Venus

Basics and Rotation

Geography and Volcanoes

Atmosphere and Inferno

Venus Basics

Its orbit lies within Earth’s orbit, placing it close to sun (47^o or less away)

Called the Morning or Evening star

Brightest object in sky after sun and moon.

Star Light, Star Bright, First Star I See Tonight, is usually the planet Venus

Surface is highly reflective (high albedo): 70% of light hitting is reflected, compared to 10% for Mercury or the Moon.

Actually brightest when it is a crescent, because it is so close.

Sister Planet?

Venus Radius: 6052 km

Venus Mass: 0.82 Earth Mass

Venus Density: 5.2 g/cm³

Venus would appear to be Earth’s closest sister.

Although it is missing a moon

At 0.7 A.U., Venus receives 2x as much sun energy (but bounces a lot back)

From early observations, we knew covered in thick clouds.

People imagined wet, humid tropical world, covered in dense foliage.

Rotation Rate Gone Backward

Then in the 1960s, radar was bounced off Venus to measure its Rotation Rate (Day)

Venus takes 243 days to turn around.

Rotation is also Retrograde. Venus spins clockwise while most everything else in the Solar system spins and orbits counter-clockwise

Venus’s Solar day is 116.7 days, shorter than its sidereal day because it goes backwards.

Likely something big hit it during the bombardment era, knocking it almost to a halt.

No Magnetic Field

Venus has no measurable magnetic field, despite being similar in size and composition to the Earth.

Believed to be strong evidence for Dynamo Theory.

Venus spins too slowly to generate a field.

Peering Into Venus

For long time, history of Venus is history of trying to see through its atmosphere

Has high, jet stream looking clouds that move around once every 4 days, confusing rotation rate.

Temperature of upper atmosphere clouds is 240K, very similar to our own.

Finally, radio observations, which can go right through clouds came into being in the 50’s

Inferno

Early radio observations measured thermal emission, showing Venus has temperature of some 730K. (about 850^o F)

Not a lush jungle but the most inhospitable planetary surface in the solar system.

Pressure estimates showed some 90 atmospheres of pressure on the surface

Majority of clouds not water at all, but sulfuric acid clouds making sulfuric acid rain.

Roughly 10 years ago, the spacecraft Magellan was sent to radar map the planet.

Venus Geography

Venus has only two "continents", Aphrodite (the largest) and Ishtar, covering only 8% of the planet.

In comparison, Earth’s continents cover 25%

Most of the surface is fairly smooth, resembling rolling plains with a few bumps.

There is no sign of Tectonic Plates:

No mid-ocean rides, long linear mountain ranges, etc.

Ishtar

Highest place on the planet is the Lakshmi Planum on Ishtar, about 1500km across

Planet’s tallest mountain, Maxwell Mons, is there, some 14 km above lowest point (Everest is 20 km above lowest point in ocean...)

A crater 100 km across lies on slope of Mons, called Cleopatra

Believed to be from a meteorite strike, although it seems to have caused some lava flow.

Aphrodite

Some thought the Aphrodite might be similar to a Mid-Atlantic Ridge, a place where two plates were spreading apart.

Close up images see criss-crossing strips of buckled land, crammed together with great energy.

Also see large cracked areas, where lava raised the land up and then receded, dropping and cracking it.

While evidence of great energies bubbling below, does not show Earth-like plates.

Volcanoes

Venus has impressive array of lava domes, areas where lava rose, formed dome, and then sank, making the domes crack.

Most Venus volcanos appear to be shield volcanos, like Hawaii, where hot spots, not plate edges, caused volcanos to form.

Largest volcanic structures are coronae, 100s of km across, upwellings from below that form large bumps.

Often lots of volcanos associated with them.

Active Surface?

No Venusian Volcano has every been seen to erupt

However, sulfur dioxide in atmosphere is large, showing large variations

Lightning discharges, like those seen on Earth in plumes of erupting volcanoes, have been seen.

Some, baffled by lack of plate tectonics, have suggested most of Venus does not regularly erupt, but instead melts down in the occasional catastrophic event. Large pieces of the planet erupting/melting at once.

Craters

Another feature of Venus are its craters.

Large ones are circular, but smaller ones (less than 15 km) are quite asymmetric

Odd shape probably results from objects breaking up in dense atmosphere before hitting surface

Are no craters smaller than 3 km across.

Counting craters gives us estimate of Venus’ surface age:

About 1 billion years old, with some parts maybe resurfaced more often (200-300 million years).

Brief Visits

Only landings on Venus were by Soviets in 1970s: The Veneras (9,10,13,14,17,18...)

Each craft lasted only a hour before melting, but did send back pictures of the surface:

Rocks seemed flat and jagged

Flat because of immense pressure
Jagged because not much erosion: Recent rocks???

Also saw granite material, similar to Earth’s crust.

Atmosphere

Venus is 96.5 Carbon Dioxide, 3.5 Nitrogen with a few traces elements.

Earth’s white clouds are caused by condensed water vapor and ices.

On Venus it is Sulfuric acid, a mixture of water and Sulfur Dioxide

Venus is so hot because of its Carbon dioxide: An excellent Greenhouse gas

Venus has more than 100,000x as much than Earth’s atmosphere

What Happened to Venus?

Initial atmospheres of Earth and Venus must have been very similar

On Earth, water condensed into Oceans, in which most Carbon Dioxide dissolved and eventually became deposited in rocks (limestone)

Take CO₂ out of rocks and Earth’s atmosphere gets 70x heavier and 98% CO₂

Runaway Greenhouse Effect

On Venus, temperature may have been high enough that no oceans formed.

Or, if oceans did form,higher temperature may have led to Runaway Greenhouse Effect

Little water evaporates, releasing little CO₂

This traps more heat, evaporating more water and releasing more CO₂

This traps more heat causing a feedback loop which will build on itself until all the oceans boil away