primary atmosphere - atmosphere it had when it formed.
– Consisted of light gases such as hydrogen and helium, methane, ammonia,
and water vapor. Nothing like what we have today.
– This atmosphere would have escaped - Earth’s gravity is not strong enough
to hold onto the light gases.
Secondary atmosphere - released from planet’s interior as a result of volcanic
activity (outgassing).
– Rich in water vapor, carbon dioxide, sulfur dioxide, and compounds
containing nitrogen.
– Water vapor condensed forming the oceans as the planet cooled.
– Much of the carbon dioxide and sulfur dioxide dissolved in the oceans or
combined with surface rocks.
– UV radiation liberated nitrogen from its chemical bonds with other elements,
forming a nitrogen-rich atmosphere.
– Oxygen is so reactive that any free oxygen which appeared would have
been removed as quickly as it formed.
– Life appears in the oceans - living organisms produce atmospheric oxygen
and eventually the ozone layer formed.
– The fact that oxygen is a major constituent of our present atmosphere is a
direct consequence of the evolution of life on Earth.
The greenhouse effect is …
A.
B.
C.
D.
Bad
Good
Neutral
Depends on the situation.
The Greenhouse Effect
• Definition - The partial trapping
of solar radiation by the
atmosphere, similar to the
trapping of heat in a greenhouse.
• Sunlight that is not reflected by
the clouds reaches Earth’s
surface, warming it up. Infrared
radiation reradiated from the
surface is partially absorbed by
carbon dioxide (and also water
vapor) in the atmosphere,
causing the overall surface
temperature to rise.
• More and more evidence is
pointing to humans increasing
this effect (increasing CO2
levels), which could have
catastrophic consequences
(rising ocean levels, etc.).
The Runaway Greenhouse Effect on
Venus
• Venus is hot because of the greenhouse effect - 730 K surface
temperature.
• Greenhouse gases, mainly water vapor and carbon dioxide, warm our
planet.
• Venus’s atmosphere is made up almost entirely of a greenhouse gas carbon dioxide. This thick blanket absorbs 99% of all radiation released
by the surface.
• Why is Venus’s atmosphere so different from Earth’s?
• On Earth, much of the greenhouse gases left the atmosphere (carbon
dioxide dissolved in oceans or combined with surface rocks). This did
not happen on Venus. If it hadn’t happened on Earth, we’d look a lot
more like Venus.
• Venus is closer to the sun, so it has a higher temperature initially. This
could have been too high for water vapor to condense into oceans. Full
greenhouse effect would have gone into operation immediately following
outgassing.
• If oceans did form, then the temperature must have been high enough to
allow a process called the runaway greenhouse effect to come into play.
The Runaway Greenhouse
Effect on Venus
• Runaway greenhouse effect:
– Imagine placing Earth at Venus’s orbit.
– The amount of sunlight hitting Earth’s surface would nearly
double, causing the planet to warm.
– More water would evaporate to the atmosphere.
– Ability of oceans and surface rocks to hold carbon dioxide
would diminish, and more carbon dioxide would enter the
atmosphere.
– Greenhouse heating would thus increase.
– Planet warms even more, resulting in the release of more
greenhouse gases, and so on. This process would “run
away” or snowball.
– Eventually, all the greenhouse gases would return to the
atmosphere.
The Runaway Greenhouse
Effect on Venus
• Greenhouse effect was even more extreme in Venus’s past when the
atmosphere contained water vapor.
– Water vapor helped temperature reach probably twice as high as
present.
– Water vapor rose high in the atmosphere due to extremely high
temperatures, where UV radiation broke it apart into hydrogen and
oxygen.
– Hydrogen escaped the atmosphere and oxygen quickly combined
with other atmospheric gases.
– All water on Venus was lost forever.
• If Earth was located between 0.7 and 1 AU, it would have experienced
the same fate as Venus.
• It is highly unlikely that global warming due to human activities will ever
send Earth down the path taken by Venus (that doesn’t mean it will
have no effect, however).
Evolution of the Martian Atmosphere
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Presumably, Mars had first a primary and then a secondary (outgassed)
atmosphere early in its history.
4 billion years ago, atmosphere rich in carbon dioxide, perhaps even blue skies
and rain. Sun was less luminous at this time, so conditions could have been fairly
comfortable - above freezing temperatures possible due to thick atmosphere.
During next billion years, most of the Martian atmosphere disappeared.
– Leaked away due to planet’s low gravity.
– Expelled by impacts with large bodies in the early solar system.
Mars cooled faster than Earth and never developed large-scale plate tectonics. So
Mars had less volcanism. So more carbon dioxide was depleted than replenished
(not as much outgassing). The level of carbon dioxide steadily declined.
Planet cooled as greenhouse gases diminished.
