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Chapter 9
Earth-Like Planets: Venus and Mars
• Venus and Mars resemble Earth
more than any other planets.
• Is it possible that life exists or
did exist on either?
• One day, will we be able to
establish a human presence on
Venus or Mars?
• Can studying these planets give
us clues to Earth’s origins or
future.
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9.1.1 Appearance of Venus
• Venus gets closer to Earth
than any other planet.
• It appears as a bright object
near the Sun after sunset or
before sunrise.
• Venus goes through phases,
like the Moon.
• Venus is shrouded in thick
clouds making it impossible
to view the surface from
Earth.
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9.1.1 Appearance of Mars
• About every 24 months
the Earth and Mars are
at their closest.
• Mars is reddish due to
iron oxides (rust) in the
soil.
• Around 1900, Percival
Lowell thought he saw
canals on Mars!
• Mars has polar ice caps,
thin clouds, and dust
storms.
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9.1.2 Rotation of the Planets
• The rotation of Mars has been measured by
observing features on the planet over a very
long time, about 200 years.
• Mars’ sidereal rotation period is 24 h 37m 23s,
about 40min longer than Earth’s.
• The rotation of Venus is measured using radar,
but unlike Mercury, the radar is used to observe
surface features and watch them rotate.
• Venus’ sidereal rotation period is 243 days!
• Venus rotates backwards (retrograde)!!
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Properties of Earth, Venus, and
Mars
• Compare some properties of these planets.
Earth
Venus
Mars
Semi major axis (AU)
1.00
0.72
1.52
Period (Earth years)
1.00
0.61
1.88
Mass (Earth = 1)
1.00
0.82
0.11
Diameter (Earth = 1)
1.00
0.95
0.53
Density (g/cm3)
5.5
5.3
3.9
Surface Gravity (Earth = 1)
1.00
0.91
0.38
Escape velocity (km/s)
11.2
10.4
5.0
Rotation period
23.9 hours
243 days
24.6 hours
Surface Area (Earth = 1)
1.00
0.90
0.28
Atmospheric Pressure (bar)
1.00
90
0.007
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9.1.3 Basic Properties
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•
•
•
•
Venus
Very similar to Earth
High geological activity
Thick atmosphere of
CO2
Surface temperature of
730K (over 850 F).
Atmospheric pressure of
90 bar, equivalent to
1km under the ocean.
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•
•
•
•
•
Mars
Smaller than Earth
Thin atmosphere
Significant past
geological activity
Probably had a thick
atmosphere and liquid
water in the past.
Could have supported
life.
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9.2 The Geology of Venus
• Being of similar size and composition to
Earth, we might expect Venus to have
similar geology.
• More spacecraft have visited Venus than
any other planet.
– 1962 U.S. Mariner 2 flyby
– 1970 Soviet Venera 7 lands on Venus and
sends back pictures for 23 minutes (heat).
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More Missions to Venus
– 1970s Venera craft
measure soil and
atmosphere.
– 1970s U.S. Pioneer
Venus -- radar map.
– 1980s Venera 15 and
16 radar orbiters.
– 1991-3 Magellan radar
maps with 100m
resolution.
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
Globe of Venus constructed
from Magellan radar maps.
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Venus Express: Orbiting Venus
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Venus Express
• Built by the European Space
Agency (ESA)
• Launched on a Russian
Soyuz rocket in November
2005.
• Arrived at Venus April 11,
2006.
• Will study Venus for 2 to 5
years, especially to
understand the greenhouse
effect.
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Radar Maps of Venus
1
N
2
S
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9.2.3 Venus: Craters and Surface
Age
• Count craters to estimate
surface age.
• Largest crater called Meade,
275km in diameter.
• Thick atmosphere stops only
small projectiles.
• Almost no craters smaller
than 10km in diameter.
• Count larger craters and
estimate surface age at 500
million years.
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9.2.4 Volcanoes on Venus
•
•
•
•
Significant volcanism.
Largest volcano, Sif
Mons, is wider but
shorter than Mauna
Kea.
Some volcanoes
produce “pancake
domes”.
Many volcanoes don’t
make it to the
surface, but push up
the crust in bulges
called coronae.
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9.3 The Massive Atmosphere of
Venus
• Atmosphere is 96% CO2,
3.5% nitrogen and trace
amounts of H2O, H2SO4.
• The CO2 traps heat on
the planet via the
greenhouse effect.
• Runaway greenhouse
effect.
• The surface temperature
is more than 700 Kelvin
(850F).
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The Birth of Venus
• Backward rotation could mean that Venus suffered a giant
impact early in its history.
• The runaway greenhouse effect explains why Venus has such a
massive atmosphere and high surface temperatures.
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9.4 The Geology of Mars
• Humans could survive
on Mars, making it
more interesting.
• Spacecraft have
visited and landed on
Mars.
– 1965: Mariner 4 fly by
– 1971: Mariner 9 orbited
– 1976: Vikings 1 and 2
landed
– 20 years pass with 2
failed missions to Mars.
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9.4.1 Spacecraft Exploration of Mars
– 1997: Pathfinder
lands on Mars and
Global Surveyor
orbits.
– Several more failed
space missions.
– 2004: 4 spacecraft to
Mars, 2 U.S., 1 ESA,
1 Japan. (Japanese
craft failed to orbit,
ESA rover, Beagle,
didn’t function).
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9.4.2 Global Properties of Mars
• Mars is half Earth’s
diameter.
• Small metal core.
• No magnetic field.
• We have good maps of
Mars showing:
– Olympus Mons: highest
peak in the solar system.
– Vallis Marineris: largest
canyon in the solar
system.
– Old highlands and
younger lowlands.
Olympus Mons
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9.4.3 Volcanoes on Mars
• The lowland plains
seem to be formed 3
to 4 billion years ago
– Lava flows
• Largest volcanoes on
the Tharsis bulge.
• 3 shown at right
• Olympus Mons is
largest, larger than
Mauna Loa in Hawaii
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9.4.4 Martian Cracks and Canyons
• Valles Marineris is
the largest canyon
in the solar system.
• Formed by cracking
in the Tharsis bulge
• L.A. to N.Y.
• Landslides in valley
are evidence of
erosion (water?).
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9.5 Martian Polar Caps and Climate
• Thin atmosphere, like
Earth’s 30km up.
• Mostly CO2.
• Clouds of dust, H2O, and
CO2.
• Seasonal ice caps of
CO2 (dry ice).
• Permanent polar caps
(south at left) have H2O.
• Ice in the soil
(permafrost).
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9.5.4 Climate Change on Mars
• Mars suffers from the runaway refrigerator
effect.
– Mars might have had a much thicker atmosphere
and milder climate in the past.
– Mars has smaller surface gravity than Venus or
Earth.
– Atmospheric gases can escape into space, cooling
off the planet (evaporative cooling).
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9.5.5 Life on Mars
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Search for Life on Mars
• Life as we know it needs water.
• If Mars had water in the past, life could have
developed.
• We’ve tried looking for signs of life in a variety
of ways, so far with negative results.
• It could be that UV light has sterilized the
surface -- could microbes be underground?
• Some scientists claim that a meteorite from
Mars shows signs of microbial life. (next)
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Martian Meteorite
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Discussion Question
• This is to review some of the material just
presented.
• First answer the following question:
– Venus, Earth, and Mars have mountains. What are
two factors that may explain why Mars has the
largest mountain, though it is the smallest of the
three planets.
• Second pass your answer to another person for
discussion and grading. Please sign your name
at the bottom of the answer you are grading.
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