Download Is there evidence of plate tectonics?

Lesson4d
Working out the global history of
Mars
• There is much less water at a deep of one
meter, near the equator of Mars, compared to
the polar regions.
• At lower latitudes (near equator) the
temperature is higher than at the poles.
• This can cause ice near the surface, at low
latitudes, to melt and then evaporate away.
• The measurement doesn’t show that there is
no underground water at low latitudes on
Mars.
• It only shows that there is little water at a
depth of one meter, at low latitudes.
Geologic history of Mars
• If you consider that a surface that has not
been destroyed for billions of years will have a
large impact crater density it is possible to tell
where most of the geologic history of Mars
has taken place.
Which hemisphere is younger
1. Northern
2. Southern
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Is there evidence of plate tectonics?
Earth
• http://www.google.com/mars/
Is there evidence of plate tectonics?
Evidence of Plate tectonics on
Mars?
1. Yes
2. No
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Evidence of Plate tectonics
• Mountain ranges along plate boundaries
• Strato-volcanoes over subduction zones.
• Although there are ridges that might be
construed as some type of mountain range,
there are no strato-volcanoes on Mars.
• All volcanoes are shield volcanoes with gently
sloping sides due to the low viscosity of the
lava.
Tectonics vs. Plate Tectonics
• Remember tectonics is changes due to
motions in the crust. This includes graben,
fissures, stress fractures, volcanism.
• Plate tectonics is the whole sale motion of
large portions of crustal materials. Like the
motion of North and South America.
• Although it is very clear that the Tharsis bulge
area has had lots of tectonism, there is little
evidence of plate tectonics on Mars.
Why is Olympus Mons so big?
.
1. Lack of thick atmosphere allows
volcanoes to grow large on Mars
2. No strato-volcanoes means lava has to
come out of shield volcanoes making
them large
3. No plate tectonics so same lithosphere
stays over the magma plume
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Plate tectonics makes new volcanoes as
crust moves over magma plume
Let’s use what we have learned
• Mars is much smaller than the Earth. This
means that is also cooled much more rapidly,
causing a thick crust to form rapidly.
When Earth’s crust was still molten, Mars
already had a well established surface.
Let’s use what we have learned
• Mars is much smaller than the Earth. This means
that is also cooled much more rapidly, causing a
thick crust to form rapidly.
• (Surface Area/Volume)earth = 3/6371 = 0.00047
• (Surface Area/Volume)mars = 3/3397 = 0.00088
• Mars cooled faster than Earth by a factor of 2.
• This set up a thick crust with little time for plate
tectonics.
• Mars was still hot on the inside but was not
able to release this heat through plate
tectonics.
• Instead a single, enormous magma plume
developed in one region of Mars. The Tharsis
Bulge.
• Mars was still hot on the inside but was not
able to release this heat through plate
tectonics.
• Instead a single, enormous magma plume
developed in one region of Mars. The Tharsis
Bulge.
• Mantle plume caused uplift of crust in the
Tharsis region. Pressure from uplift allowed
shield volcanoes to form, which contributed to
layers of lava on top of the Tharsis region.
• This created great overlying weight in region.
• Eventually the magma plume subsided and
the weight of the Tharsis region caused
portions of the bulge to sink down.
• http://www.google.com/mars/
• Eventually the magma plume subsided and
the weight of the Tharsis region caused
portions of the bulge to sink down.
• http://www.google.com/mars/
• The result of portions sinking is an enormous
graben feature that formed Valles Marineris.
• Uplift and retreat of Tharsis region likely
happens multiple times.
• This activity pumped energy into the Tharsis
region in the form of stress heat and
underground magma chambers.
• There are signs of lava flow away from the
Tharsis region. There is also evidence of water
flow that originated from heated underground
ice deposits.
• http://www.google.com/mars/
What about the North?
• There is a strange dichotomy between the
northern and southern hemispheres on Mars.
• The crust in the Southern hemisphere is very
thick and old. The Northern hemisphere is
very low.
• What about relative age of the regions?
Possibilities
1) Giant collision with asteroid that removed
the crust in the Northern Hemisphere.
This is similar to the theory that Earth had
two moons which gently collided and spread
a thicker crust onto the farside of the Moon.
Possibilities
1) Giant collision with asteroid that removed
the crust in the Northern Hemisphere.
2) Entire global lithosphere of Mars rotated.
This could happen if magma plume builds up
crust from underneath very early on in Mars
history.
Possibilities
1) Giant collision with asteroid that removed
the crust in the Northern Hemisphere.
