Venus
&
Mars
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Nearest Planets
Venus and Mars resemble
Earth more than any
other planets
But the three are very
different from each other
Venus and Mars are among
the brightest objects in the night sky
Intriguing questions:
Will we someday be able to visit Venus or Mars?
Does alien life exists on either?
Can studying these planets give us
clues to Earth’s origin, or future?
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Venus
Mars
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Appearance of Venus
Venus gets closer to Earth (about 40 million
km) than does any other planet
Venus sometimes appears as a bright
object near the Sun after sunset (an “evening
star”) or before sunrise (a “morning star”)
Galileo discovered that Venus goes through
phases, like the Moon
Venus is shrouded by thick
clouds, making it impossible
to view its surface, even with
cameras in orbit around the
planet
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Appearance of Mars
About every 24 months the Earth and Mars
are at their closest (~56 million km apart)
Mars is reddish due to the presence of iron
oxides (rust) in its soil
Around 1900, Percival Lowell thought he saw
canals on Mars!
It is now generally accepted that what he saw was
an optical illusion
Two faces of Mars
Mars has polar ice
caps, thin clouds,
and dust storms
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Rotation of Venus and Mars
The rotation (spinning) 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 40 minutes longer than Earth’s
The rotation (spinning) of Venus is measured
using radar, but (unlike Mercury) the radar is
used to observe the motion of Venus’ surface
features
Venus’ sidereal rotation period is 243 days!
Surprise: Venus spins in a backward or retrograde
direction!!
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Properties of Earth, Venus, and Mars
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Basic Properties
Venus
Similar to Earth in
size, mass, and high
geological activity
Has thick atmosphere
consisting mostly of
carbon dioxide (CO2)
Atmospheric pressure
almost 100 times
greater than Earth’s
Surface very hot, with
temperature of 730K
(over 850° F)
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Mars
Rather small in mass
and size compared to
Earth
May have had
significant geological
activity in the past
Has thin atmosphere
Probably had thick
atmosphere and liquid
water in the past
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Which could have
supported life
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The Geology of Venus
Venus being similar size and
composition to the Earth, we might
expect the two planets to have similar
geology
This is partly true, but Venus does not
exhibit the same kind of plate tectonics as
the Earth
Early missions to Venus:
1962: U.S. Mariner 2 did a flyby
1970: Soviet Venera 7 became the first craft to
land on Venus and broadcast back pictures for 23
minutes (before succumbing to heat)
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Missions to Venus
More spacecraft have
visited Venus than
any other planet
Additional missions to Venus
Images taken by Venera 13
1970: more Venera probes made photographs as
well as analyzed the soil and atmosphere
1970s: U.S. Pioneer Venus orbiter made the first
crude global radar map
1980s: Soviet Venera 15 and 16 radar orbiters
made better radar maps
1991-1993: U.S. Magellan spacecraft made radar
maps with 100-m resolution
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Radar Maps of Venus
1
N
2
S
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Craters and Surface Age of Venus
Crater counting used to
estimate surface age
Largest crater, called Meade,
275km in diameter
Thick atmosphere seems to
stop only small projectiles
Almost no craters smaller than
10 km in diameter
Counting of larger craters
suggests surface age of
roughly 500 million years
Probable indication of persistent
geological activity
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Volcanoes on Venus
Significant volcanism
Largest volcano, called
Sif Mons, wider but lower
than Mauna Loa in Hawaii
Some volcanoes have
shapes like “pancake
domes”
Volcanic bulges called
coronae are common
Computer-generated view of Sif Mons
Pancake-dome volcanoes
These are produced by
hot magma from the
planet’s interior which
does not make it to the
surface
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The Massive Atmosphere of Venus
Atmosphere is 96% carbon dioxide, 3.5%
nitrogen, and very little else
The CO2 traps heat on the planet via the
greenhouse effect
Surface temperature
is more than 700 K
(850°F)
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Birth of Venus
Backward rotation of Venus may have been
caused by a giant impact early in its history
The massive atmosphere and high surface
temperature of Venus may have been caused
by runaway greenhouse effect
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The Geology of
Mars
Mars is more hospitable
than Venus to humans,
making Mars more interesting
Early missions to Mars:
1965: Mariner 4 fly by
1971: Mariner 9 was the first to orbit
1976: Vikings 1 and 2 landed
20 years passed with 2 failed missions
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Spacecraft Exploration of
Mars
Recent missions:
1997: Pathfinder landed
and Mars Global Surveyor
orbited
2002: Another craft orbited
2004: 2 U.S., 1 European Space Agency, and 1
Japanese craft sent (Japanese craft failed to orbit
and ESA rover, Beagle, didn’t function, but U.S.
landers succeeded)
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Global Properties of Mars
Mars’ diameter is half Earth’s
A small metal core, but no magnetic field
We have good maps of Mars’s surface
showing:
Olympus Mons: highest peak in the solar system
Vallis Marineris: largest canyon in the solar system
Computer-generated rendering of
Highlands and lowlands
Highlands believed to be older
than lowlands
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Olympus Mons
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Topographic Map of Mars
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Volcanoes on Mars
The lowland plains may
have 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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Martian Cracks and Canyons
Valles Marineris is the
largest canyon in the
solar system
Formed by tectonic
cracking in the Tharsis
bulge
L.A. to N.Y.
Landslides in valley are
evidence of erosion
(water?)
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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) have H2O
Ice in the soil (permafrost)
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Climate Change on Mars
Mars suffers from 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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Life on Mars
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Search for Life on Mars
Life as we know it needs water
If Mars had had water in the past, life could
have existed there
Scientists have tried looking for signs of life
on Mars in a variety of ways, so far with
negative results
It could be that UV light has sterilized the
surface, but could microbes live below?
Some scientists claimed that a meteorite from
Mars shows possible evidence of life in its
past (see next slide)
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Martian Meteorite
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