Comparative%20Planetology%20of%20Venus%20and%20Mars

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Comparative Planetology of Venus and Mars
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• Chapter 22

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Guidepost
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• You have been to the moon and to Mercury, and now
  you are going to find Venus and Mars dramatically
  different from those small, inactive and airless
  worlds. Venus and Mars have internal heat and
  atmospheres. The internal heat means they are
  geologically active, and the atmospheres mean
  they have weather. As you explore, you will
  discover answers to six essential questions
• Why is the atmosphere of Venus so thick?
• What is the hidden surface of Venus really like?
• How did Venus form and evolve?
• Why is the atmosphere of Mars so thin?
• What is the evidence that Mars once had water on
  its surface?
• How did Mars form and evolve?

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Guidepost (continued)
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• The comparative planetology questions that need
  to always be on your mind when you explore
  another world are these
• How and why is this world similar to Earth?
• How and why is this world different from Earth?
  You will see that small initial differences can
• have big effects.
• You are a planet-walker, and you are becoming an
  expert on the kind of planets you can imagine
  walking on. But there are other worlds beyond
  Mars in our solar system so peculiar they have no
  surfaces to walk on, even in your imagination.
  You will explore them in the next two chapters.

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Outline
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I. Venus A. The Atmosphere of Venus B. The
Venusian Greenhouse C. The Surface of Venus D.
Volcanism on Venus E. The Rotation of Venus F.
A History of Venus II. Mars A. No Canals on
Mars B. The Atmosphere of Mars C. The Geology
of Mars D. Finding the Water on Mars E. A
History of Mars III. The Moons of Mars A.
Origin and Evolution
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Venus and Mars
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Two most similar planets to Earth

• Similar in size and mass

• Atmosphere

• Similar interior structure

• Same part of the solar system

Yet, no life is possible on either one of them.
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The Atmosphere of Venus
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4 thick cloud layers (? surface invisible to us
from Earth).
Very stable circulation patterns with high-speed
winds (up to 240 km/h)
Extremely inhospitable
96 carbon dioxide (CO2)
Very efficient greenhouse!
3.5 nitrogen (N2)
Rest water (H2O), hydrochloric acid (HCl),
hydrofluoric acid (HF)
Extremely high surface temperature up to 745 K
(880 oF)
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The Surface of Venus
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Early radar images already revealed mountains,
plains, and craters.
More details from orbiting and landing spacecraft
Venera 13
Colors modified by clouds in Venuss atmosphere
After correction for atmospheric color effect
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Radar Map of Venuss Surface
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Surface features shown in artificial colors

• Scattered impact craters

• Volcanic regions

• Smooth lava flows

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Lava Flows
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Young, uneven lava flows (shown Lava flow near
Flagstaff, AZ) show up as bright regions on radar
maps.
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Surface Features on Venus
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Maxwell Montes are 50 higher than Mt. Everest!
Smooth lowlands
Highland regions
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Craters on Venus
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Nearly 1000 impact craters on Venuss surface
? Surface not very old
No water on the surface thick, dense atmosphere
? No erosion
? Craters appear sharp and fresh
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Volcanism on Earth
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Volcanism on Earth is commonly found along
subduction zones (e.g., Rocky Mountains).
This type of volcanism is not found on Venus or
Mars.
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Shield Volcanoes
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Found above hot spots
Fluid magma chamber, from which lava erupts
repeatedly through surface layers above
All volcanoes on Venus and Mars are shield
volcanoes.
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Shield Volcanoes (2)
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Tectonic plates are moving over hot spots
producing shield volcanoes ? Chains of volcanoes
Example The Hawaiian Islands
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Volcanism on Venus
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Sapas Mons (radar image)
400 km (250 miles)
2 lava-filled calderas
Lava flows
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Volcanic Features on Venus
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Aine Corona
Baltis Vallis 6800 km long lava flow channel
(longest in the solar system!)
Coronae Circular bulges formed by volcanic
activity
Some lava flows collapsed after molten lava
drained away
Pancake Domes
Associated with volcanic activity forming coronae
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The Rotation of Venus
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• Almost all planets rotate counterclockwise, i.e.
  in the same sense as orbital motion.

• Exceptions Venus, Uranus and Pluto

• Venus rotates clockwise, with period slightly
  longer than orbital period.