2) Entire global lithosphere of Mars rotated.
This could happen if magma plume builds up
crust from underneath very early on in Mars
history.
3) Could the Northern hemisphere be an
ancient ocean bed?
• What would be required for the Northern
Hemisphere to be the floor of an ancient
ocean?
What would be required for Mars
to have an ocean?
1.
2.
3.
4.
Lots of water
Thick atmosphere
No polar ice caps
Living creatures
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Did Mars ever have a thicker
atmosphere?
Evidence would be if there was ever standing
water on Mars.
Spirit and Opportunity think so.
Both landed on Mars in January 2004.
Opportunity is still actively researching Mars.
Spirit/Opportunity rovers
Hematite signal from orbit
Hematite is an iron compound which typically
forms in the presence of water
The
Opportunity
landing site
on Meridiani
Planum
Eagle Crater
Inside Eagle Crater
Blue Berries – Hematite spheres
Abrasion tool collects samples
Spectral analysis confirms Hematite
Analysis of rock shows Sulfur, Chlorine and
Bromine: All of which are soluble in water
High sulfur suggests basalt rocks (Jarosite)
that have been dissolved in water
Triple combination
• The triple combination of sulfur, chlorine and
bromine make the evidence clear that water
was present in the rocks.
• These three chemicals are deposited when
water evaporates and leaves them behind.
• Combining this with the Hematite which
formed within the sedimentary rock layers is
conclusive that water was on the surface at
Meridiani Planum.
Standing water about 3.7 billion
years ago.
• If there was standing water in the distant past
on the surface of Mars, then what happened
to the atmosphere?
Mars Science Laboratory
Landed on Aug 5, 2012
Determine
whether life
ever arose on
Mars
Characterize
the climate
Characterize
the geology
Landing site for MSL – Gale Crater
• Landing site
Smectite clay on earth from 100
million years ago.
Model for the
formation of
Gale crater.
Running water on Mars
So it appears that there was standing water and
running water on Mars in the distant past.
Why is the Martian atmosphere so very thin
Today?
• What does having an atmosphere depend
on?
• Venus has a very thick atmosphere. The Earth
has a substantial atmosphere and Mars has a
very thin, low density atmosphere.
• Mercury and the Moon have no atmosphere.
.
1. The planet must be
massive
2. The planet must
have water
3. The planet must
have plate tectonics
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• The Earth has an atmosphere but the Moon
does not.
• The mass of the planet dictates the force of
gravity present to hold the gasses from
escaping into outer space.
• The Earth is not massive enough to hold
Helium in the atmosphere. It escapes into
space.
• Mercury has no atmosphere but Saturn’s
largest moon, Titan, which is virtually the
same size as Mercury, has a thick atmosphere.
Titan has 1.45 times the Earth’s
atmospheric pressure
• Mercury has no atmosphere but Saturn’s
largest moon, Titan, which is virtually the
same size as Mercury, has a thick atmosphere.
• Given this evidence, what else must having an
atmosphere also depend on?
.
1. Must also depend
on having water
2. Must also depend
on having geologic
activity
3. Must also depend
on the temperature
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• It depends on the temperature of the planet.
• Near the Sun, where temperatures are high it
is easier for atmospheric gasses to escape into
outer space because the gasses are heated
and therefore moving rapidly.
• Far from the Sun, where temperatures are
very low, the atmospheric gas motions are
very slow and easy for a smaller planet to
hang onto.
• Mars is massive enough and far enough from
the Sun that it should have held on to much of
its atmosphere.
• There is evidence that Mars use to have a
thicker atmosphere, especially if there was
once liquid water on the surface.
• Also, volcanism releases gasses such as CO2,
H2O and methane (CH4). This would add to
the atmosphere.
• Something else must have happened.
Magnetic Field of the Earth
• Magnetic fields arise from the motion of
charges.
• When free electrons flow through a wire, the
wire produces a magnetic field.
Compasses positioned around
current carrying wire.
The right hand rule.
Inside this current loop, which way does
the magnetic field point?
Current
.
1.
2.
3.
4.
5.
6.
To the left
To the right
Up
Down
Out of the screen
Into the screen
The Earth has a liquid core with
free electrons circulating around.
• When the molten material cools and sinks
back toward the inner core, the rapid rotation
of the Earth causes the flow to follow a helical
path.
No, or slow, rotation
Rapid rotation
Field is actually very complex
• So to have a magnetic field for a planet, you
need a molten core and you need the planet
to rotate quickly in order to set up the
currents.
