Venus and Mercury
The Inner Planets
Venus
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Only a bit smaller than Earth
Nearest planet (26 million miles)
Shows phases as it orbits the Sun
Most circular orbit
Atmosphere mostly Carbon Dioxide
– 90 times as dense as Earth
– Runaway Greenhouse Effect
– Surface Temperature 450 C (850 F)
Venus’ Odd Rotation
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Venus Rotates in 243 days relative to the stars
Longer than its year (225 days)
Solar Day = 117 days
You can jog as fast as Venus rotates
Opposite to all other planets’ rotation
(retrograde)
• Why?
– Mega-Impact?
– Tidal Effects?
– Possible tidal lock on Earth?
Venus from Pioneer Venus
Orbiter
Polar
View of
Venus
Topography of Venus
Earth Elevations
Radar
Map of
Venus
Radar
Map of
Venus
Surface of Venus
Venus
has
Craters
Venus is Volcanic
“Pancake” Volcanoes
Channel
on
Venus
Coronas
How Coronas Form?
Venus Differs from Earth in One
Important Way:
• Venus has no Plate Tectonics
• Earth’s internal heat causes hot material to
rise within Earth and plates to move
• Venus’ crust is too rigid
• Heat builds up and escapes in planet-wide
volcanic activity
• Last event about 600 million years ago
Why Venus Has No Plate
Tectonics
• It’s Hot! We’d expect hot rocks to be less
rigid, not more!
• But it’s also dry! Dry rocks are ten times
stronger at high temperatures than wet
rocks.
• Earth is wet. That affects not just the
outside, but the inside as well.
Did Venus Once Have Oceans?
• Resurfacing means no visible trace of
early history
• Deuterium: Ghost of a Vanished Ocean?
– Deuterium is hydrogen with one neutron
– Twice as heavy as normal hydrogen
– Less likely to escape
– D/H on Venus 100x that of Earth
• Photodissociation + Stripping
– Venus lacks magnetic field
Venus and the Da Vinci Code
• 13 Venus years = 13 x 224.7 = 2921.1 days
• 8 Earth Years = 8 x 365.25 = 2922 days
• 5 Synodic periods of Earth and Venus
= 5 x 583.9 = 2919.5 days
• 1/5 of a circle = 72 degrees
• In 583.9 days, Earth moves 360 + 215.5
degrees (almost 360 + 3 x 72)
• In 583.9 days, Venus moves 720 + 215.5
degrees
Venus and Earth
• 13 Venus years = 13 x 224.7 = 2921.1
days
• 8 Earth Years = 8 x 365.25 = 2922 days
• 5 Synodic periods of Earth and Venus
= 5 x 583.9 = 2919.5 days
• 12 Venus rotations = 2916 days
• Venus shows nearly the same face to
Earth every 584 days.
Venus and Earth
Mercury
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Closest planet to Sun
Very elliptical orbit (e = .205)
Only 3000 miles (5000 km) in diameter
Rotation locked to Sun: 3 rotations in 2
orbits (58 days)
• Moon-like on the outside (craters)
• Earth-like on the inside (dense core,
magnetic field)
Mercury’s
Rotation
Mercury’s Interior
• Density = 5.43 (Earth = 5.5)
• Much smaller than Earth, less pressure
inside
• Core about 2/3 of Mercury’s diameter
(Earth = ½)
• Mega-Impact may have blown away
silicate outer part
Mercury
from Mariner
10, 1973
Mercury has Craters
• Not as dense
as on Moon
• Most of
Mercury
covered with
lava plains
(intercrater
plains)
Mercury
has a
huge
impact
basin, the
Caloris
Basin
Because of Mercury’s
locked rotation, it has two
“hot poles”
And Mercury has the last thing
you’d expect to find: ice caps
Mercury’s ice seems to be
hiding in polar craters
We’re
B-a-a-ck
“Spider” Crater
Caloris
Basin
Volcano on
Mercury