Venus and Mercury

The Inner Planets

Venus• 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• 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• 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