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In bulk, Venus and Earth are twin worlds.
– nearly the same size
– nearly the same density
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This similarity leads to an expectation of similar evolutionary histories.
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They are made of the same mix of rock and metal, must have had similar outgassing and impact histories...

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In bulk, Venus and Earth are twin worlds
– nearly the same size
– nearly the same density
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This similarity leads to an expectation of similar evolutionary histories.
One look from the outside suggests that they have achieved substantially different outcomes.

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Venus has an atmosphere mostly composed of Carbon
Dioxide (96% CO
2
4% Nitrogen) with a surface pressure nearly
100 times that of Earth.
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Clouds composed of droplets of sulfuric acid hide 100% of the surface 100% of the time.
The surface temperature is 750K
(900 F)
3

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Venus has an atmosphere mostly composed of Carbon
Dioxide with a surface pressure nearly 100 times that of Earth.
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Clouds composed of droplets of sulfuric acid hide 100% of the surface 100% of the time.
●
The surface temperature is 750K
(900 F)
4

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The temperature of a planet is a balance between the incoming solar radiation and the outgoing blackbody “glow”, primarily at infrared wavelengths.
– Ignoring subtleties like reflectivity
(albedo) and infrared emission efficiency, the temperature of a rotating planet is 280K divided by the square root of the planet's distance from the sun in AU.
– The Earth's equilibrium temperature should be right around freezing.
Venus should be about 330K or
140F.
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Accounting for Venus' high reflectivity this simple calculation suggests a more Earthlike temperature.
–
1950's science fiction movies depicted a tropical Venus.
T =
280K
  AU 
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6

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Currently, on Earth, we are concerned about an atmospheric abundance of Carbon Dioxide of a fraction of a percent.
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The Earth's greenhouse is essential for life on Earth.
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Accounting for albedo, the Earth's equilibrium temperature would be well below freezing without it.
Contributors include water vapor, carbon dioxide and methane among others.
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Carbon Dioxide is transparent to visible light.
–
Light enters the atmosphere and warms the surface
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The warm surface emits infrared light
– Carbon dioxide traps infrared
– Temperatures increase until enough infrared leaks out to balance incoming sunlight energy.
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Liquid water on Earth an lots of it!
–
Earth was initially a twin to Venus with a crushing
Carbon Dioxide atmosphere, but oceans removed Earth's CO
2 leaving Nitrogen.
– Venus being closer to the Sun may
●
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● have received less water initially had a hard time forming oceans due to the higher temperatures (and the greenhouse effect of the water vapor itself).
Where is the water today?
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Ultraviolet light from the Sun can break up water molecules
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The hydrogen escapes due to its low mass.
“Heavy” hydrogen (deuterium) lingers
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On Venus deuterium is 10 times more abundant than on Earth, suggesting that it indeed has forever lost an ocean worth of water.
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The Venusian clouds forever hide the surface of Venus from prying eyes at visible wavelengths
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But radio light can penetrate the clouds.
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The Venusian clouds forever hide the surface of Venus from prying eyes at visible wavelengths
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But radio light can penetrate the clouds.
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An Earth-based or Venus-orbiting radio transmitter can send out a pulse of radio light (like a flash picture).
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The time delay for the pulse coming back reveals surface topography.
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The brightness of the returned signal reveals surface roughness.
– A rotating planet combined with the Doppler effect permits a reconstruction of a detailed picture.
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Venus does not display the clear distinction between “highlands” and
“lowlands” seeing on the Earth.
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Spacecraft from Earth have visited the Venusian surface
–
Given the temperature, survival is brief – a few hours
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Spacecraft from Earth have visited the Venusian surface
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Given the temperature, survival is brief – a few hours
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Spacecraft from Earth have visited the Venusian surface
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Given the temperature, survival is brief – a few hours
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Large (>35 kilometer) impact craters uniformly pepper the surface of Venus. 1000 of them, compared with about 200 on Earth.
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Venus' thick atmosphere stops smaller impactors.
– The uniformity points to a global resurfacing event ½ billion years ago.
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Volcanoes are common on Venus.
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Due to the lack of weather, erosion is slow. The volcanism may look
“recent” simply because of this lack of erosion.
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Most impact craters are undisturbed.
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Some astronomers argue that Venus is currently geologically dead.
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Overall, the surface of Venus looks quite young, consistent with a global resurfacing ½ billion years ago.
At the same time, much of the “action” seems to have occurred at this distant time, and Venus is likely not quite so active today.
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Overall, the surface of Venus looks quite young, consistent with a global resurfacing ½ billion years ago.
At the same time, much of the “action” seems to have occurred at this distant time, and Venus is likely not quite so active today.
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One theory:
Earth dissipates its internal heat through plate tectonics and volcanism at plate boundaries.
Venus does not have a crust that accommodates this process. Heat builds up leading to catastrophic resurfacing at billion-year intervals.
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