Could have depleted atmosphere of carbon dioxide in a few hundred million years.
Water froze out of the atmosphere as the temperature continued to fall.
Eventually, even the remaining carbon dioxide began to freeze out, particularly at
the poles.
Mars is now a cold, dry planet with most of its original atmospheric gases now
residing in or under the barren surface.
Evolution of the Martian Atmosphere
• Mars continues to cool, slowly losing what remains of its thin
atmosphere, and no natural event can reverse the process.
• Scientists are considering “terraforming” Mars - transforming its
present inhospitable climate into an Earth-like environment.
• Would have to introduce greenhouse gases to reverse the chain
of events just described.
– Melt polar caps using giant orbiting mirrors.
– Deflect ammonia-rich asteroids to collide with Mars.
– Build greenhouse emitting factories on Mars.
– Process (which is not feasible with today’s technology)
would take several centuries for natural processes to take
over.
– Would then take up to 2 thousand years to reach an Earthlike climate.
Lunar Air?
• The moon has no atmosphere - it escaped a long time ago!
• The moon’s escape speed is only 2.4 km/s (compared to the
earth’s 11.2 km/s), so it couldn’t hang on to any atmosphere it
may have once had.
• Due to the lack of atmosphere (which helps moderate
temperatures), the moon experiences a wide range of surface
temperatures, from 100 K at night to over 400 K during the day
(from well below water’s freezing point to well above waters
boiling point).
• While most of the moon’s surface is bone dry, a good amount of
water ice might be located at the lunar poles, where the sun
never gets very high above horizon, making temperatures there
much cooler (never exceeds about 100 K).
Seismology
• Earthquakes - sudden dislocation of rocky material
near Earth’s surface.
• Seismic waves - systematic waves that move
outward from the site of an earthquake, which can be
measured using a seismograph.
– P-waves - pressure waves, which expand and
compress the core or mantle as they move at
speeds of 5 to 6 km/s. They travel through both
liquids and solids.
– S-waves - shear waves, cause side to side motion
as they move at speeds of 3 to 4 km/s. They
cannot travel through liquid.
Seismology
• Using the different
arrival times of the
waves, geologists can
infer the density of
matter in the interior.
• This is how we know
what the interior of the
earth is like.
Modeling Earth’s Interior
• The outer core is molten - as evidenced by the lack of s-waves which
pass through from one side of the planet to the other after an
earthquake. The radius of this outer core is about 1300 km.
• The mantle is about 3000 km thick and accounts for the bulk of Earth’s
volume.
• Average thickness of crust is only 15 km.
• Density and temperature both increase with depth.
• High central density implies the presence of large amounts of nickel
and iron.
• Inner core is solid and metallic, outer core is molten and metallic.
• Mantle is mostly rocky (compounds of silicon and oxygen).
• We have not been able to drill more than 10 km, so we don’t have a
direct mantle sample. However, lava brings up mantle material in
volcanoes. Therefore, mantle material probably resembles the basalt
found near volcanoes.
Differentiation
• Earth is not a homogeneous ball of rock - it has a layered structure.
– Low-density rocky crust at the surface.
– Intermediate-density rocky material in the mantle.
– High-density metallic core.
• Variation in density and composition is known as differentiation.
• Why not uniform composition?
– In distant past, much of Earth was molten.
– Higher-density material settled to the core, with lighter-density material
rising to the top.
– Heating sources:
• Bombardment of interplanetary debris when Earth was young,
which would have generated enough heat to melt the planet.
• Radioactivity (release of energy by certain unstable elements such
as uranium) built up heat in the interior as it takes a long time to
travel to the surface through the rock. Enough of the early planet
was radioactive to at least keep the planet at most semi-solid for a
billion years.
The Lunar Interior
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The low average density of the moon suggests it contains fewer heavy elements
(like iron) than does Earth.
Seismic instruments were left on the moon by astronauts and indicate only weak
“moonquakes” deep within the lunar interior. These contain about as much
energy as a firecracker and have little effect.
The moon is “geologically dead.”
From the weak moonquakes, it has been inferred that:
– Moon is almost uniform in density, but is chemically differentiated (chemical
properties change from core to surface).
– Central core - 300 km in radius.
– 400 km thick inner mantle of semisolid rock similar to Earth’s upper mantle.
– 900 km thick outer mantle of solid rock.
– 60 to 150 km thick crust.
Core is probably more iron rich than the rest (nothing compared to Earth’s
though).
Central temperature is too cool to melt rock, although there is evidence that the
inner core may be partially molten.
The crust is thicker on the far side of the moon - the earth’s gravity pulled more
mantle to the near side of the moon, leaving the far side with more crust as the
moon cooled and solidified.
Surface Activity on Earth
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Earthquakes
Volcanoes
Wind erosion
Water erosion
Continental
Drift
• Plates (slabs of Earth’s surface) are constantly in
motion - called continental drift.
