Lecture 16
Venus--Earth's Evil Twin
(chapter 6--Jakosky)

Venus has been called "Earth's twin" or more appropriately, "Earth's Evil Twin".

What are some of the similarities and differences between Venus and Earth?



Location, Location, Location

Although Venus is similar to Earth in many ways, its slightly closer proximity to the Sun (0.723 AU) creates a harsh atmosphere incapable of supporting life.

Venus' atmosphere

Water loss on Venus

Scientists estimate the amount of water initially present on Venus to be at least enough to create a global layer about 3 m thick---about 0.15% of the Earth's water inventory.

But Venus has lost its water over time.  The process by which this happened is called Photodissociation.

Venus' Atmosphere

1.  An H2O molecule (present in atmosphere as water vapor) is broken down by sunlight.
2.  The hydrogen atoms escape to space.
3.  The oxygen atom oxidizes surface minerals.

This does not occur on the Earth because the Earth is cooler (so water vapor stays lower in the atmosphere) and because the temperature increases sharply at 10km (the tropopause) and increases throughout the Stratosphere. Therefore, all the water vapor condenses at the lower (cooler) 10-35 km altitudes and so very little water rises all the way to the Ionosphere (> 50 km --above the Ozone layer) where it would be photodissociated by the Sun's UV.

Earth's Atmosphere

Take an Earth-like planet, put it where Venus is and see what happens:

1.  Atmosphere would begin to warm up because of the higher input of solar energy--->temperature would increase about 10% or ~ 30 K.

2.  Higher temperatures mean the atmosphere would be capable of holding more water vapor--->some of the oceans' water begin to evaporate.

3.  Water vapor--a greenhouse gas--absorbs heat energy--->leading to an increase in atmospheric water vapor--->leading to an increase in atmospheric temperature.

4.  This increase in atmospheric temperature leads to the atmosphere being able to hold more water vapor--->oceans evaporate.

5.  And so on.....--->Runaway Greenhouse.

The system would stabilize once all of the surface water was absorbed and the temperature was very high.

This scenario is in fact what has happened to Venus.

Why doesn't Earth show the same Runaway Greenhouse Effect?

Location, Location, Location

1.  Lower terrestrial temperatures mean the atmosphere capable of holding less water vapor.

2.  Earth's atmosphere is stable at relatively low water abundances and temperatures.

3.  Earth needs 40% more sunlight to trigger a Runaway Greenhouse.

4.  Earth needs to be closer to the Sun or the Sun needs to put out more energy for a Runaway Greenhouse to develop.

5.  The Sun's output does increase with time so one day (~2 b.y.), Earth will develop a Runaway Greenhouse.

Surface of Venus

surface of Venus from Russian probe.

Because of Venus' thick atmospheric cloud cover, radar images are the only way to reveal the surface of Venus.

The Magellan spacecraft has provided scientists with the best results.  Magellan was launched in 1990, orbited several years and then crashed into the surface.  Data analysis is ongoing.


impact crater on Venus Why?

*Geological processes on Earth able to destroy craters more rapidly.

Interior of Venus

Venus has no plate tectonics because no subduction and spreading zones have been picked up on radar.
Volcano on Venus

There is evidence for catastrophic resurfacing every 500 m.y.

1.  The lack of water makes the mantle viscous, prevents convection, surface stagnant.

2.  Heat builds up in mantle.

3.  High heat melts mantle, massive global volcanism occurs every 500 m.y.

massive global volcanism

4.  Mantle cools, becomes stagnant again.

Venus exobiology

Possibly, 4 b.y.a., Venus might not have been so hellish.  Surface temperature might have been conducive to life.

Perhaps, life existed on Earth, Mars and Venus simultaneously, 4 b.y.a.

But over time--->

Venus heated up and became too hot

Mars cooled off and became too cold

Earth remained just right for life based on liquid water.

The habitable zone for constant liquid water in our solar system -Venus is too hot, Mars is too cold, the Earth is just right!

Life could have originated on all 3 planets independently or carried by meteorites ejected from impacts.

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