• Venus has no measureable magnetic field.
• It rotates on its axis, once every 243 Earth
days.
• Does Venus have a liquid core?
.
1. No
2. Yes
3. Maybe
Impossible to say for sure.
• Venus does show signs of geologic activity and
it is the same size as the Earth.
• It likely has a liquid core like the Earth does.
• But the rotation is so slow that it can not set
up helical currents that would produce a net
magnetic field.
• Mars has a rotation period of 24 hours and 37
minutes. (Virtually the same as Earth).
• It’s magnetic field is 800 times weaker than
the Earth’s magnetic field.
• What can you conclude from this?
• Mars probably no longer has a liquid core.
• It is likely that it cooled to the point of being a
solid.
• How will this effect things on Mars?
The Earth gives us clues – The Aurora Borealis
Fast moving, charged particles from the sun (solar
wind) are defected by the Earth’s magnetic field.
Some particles are redirected to the Earth’s poles.
• When these charged particles hit the
molecules in the Earth’s atmosphere the air
molecules become ionized (lose an electron).
• The electrons recombine with the ions and
this produces light (electromagnetic
radiation).
• This is why the Aurora Borealis is typically
seen over the poles of the Earth.
The magnetic shield
• The magnetic field of the Earth prevents most
of the solar wind from reaching the Earth’s
atmosphere. They are generally blocked at a
distance which is over 10 times the height of
the atmosphere.
• This prevents the top of the atmosphere from
being completely ionized.
What about Mars?
• Mars has only an extremely weak magnetic
field.
• As a result there is almost nothing to prevent
the top of the atmosphere from becoming
ionized.
• Now consider what will happen next.
What happens to these positively
charged particles?
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They repel each other and spread
out
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• With the repulsion and the energy added by
the impact of the solar wind, molecules gain
enough speed to escape the planet.
• Over very long periods of time, (hundreds of
millions of years) the atmosphere of Mars is
lost to outer space.
• With the repulsion and the energy added by
the impact of the solar wind, molecules gain
enough speed to escape the planet.
• Over very long periods of time, the
atmosphere of Mars is lost to outer space.
• But there is a problem with this model.
Venus has
90 times the
Earth’s
atmosphere
Mars has
0.006 times the
Earth’s
atmosphere
Define the Problem
1. Venus is hotter than Mars, so why
does it have a thick atmosphere?
2. Mars has had volcanic activity like
Venus, it should have a thick
atmosphere
3. Venus has no magnetic field, why
does it have a thick atmosphere?
• When a planet has a thick atmosphere, like
Venus does, some molecules are ionized and
lost, but those that are not lost build an
ionosphere around the planet.
• Since these ions are moving charges they
create a magnetic field that acts as a shield
against the incoming solar wind.
• Some atmosphere is lost, but at a very slow
rate.
• Mars with a thin atmosphere has less of an
ionosphere to protect it.
Mars does have weak, local
magnetic fields in the rock
The magnetic field is locked in the rocks in the
southern hemisphere, but not really seen in
the Northern hemisphere.
Why is this?
• Over time Mars has lost most of its original
atmosphere through these types of processes.
A Possible History of Mars
• Mars is small relative to the Earth and
therefore cooled more quickly.
• It formed a thick crust which locked in the
internal heat and prevented plate tectonics
from occurring.
• This early crust is still seen in the Southern
Hemisphere where the surface has a large
crater density
• In the first billion years, one extremely large
magma plume forms and begins to uplift the
Tharsis bulge area.
• Volcanoes begin to form and release gasses
which build up the atmosphere.
• The atmosphere becomes thick enough for
liquid water to exist on the surface.
• At the same time, tectonic activity and magma
reservoirs heat water ice in the crust allowing
it to flow and accumulate in the Northern
Hemisphere lowlands.
• As time goes by Mars interior cools some
more and the major geologic activity begins to
shut down.
• As this is happening Mars core solidifies and it
loses its protective magnetic field.
• As the atmosphere thins Mars becomes cold
and there is less opportunity for liquid water.
• Volcanism begins to wane and there is less
out-gassing to replenish the atmosphere.
• About 2-3 billion years ago, Mars becomes a
cold, dry planet with only smaller localized
volcanism.
What about life?
Best possibility
H2O + CO2 – CH4 +O2
Geologic or Biologic?
• Heat of summer is affecting the surface of
Mars in some way.
• If methane is from a geologic process, it must
be in the presence of liquid water.
• It could also arise from microbes in
underground water reservoirs that become
active in the summer.
• The Martians may still be around.