1 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...

2 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.

3 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)

4 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)

5 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.

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7 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.

8 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.

9 Why 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.

10 Radar Mapping The Venusian clouds forever hide the surface of Venus from prying eyes at visible wavelengths  But radio light can penetrate the clouds.

11 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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16 Venus does not display the clear distinction between “highlands” and “lowlands” seeing on the Earth.

17 Landers Spacecraft from Earth have visited the Venusian surface  Given the temperature, survival is brief – a few hours

18 Landers Spacecraft from Earth have visited the Venusian surface  Given the temperature, survival is brief – a few hours

19 Landers Spacecraft from Earth have visited the Venusian surface  Given the temperature, survival is brief – a few hours

20 Venusian Surface History Large (>35 kilometer) impact craters uniformly pepper the surface of Venus 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.

21 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.

22 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.

23 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.

24 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.