• The study of plate motion is called plate tectonics.
• Plates and continents are not the same thing!
• Plate motions have created mountains, oceanic
trenches, and other large scale features.
• When crustal rock shifts, we have earthquakes.
• When mantle material upwells, we have volcanoes.
Continental Drift
• Plates move at an extremely slow rate (few cm per year). Over
a period of 200 million years (5% of Earth’s age) two continents
could separate by 4000 km (width of the Atlantic Ocean).
• When plates collide, mountains form.
• Plates can also slide along one another (rather than colliding
head-on). This motion causes fault lines and earthquakes
(motion is jerky rather than smooth).
• Plates can also move apart. When this happens, new mantle
material wells up between them, forming midocean ridges. This
is why the Atlantic seafloor is slowly growing in size since the
North and South American plates are moving away from the
Eurasian and African plates.
What Drives the Plates?
• Convection!
• Each plate is made up of crust plus small portion of upper
mantle.
• Below the plates, at a depth of maybe 50 km, the temperature is
high enough that the mantle is soft enough to flow, very slowly,
although it is not molten.
• We have warm matter underlying cool matter, perfect conditions
for convection.
• The circulation is extremely sluggish - semisolid rock takes
millions of years to complete one convection cycle.
• Early on, geologists believe we had one massive supercontinent
- Pangaea.
• This process has likely happened several times in the past, and
will probably repeat in the distant future as the landmasses
continue to move.
Lecture Tutorial: Earth’s Changing Surface
(p. 99)
• Work with a partner!
• Read the instructions and questions
carefully.
• Discuss the concepts and your answers
with one another. Take time to
understand it now!!!!
• Come to a consensus answer you both
agree on.
• If you get stuck or are not sure of your
answer, ask me or another group.
Plate Tectonics on the Moon
• No evidence of plate tectonics on the moon today.
– No obvious fault lines.
– No significant seismic activity.
– No ongoing mountain building.
• Plate tectonics requires a relatively thin outer rocky layer and a
soft convective region under it to make the pieces move. The
moon has neither.
• Thick crust and solid upper mantle of moon make it impossible
for pieces of the surface to move relative to one another.
Magnetospheres
• Definition - the region around a planet
that is influenced by that planet’s
magnetic field.
• Forms a buffer zone between the planet
and the high-energy particles of the
solar wind.
Earth’s Magnetosphere
• Earth’s magnetic field
extends far above the
atmosphere and completely
surrounds our planet.
• The magnetic field lines,
which indicate the strength
and direction of the field at
any point in space, run from
north to south.
• The north and south
magnetic poles, where the
axis of an imaginary bar
magnet within our planet
intersects Earth’s surface, are
very roughly aligned with
Earth’s spin axis.
Earth’s Magnetosphere
• Van Allen Belts
– Most pronounced near Earth’s equator and
surround the planet.
– Two doughnut shaped zones in Earth’s inner
magnetosphere of high-energy charged particles.
– Located at 3000 km and 20,000 km above the
surface.
– Particles that make up the belts originate in the
solar wind. Charged particles will be attracted by
Earth’s magnetic field, which can then trap them.
Earth’s
Magnetosphere
• Aurora
– Particles from the Van Allen Belts escape from the
magnetosphere near Earth’s north and south
magnetic poles.
– At these locations, the magnetic field lines cross
the atmosphere.
– As the charged particles collide with air molecules,
we get a light show called an aurora (called the
Northern Lights in the northern hemisphere).
– Collisions excite atmospheric atoms, which then
emit light as they go back to their ground states.
Earth’s Magnetosphere
• The stream of solar wind particles can affect the magnetosphere.
– Toward the sun, the magnetosphere is squeezed inward toward
Earth’s surface.
– The opposite side has a long tail extending hundreds of thousands
of km into space.
• The magnetosphere shields us from the potentially destructive charged
particles of the solar wind. Without the magnetosphere, life may not
have been able to exist here.
• Earth’s magnetic field is not an intrinsic part of our planet.
• It is continually generated in Earth’s core and exists only because the
planet is rotating.
• Earth’s magnetism is produced by the spinning, electrically conducting,
liquid metal core.
• Both rapid rotation and a conducting liquid core are needed to produce
this magnetism.
Lunar Magnetism
• No Earth-based observation or spacecraft
measurement has ever detected any lunar
magnetic field.
• As just discussed, planetary researchers
believe planetary magnetism requires a
rapidly rotating liquid metal core.
• The moon rotates slowly and the core is likely
neither molten nor rich in metals, so we would
not expect to see a lunar magnetic field.
Formation of the Earth - Moon
System
• 4.6 billion years ago, the Earth formed by
accretion in the solar nebula.