Possible reasons

• Off-center collision with massive protoplanet

• Tidal forces of the sun on molten core

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A History of Venus
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Complicated history still poorly understood.
Very similar to Earth in mass, size, composition,
density,
but no magnetic field ? Core solid?
? Solar wind interacts directly with the
atmosphere, forming a bow shock and a long ion
tail.
CO2 produced during outgassing remained in
atmosphere (on Earth dissolved in water).
Any water present on the surface rapidly
evaporated ? feedback through enhancement of
greenhouse effect
Heat transport from core mainly through magma
flows close to the surface (? coronae, pancake
domes, etc.)
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Mars
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• Diameter 1/2 Earths diameter

• Very thin atmosphere, mostly CO2

• Rotation period 24 h, 40 min.

• Axis tilted against orbital plane by 25o, similar
  to Earths inclination (23.5o)

• Seasons similar to Earth ? Growth and shrinking
  of polar ice cap

• Crust not broken into tectonic plates

• Volcanic activity (including highest volcano in
  the solar system)

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Tales of Canals and Life on Mars
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Early observers (Schiaparelli, Lowell) believed
to see canals on Mars
This, together with growth/shrinking of polar
cap, sparked imagination and sci-fi tales of life
on Mars.
We know today canals were optical illusion do
not exist!
No evidence of life on Mars
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The Atmosphere of Mars
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Very thin Only 1 of pressure on Earths surface
95 CO2
Even thin Martian atmosphere evident through haze
and clouds covering the planet
Occasionally Strong dust storms that can
enshroud the entire planet
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The Atmosphere of Mars (2)
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Most of the Oxygen bound in oxides in rocks
? Reddish color of the surface
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History of Marss Atmosphere
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Atmosphere probably initially produced through
outgassing
Loss of gasses from a planets atmosphere
Compare typical velocity of gas molecules to
escape velocity
Gas molecule velocity greater than escape
velocity ? gasses escape into space
Mars has lost all lighter gasses retained only
heavier gasses (CO2)
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History of Marss Atmosphere
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Gases bound in the polar cap are returned to the
atmosphere each spring in spots and fans.
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The Geology of Mars
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Giant volcanoes
Valleys
Impact craters
Reddish deserts of broken rock, probably smashed
by meteorite impacts
Vallis Marineris
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The Geology of Mars (2)
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Possibly once filled with water
Northern Lowlands Free of craters probably
re-surfaced a few billion years ago
Southern Highlands Heavily cratered probably 2
3 billion years old
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Volcanism on Mars
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Volcanoes on Mars are shield volcanoes.
Olympus Mons
Highest and largest volcano in the solar system
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Volcanism on Mars (2)
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Tharsis rise (volcanic bulge)
Nearly as large as the U.S.
Rises 10 km above mean radius of Mars
Rising magma has repeatedly broken through crust
to form volcanoes.
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Finding the Water on Mars
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No liquid water on the surface
Would evaporate due to low pressure
But evidence for liquid water in the past
Outflow channels from sudden, massive floods
Collapsed structures after withdrawal of
sub-surface water
Valleys resembling meandering river beds
Gullies, possibly from debris flows
Central channel in a valley suggests long-term
flowing water
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Finding the Water on Mars (2)
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Formations possibly arising from broken pack ice,
covered by dust and volcanic ash
Cracks in the surface near the pole, probably due
to seasonal expansion and contraction of ice
under the surface
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Finding the Water on Mars (3)
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Hematite concretions in Martian Rocks,
photographed by the Mars Rover Opportunity
Sedimentary rock layers, formed by rapidly
flowing water
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Finding the Water on Mars (4)
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Galle, the happy face crater
Meteorite ALH84001
Identified as ancient rock from Mars
Some minerals in this meteorite were deposited in
water ? Martian crust must have been richer in
water than it is today.
Large impacts may have ejected rocks into space.
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The History of Mars
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• Differentiated when it formed a hot, molten core

• Cooled off rapidly

? Mostly solid core can not produce an overall
magnetic field

• Three main periods

Noachian Period 4.3 3.7 bil. years ago Heavy meteorite bombardment Flooding of plains by great lava flows Liquid surface water could have been present.
Hesperian period 3.7 3 bil. years ago Part of surface covered by lava flows Outflow channels may have formed Formation of the Tharsis rise might have decreased the orbital tilt of the rotation axis from 45o to the present 25o
Amazonian Period 3 bil. yrs. ago today Mostly uneventful Thick crust prevents geological activity.
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The Moons of Mars
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Two small moons Phobos and Deimos
Too small to pull themselves into spherical shape
Typical of small, rocky bodies Dark grey, low
density
Phobos
Very close to Mars orbits around Mars faster
than Mars rotation
Probably captured from outer asteroid belt
Deimos