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Venus vs. Earth

● In bulk, Venus and Earth are twin worlds.

– nearly the same size

– nearly the same density

● This similarity leads to an expectation of similar evolutionary histories.

– They are made of the same mix of rock and metal, must have had similar outgassing and impact histories...

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Venus vs. Earth

● In bulk, Venus and Earth are twin worlds

– nearly the same size

– nearly the same density

● 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: Evidently Not a Twin

● Venus has an atmosphere mostly composed of Carbon Dioxide (96% CO

2 4% Nitrogen)

with a surface pressure nearly 100 times that of Earth.

● Clouds composed of droplets of sulfuric acid hide 100% of the surface 100% of the time.

● The surface temperature is 750K (900 F)

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Venus: Evidently Not a Twin

● Venus has an atmosphere mostly composed of Carbon Dioxide with a surface pressure nearly 100 times that of Earth.

● Clouds composed of droplets of sulfuric acid hide 100% of the surface 100% of the time.

● The surface temperature is 750K (900 F)

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Venus: Hotter than Expected

● 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.

● 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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Venus: The Greenhouse on Steroids● Currently, on Earth, we are concerned about an atmospheric

abundance of Carbon Dioxide of a fraction of a percent.

– The Earth's greenhouse is essential for life on Earth.

● 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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The Greenhouse Effect

● Carbon Dioxide is transparent to visible light.

– Light enters the atmosphere and warms the surface

– The warm surface emits infrared light

– Carbon dioxide traps infrared

– Temperatures increase until enough infrared leaks out to balance incoming sunlight energy.

9Why the Difference between Earth and Venus??

● 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

● 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?

– Ultraviolet light from the Sun can break up water molecules

● The hydrogen escapes due to its low mass.

● “Heavy” hydrogen (deuterium) lingers

– 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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Radar Mapping● The Venusian clouds forever hide the surface of Venus from prying

eyes at visible wavelengths

– But radio light can penetrate the clouds.

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Radar Mapping● The Venusian clouds forever hide the surface of Venus from prying

eyes at visible wavelengths

– But radio light can penetrate the clouds.

● An Earth-based or Venus-orbiting radio transmitter can send out a pulse of radio light (like a flash picture).

– The time delay for the pulse coming back reveals surface topography.

– 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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Landers● Spacecraft from Earth have visited the Venusian surface

– Given the temperature, survival is brief – a few hours

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Landers● Spacecraft from Earth have visited the Venusian surface

– Given the temperature, survival is brief – a few hours

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Landers● Spacecraft from Earth have visited the Venusian surface

– Given the temperature, survival is brief – a few hours

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Venusian Surface History● Large (>35 kilometer) impact craters uniformly pepper the surface

of Venus. 1000 of them, compared with about 200 on Earth.

– Venus' thick atmosphere stops smaller impactors.

– The uniformity points to a global resurfacing event ½ billion years ago.

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Venusian Volcanoes● Volcanoes are common on Venus.

– Due to the lack of weather, erosion is slow. The volcanism may look “recent” simply because of this lack of erosion.

– Most impact craters are undisturbed.

– Some astronomers argue that Venus is currently geologically dead.

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The Venusian Surface● 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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The Venusian Surface● 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.