• The earth and moon are too dissimilar in both
density and composition to have formed from the
same pre-planetary matter (coformation theory).
• The mantles of the two however are quite similar,
so they did not form totally independently of one
another either (capture theory).
• Favored theory of the day - impact theory.
Formation of the
Earth - Moon
System
• Impact theory:
– Glancing collision between Mars-sized object
and a young molten Earth.
– Most of the bits of Earth that would have been
blasted into space could have recombined
into a stable orbit (as evidenced by computer
simulations).
– This material would have then coalesced into
our moon.
Evolution of the Earth - Moon System
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Earth and Moon were at least partially solidified 4.4 billion years ago (have found rocks of
that age).
1st billion years - Earth at least partially molten.
3.9 billion years ago - heavy bombardment ceased.
Earth cooled from the outside in (can transfer heat to space more readily closer to the
surface).
Moon (being smaller in size) lost its heat rapidly to space.
1st half-billion years - heavy bombardment kept much of the moon molten (not the deep
interior as rock doesn’t conduct heat very well).
When heavy bombardment ceased, moon was left with a solid crust with numerous large
basins.
Crust became the highlands.
Basins flooded with lava and became the maria.
Between 3.9 and 3.2 billion years ago, lunar volcanism filled the basins with the basaltic
material we see today.
Lunar volcanism ceased 3.2 billion years ago.
Not all basins were filled by lava - some still are simply craters.
Due to Earth’s gravitational pull, the lunar crust is thicker on the far side as compared to
the near side. As a result, little volcanic activity occurred on the far side (more crust for
lava to travel through).
The lunar crust is now too thick for volcanism or plate tectonics to occur.
The lunar surface has not changed much in the past 3 billion years. Only changes that
have occurred were due to erosion from meteoritic bombardment.
Volcanism on Mars
• Contains largest known volcanoes in the solar system.
• Largest is Olympus Mons - 700 km in diameter at its base slightly smaller than the state of Texas. Rises to a height of 25
km above surrounding plains.
• Do not know if any are still active, but from cratering rates, some
erupted as recently as 100 million years ago.
• Great height of volcanoes is a result of planet’s low surface
gravity. A mountain can only be as tall as it can support (lower
gravity means the lava weighs less). Martian surface gravity is
40% of Earth’s, letting its volcanoes rise 2.5 times higher.
The Martian Grand Canyon
• Valles Marineris
• Running water played no part in its formation (not like an Earth
canyon!).
• Formed by same crustal forces that pushed the Tharsis region
upward, causing the surface to split and crack.
• Cratering studies suggest it’s at least 2 billion years old.
• Runs for almost 4000 km, extends 1/5 of the way around the
planet.
• Widest point is 120 km across, gets as deep as 7 km. Earth’s
Grand Canyon could fit into one of its side “tributary” cracks.
• Large enough to be seen from Earth.
• The crustal forces however did not develop into full-fledged
plate motion as on Earth (not plate tectonics!).
Water
on
Mars?
Evidence for Past Water on Mars
• Two types of flow features:
– Runoff channels - in southern highlands, extensive systems of
interconnecting, twisting channels that merge into larger, wider
channels. Dried up river beds. From a time when liquid water was
widespread (4 billion years ago).
– Outflow channels - probably relics of catastrophic flooding long ago.
Appear only in equatorial regions. Do not form as an extensive
system. Probably paths taken by water draining from southern
highlands to the northern plains. Formed about 3 billion years ago.
Largest flowed at a hundred times the flow rate of the Amazon River.
• Mars may have enjoyed an extended early period during which rivers,
lakes, and maybe even oceans adorned its surface.
• Data from the most recent landers suggest at least some parts of the
planet experienced long periods in the past during which liquid water
existed on the surface.
• The extent and nature of the water is still heavily debated.
Where is the Water Today?
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Amount of water vapor in Martian atmosphere is tiny.
Evidence for liquid water on Mars found by the Pheonix lander.
Most likely water is now locked in a layer of permafrost, with more contained in
the polar caps.
– “Seasonal caps” very in size as atmospheric carbon dioxide alternately
freezes and evaporates during the winter and summer months.
– Permanently frozen “residual caps” are composed of water ice.
• Thickness of the caps is unknown, but they could be a major storehouse
for water on Mars.
4 billion years ago, running water that formed the channels began to freeze,
forming the permafrost and drying out the river beds.
Mars remained frozen for a billion years until volcanic activity heated large
regions of the surface, melting the permafrost, and causing floods that created
the outflow channels.
When volcanic activity subsided, the water refroze, and Mars again became dry.
While the total amount of water frozen in the permafrost is still unknown, it is
likely that if all the water melted, it would cover the surface to a depth of several
meters.
The Face on Mars - Then
The Face on Mars - Now