Lesson 5

Venus

Venus from Earth

At its brightest, Venus is about 60 times brighter than the brightest stars.

Venus is the morning or evening “star”

• Venus is never more than 47 degrees from the Sun.

Why does Venus look this way?

.

1 2 3 4

25% 25%25%25%

1. The Earth is casting a shadow on Venus

2. Mercury is casting a shadow on Venus

3. We are seeing the day and night side of Venus

4. There are clouds blocking part of Venus

Venus phases

• What is Venus’s phase when it is closest to the Earth?

.

1 2 3

33% 33%33%1. New Venus2. Full Venus3. Half-lit

When Venus is close to the Earth we only see the night side of Venus. When it is on the other side of the Sun we see the full

Venus

• Galileo was the first to see the phases of Venus, using a small telescope. (1610)

• The changing size and phases was proof that Venus didn’t orbit the Earth. Instead, Venus had to orbit the Sun.

Venus – Basic Properties

Venus – Basic Properties

• Diameter is 650 km less than Earth’s• Mass is 81.5% of Earth’s • Virtually a twin to Earth in size and mass.• Venus has no moon• Venus atmosphere is 90 times that of Earth• Surface temperature is ~ 870o F everywhere.• Always under cloud cover

• Venus rotates on its axis very slowly (225 Earth Days) and in the opposite direction of that of the other planets.

• On Venus the Sun would rise in the west and set in the East.

• On Venus, a year is 1.92 Venus days long. So if you were on Venus you would have about 2 days each year.

• The slow rotation and small differences in surface temperature around the planet means wind speeds are very slow on Venus. Virtually no wind erosion as on Mars.

• We saw on Mars that there has been a great deal of wind erosion.

• Venus has virtually one global temperature with only small variations.

• Would you expect Venus to also have lots of wind erosion on its surface?

.

1 2 3

33% 33%33%

1. Yes. Venus has a thick atmosphere and is very hot. This should cause enormous winds

2. Yes. Since Venus rotates slowly winds should be able to blow from the poles to the equator

3. No. There are no temperature differences so wind will not develop.

• Venus rotates on its axis very slowly and in the opposite direction of that of the other planets.

• On Venus the Sun would rise in the west and set in the East.

• On Venus, a year is 1.92 Venus days long. So if you were on Venus you would have about 2 days each year.

• The slow rotation and small differences in surface temperature around the planet means wind speeds are very slow on Venus. Virtually no wind erosion as on Mars.

Venus from outer space

• Venus is always shrouded in thick clouds. The atmosphere is almost completely carbon dioxide (CO2).

• The cloud decks seen from space are composed mostly of sulfur dioxide (SO2).

• There is virtually no water in the atmosphere of Venus.

Topographical Map

Red highest elevation, blue lowest

Earth topographical map

Earth Active Volcanoes

Venus volcanoes

Is there plate tectonics occurring on Venus?

Is there plate tectonics occurring on Venus?

1 2

50%50%1. Yes2. No

What type of volcanoes do you expect?

What type of volcanoes are on Venus?

1 2 3

33% 33%33%1. Strato-volcanoes2. Shield volcanoes3. A mixture of the

two.

Venus volcanoes

Gula Mons

Maat Mons

All Venus volcanoes are shield.

• Large shield volcanoes• Pancake Domes (smaller, sometimes form

along fissures)• Corona (uplifts in crust causing tectonic

features)• Arachnoids (spider-like volcanoes that have

sunk back into the surface)

Large shield volcano

Lava Flow and

channels

Mylitta Fluctus

Lava Flow and channels

Atla Regio about 1000 km long

Helen Planitia

Long lava flow channels (~1200 km

long) which show signs of being

covered by later lava flow.

Pancake volcanoes forming in a line along fissure

Flat top, pancake domes suggest that the lava was of higher viscosity

• If pancake domes form along crustal fissures, why might they have higher viscosity?

.

1 2

50%50%1. There must be a subduction zone that mixes crustal material into the mantle magma.

2. Mantle magma emerging up through the fissure probably melts the crust which makes the magma of higher viscosity

Corona features Caused by hot mantle

material uplifting

and sinking over and

over again.

Arachnid volcanoes are probably volcanic domes which sunk back down when magma retreated

About 1000 impact craters on Venus

Impact craters• On Venus there are no impact craters smaller

than 3 km. Most are around 10 km or bigger.

• Why do you think this is?

.

1 2 3

33% 33%33%1. All the smaller craters are

destroyed by geologic activity2. Smaller meteors can’t make it

through the thick Venus atmosphere

3. Venus only gets hit by big objects.

Distribution of Impact craters

Impact craters• On Venus there are no impact craters smaller

than 3 km. Most are around 10 km or bigger. This is due to the protection of the extremely thick atmosphere.

• The impact craters are randomly distributed across the surface of Venus.

• Density is about 1.9 craters/million km2. This is the same everywhere on the surface.

• What does this suggest?

.

1 2 3

33% 33%33%1. The surface of Venus is the

same age globally.2. Younger regions on the

surface have been hit more frequently in the recent past.

3. Venus is similar to the highlands of the Moon

Lunar Site Crater Density Age(10km/1 million km2) (billions of years)

Fra Mauro 550 3.94Cayley 430 3.77Apollo 11 120 3.55Apollo 15 97 3.40Apollo 12 70 3.28Copernicus crater rim 20 0.90

Impact craters• On Venus there are no impact craters smaller

than 3 km. Most are around 10 km or bigger. This is due to the protection of the extremely thick atmosphere.

• The impact craters are randomly distributed across the surface of Venus.

• Density is about 1.9 craters/million km2

• Lunar density for a 0.9 billion years old surface is 20 craters/million km2

Piecing the evidence together.

• Few impact craters that are randomly distributed.

• No plate tectonics. • Volcanoes are randomly distributed across the

surface.

• What does this suggest about the surface of Venus?

.

1 2 3

33% 33%33%

1. There is a combination of very old surfaces and very young surfaces on Venus

2. Craters near volcanoes are destroyed while other places retain their ancient craters

3. Venus must have eruptions on a global scale that resurfaces the entire planet.

Piecing the evidence together.

• Few impact craters that are randomly distributed. (Means the global surface of Venus is about the same age).

• No plate tectonics. (Means pressure release has to come in the form of magma plumes)

• Volcanoes are randomly distributed across the surface. (Means that eruptions are not just in local regions like on Mars)

Geology on Venus• Venus appears to resurface nearly the entire

planet at the “same” time, about 200 – 600 million years ago. (Possibly over a 100 million year period)

• Since there is no plate tectonics, pressure and temperature builds inside of Venus and is finally released globally, resurfacing the entire planet.

• Activity then slows until pressure rebuilds.• This is likely a cyclic process.