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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.